u-dma-buf(User space mappable DMA Buffer)

Overview

Introduction of u-dma-buf

u-dma-buf is a Linux device driver that allocates contiguous memory blocks in the kernel space as DMA buffers and makes them available from the user space. It is intended that these memory blocks are used as DMA buffers when a user application implements device driver in user space using UIO (User space I/O).

A DMA buffer allocated by u-dma-buf can be accessed from the user space by opening the device file (e.g. /dev/udmabuf0) and mapping to the user memory space, or using the read()/write() functions.

CPU cache for the allocated DMA buffer can be disabled by setting the O_SYNC flag when opening the device file. It is also possible to flush or invalidate CPU cache while retaining CPU cache enabled.

The physical address of a DMA buffer allocated by u-dma-buf can be obtained by reading /sys/class/u-dma-buf/udmabuf0/phys_addr.

The size of a DMA buffer and the device minor number can be specified when the device driver is loaded (e.g. when loaded via the insmod command). Some platforms allow to specify them in the device tree.

Architecture of u-dma-buf

Figure 1. Architecture

Supported platforms

• OS : Linux Kernel Version 3.6 - 3.8, 3.18, 4.4, 4.8, 4.12, 4.14, 4.19, 5.0 - 5.10 (the author tested on 3.18, 4.4, 4.8, 4.12, 4.14, 4.19, 5.4, 5.10).
• CPU: ARM Cortex-A9 (Xilinx ZYNQ / Altera CycloneV SoC)
• CPU: ARM64 Cortex-A53 (Xilinx ZYNQ UltraScale+ MPSoC)
• CPU: x86(64bit) However, verification is not enough. I hope the results from everyone. In addition, there is a limit to the following feature at the moment.
  • Can not control of the CPU cache by O_SYNC flag . Always CPU cache is valid.
  • Can not various settings by the device tree.

Note: udmabuf to u-dma-buf

Why u-dma-buf instead of udmabuf

The predecessor of u-dma-buf is udmabuf. The kernel module name has been changed from "udmabuf" to "u-dma-buf". The purpose of this is to avoid duplicate names because another kernel module with the same name as "udmabuf" has been added since Linux Kernel 5.x.

Changes from udmabuf to u-dma-buf

Usage

Compile

Makefile

This repository contains a Makefie. Makefile has the following Parameters:

Cross Compile

If you have a cross-compilation environment for target system, you can compile with:

shell$ make ARCH=arm KERNEL_SRC=/home/fpga/src/linux-5.10.120-zynqmp-fpga-generic all

The ARCH variable specifies the architecture name.
The KERNEL_SRC variable specifies the Linux Kernel source code path.

Self Compile

If your target system is capable of self-compiling the Linux Kernel module, you can compile it with:

shell$ make all

You need the kernel source code in /lib/modules/$(shell uname -r)/build to compile.

Build in Linux Source Tree

It can also be compiled into the Linux Kernel Source Tree.

Make directory in Linux Kernel Source Tree.

shell$ mkdir <linux-source-tree>/drivers/staging/u-dma-buf

Copy files to Linux Kernel Source Tree.

shell$ cp Kconfig Makefile u-dma-buf.c <linux-source-tree>/drivers/staging/u-dma-buf

Add u-dma-buf to Kconfig

shell$ diff <linux-source-tree>/drivers/staging/Kconfig
  :
+source "drivers/staging/u-dma-buf/Kconfig"
+

Add u-dma-buf to Makefile

shell$ diff <linux-source-tree>/drivers/staging/Makefile
  :
+obj-$(CONFIG_U_DMA_BUF) += u-dma-buf/

Set CONFIG_U_DMA_BUF

For make menuconfig, set the following:

Device Drivers --->
  Staging drivers --->
    <M> u-dma-buf(User space mappable DMA Buffer) --->

If you write it directly in defconfig:

shell$ diff <linux-source-tree>/arch/arm64/configs/xilinx_zynqmp_defconfig
   :
+CONFIG_U_DMA_BUF=m

Install

Installation with the insmod

Load the u-dma-buf kernel driver using insmod. The size of a DMA buffer should be provided as an argument as follows. The device driver is created, and allocates a DMA buffer with the specified size. The maximum number of DMA buffers that can be allocated using insmod is 8 (udmabuf0/1/2/3/4/5/6/7).

zynq$ insmod u-dma-buf.ko udmabuf0=1048576
u-dma-buf udmabuf0: driver version = 4.4.1
u-dma-buf udmabuf0: major number   = 248
u-dma-buf udmabuf0: minor number   = 0
u-dma-buf udmabuf0: phys address   = 0x1e900000
u-dma-buf udmabuf0: buffer size    = 1048576
u-dma-buf u-dma-buf.0: driver installed.
zynq$ ls -la /dev/udmabuf0
crw------- 1 root root 248, 0 Dec  1 09:34 /dev/udmabuf0

In the above result, the device is only read/write accessible by root. If the permission needs to be changed at the load of the kernel module, create /etc/udev/rules.d/99-u-dma-buf.rules with the following content.

SUBSYSTEM=="u-dma-buf", GROUP="root", MODE="0666"

The module can be uninstalled by the rmmod command.

zynq$ rmmod u-dma-buf
u-dma-buf u-dma-buf.0: driver removed.

Installation with the Debian package

For details, refer to the following URL.

• https://github.com/ikwzm/u-dma-buf-kmod-dpkg

Configuration via the module parameters

The u-dma-buf kernel module has the following module parameters:

udmabuf[0-7]

This parameter specifies the capacity of the u-dma-buf to be created in bytes. The number of u-dma-buf that can be created with this parameter is 8. The device name will be udmabuf[0-7]. If this parameter is 0, the u-dma-buf is not created.

info_enable

This parameter specifies whether or not detailed information about when the u-dma-buf was created should be displayed.

dma_mask_bit

** Note: The value of dma-mask is system dependent. Make sure you are familiar with the meaning of dma-mask before setting. **

bind

This parameter specifies the parent device of the u-dma-buf. If this parameter is an empty string (default value), u-dma-buf is created as a new platform device. If a parent device name is specified for this parameter, u-dma-buf is created as its child device.

The format of the string specified in this parameter is "<bus>/<device-name>".

The <bus> is the bus name, currently pci is supported. The bus name can be omitted. If omitted, it will be the platform bus.

The <device-name> specifies the name of the device under bus management.

For example, to designate "0000:00:15.0" under the pci bus as the parent device, do the following

shell$ sudo insmod u-dma-buf.ko udmabuf0=0x10000 info_enable=3 bind="pci/0000:00:15.0" 
[13422.022482] u-dma-buf udmabuf0: driver version = 4.4.1
[13422.022483] u-dma-buf udmabuf0: major number   = 238
[13422.022483] u-dma-buf udmabuf0: minor number   = 0
[13422.022484] u-dma-buf udmabuf0: phys address   = 0x0000000070950000
[13422.022485] u-dma-buf udmabuf0: buffer size    = 65536
[13422.022485] u-dma-buf udmabuf0: dma device     = 0000:00:15.0
[13422.022486] u-dma-buf udmabuf0: dma bus        = pci
[13422.022486] u-dma-buf udmabuf0: dma coherent   = 1
[13422.022487] u-dma-buf udmabuf0: dma mask       = 0x00000000ffffffff
[13422.022487] u-dma-buf udmabuf0: iommu domain   = NONE
[13422.022487] u-dma-buf udmabuf0: quirk mmap     = 0
[13422.022488] u-dma-buf: udmabuf0 installed.

quirk_mmap_mode

This parameter specifies the default value of quirk-mmap-mode. quirk-mmap is described in detail below.
If this parameter is 1, quirk-mmap is prohibited.
If this parameter is 2, quirk-mmap is used.
If this parameter is 3, quirk-mmap is not used if the device has a dma-cohrent of true, and quirk-mmap is used only if dma-coherent is false.

If the architecture is ARM or ARM64, this parameter defaults to 2.
If the architecture is other than the above, this parameter defaults to 3.

Configuration via the device tree file

In addition to the allocation via the insmod command and its arguments, DMA buffers can be allocated by specifying the size in the device tree file. When a device tree file contains an entry like the following, u-dma-buf will allocate buffers and create device drivers when loaded by insmod.

		udmabuf@0x00 {
			compatible = "ikwzm,u-dma-buf";
			device-name = "udmabuf0";
			minor-number = <0>;
			size = <0x00100000>;
		};

zynq$ insmod u-dma-buf.ko
u-dma-buf udmabuf0: driver version = 4.4.1
u-dma-buf udmabuf0: major number   = 248
u-dma-buf udmabuf0: minor number   = 0
u-dma-buf udmabuf0: phys address   = 0x1e900000
u-dma-buf udmabuf0: buffer size    = 1048576
u-dma-buf amba:udmabuf@0x00: driver installed.
zynq$ ls -la /dev/udmabuf0
crw------- 1 root root 248, 0 Dec  1 09:34 /dev/udmabuf0

The following properties can be set in the device tree.

• compatible
• size
• minor-number
• device-name
• sync-mode
• sync-always
• sync-offset
• sync-size
• sync-direction
• dma-coherent
• dma-mask
• quirk-mmap-off
• quirk-mmap-on
• quirk-mmap-auto
• memory-region

compatible

The compatible property is used to set the corresponding device driver when loading u-dma-buf. The compatible property is mandatory. Be sure to specify compatible property as "ikwzm,u-dma-buf" (for u-dma-buf.ko) or "ikwzm,udmabuf-0.10.a" (for udmabuf.ko).

size

The size property is used to set the capacity of DMA buffer in bytes. The size property is mandatory.

		udmabuf@0x00 {
			compatible = "ikwzm,u-dma-buf";
			size = <0x00100000>;
		};

If you want to specify a buffer size of 4GiB or more, specify a 64bit value as follows. A 64-bit value is expressed by arranging two in the order of upper 32 bits and lower 32 bits.

		udmabuf@0x00 {
			compatible = "ikwzm,u-dma-buf";
			size = <0x01 0x00000000>;  // size = 0x1_0000_0000
		};

minor-number

The minor-number property is used to set the minor number. The valid minor number range is 0 to 255. A minor number provided as insmod argument will has higher precedence, and when definition in the device tree has colliding number, creation of the device defined in the device tree will fail.

The minor-number property is optional. When the minor-number property is not specified, u-dma-buf automatically assigns an appropriate one.

		udmabuf@0x00 {
			compatible = "ikwzm,u-dma-buf";
			minor-number = <0>;
			size = <0x00100000>;
		};

device-name

The device-name property is used to set the name of device.

The device-name property is optional. The device name is determined as follow:

1. If device-name property is specified, the value of device-name property is used.
2. If device-name property is not present, and if minor-number property is specified, sprintf("udmabuf%d", minor-number) is used.
3. If device-name property is not present, and if minor-number property is not present, the entry name of the device tree is used (udmabuf@0x00 in this example).

		udmabuf@0x00 {
			compatible = "ikwzm,u-dma-buf";
			device-name = "udmabuf0";
			size = <0x00100000>;
		};

sync-mode

The sync-mode property is used to configure the behavior when u-dma-buf is opened with the O_SYNC flag.

• sync-mode=<1>: If O_SYNC is specified or sync-always property is specified, CPU cache is disabled. Otherwise CPU cache is enabled.
• sync-mode=<2>: If O_SYNC is specified or sync-always property is specified, CPU cache is disabled but CPU uses write-combine when writing data to DMA buffer improves performance by combining multiple write accesses. Otherwise CPU cache is enabled.
• sync-mode=<3>: If O_SYNC is specified or sync-always property is specified, DMA coherency mode is used. Otherwise CPU cache is enabled.

The sync-mode property is optional. When the sync-mode property is not specified, sync-mode is set to <1>.

		udmabuf@0x00 {
			compatible = "ikwzm,u-dma-buf";
			size = <0x00100000>;
			sync-mode = <2>;
		};

Details on O_SYNC and cache management will be described in the next section.

sync-always

If the sync-always property is specified, when opening u-dma-buf, it specifies that the operation specified by the sync-mode property will always be performed regardless of O_SYNC specification.

The sync-always property is optional.

		udmabuf@0x00 {
			compatible = "ikwzm,u-dma-buf";
			size = <0x00100000>;
			sync-mode = <2>;
			sync-always;
		};

Details on O_SYNC and cache management will be described in the next section.

sync-offset

The sync-offset property is used to set the start of the buffer range when manually controlling the cache of u-dma-buf.

The sync-offset property is optional. When the sync-offset property is not specified, sync-offset is set to <0>.

Details on cache management will be described in the next section.

sync-size

The sync-size property is used to set the size of the buffer range when manually controlling the cache of u-dma-buf.

The sync-size property is optional. When the sync-size property is not specified, sync-size is set to <0>.

Details on cache management will be described in the next section.

sync-direction

The sync-direction property is used to set the direction of DMA when manually controlling the cache of u-dma-buf.

• sync-direction=<0>: DMA_BIDIRECTIONAL
• sync-direction=<1>: DMA_TO_DEVICE
• sync-direction=<2>: DMA_FROM_DEVICE

The sync-direction property is optional. When the sync-direction property is not specified, sync-direction is set to <0>.

		udmabuf@0x00 {
			compatible = "ikwzm,u-dma-buf";
			size = <0x00100000>;
			sync-offset = <0x00010000>;
			sync-size = <0x000F0000>;
			sync-direction = <2>;
		};

Details on cache management will be described in the next section.

dma-coherent

If the dma-coherent property is specified, indicates that coherency between DMA buffer and CPU cache can be guaranteed by hardware.

The dma-coherent property is optional. When the dma-coherent property is not specified, indicates that coherency between DMA buffer and CPU cache can not be guaranteed by hardware.

		udmabuf@0x00 {
			compatible = "ikwzm,u-dma-buf";
			size = <0x00100000>;
			dma-coherent;
		};

Details on cache management will be described in the next section.

dma-mask

** Note: The value of dma-mask is system dependent. Make sure you are familiar with the meaning of dma-mask before setting. **

		udmabuf@0x00 {
			compatible = "ikwzm,u-dma-buf";
			size = <0x00100000>;
			dma-mask = <64>;
		};

quirk-mmap-off

If the quirk-mmap-off property is specified, quirk-mmap. is not used.

quirk-mmap-on

If the quirk-mmap-on property is specified, quirk-mmap. is used.

quirk-mmap-auto

If the quirk-mmap-auto property is specified, quirk-mmap is not used if the device has a dma-cohrent of true, and quirk-mmap is used only if dma-coherent is false.

memory-region

Linux can specify the reserved memory area in the device tree. The Linux kernel excludes normal memory allocation from the physical memory space specified by reserved-memory property. In order to access this reserved memory area, it is necessary to use a general-purpose memory access driver such as /dev/mem, or associate it with the device driver in the device tree.

By the memory-region property, it can be associated the reserved memory area with u-dma-buf.

	reserved-memory {
		#address-cells = <1>;
		#size-cells = <1>;
		ranges;
		image_buf0: image_buf@0 {
			compatible = "shared-dma-pool";
			reusable;
			reg = <0x3C000000 0x04000000>; 
			label = "image_buf0";
		};
	};
	udmabuf@0 {
		compatible = "ikwzm,u-dma-buf";
		device-name = "udmabuf0";
		size = <0x04000000>; // 64MiB
		memory-region = <&image_buf0>;
	};

In this example, 64MiB of 0x3C000000 to 0x3FFFFFFF is reserved as "image_buf0". In this "image_buf0", specify "shared-dma-pool" in compatible property and specify the reusable property. By specifying these properties, this reserved memory area will be allocated by the CMA. Also, you need to be careful about address and size alignment.

The above "image_buf0" is associated with "udmabuf@0" with memory-region property. With this association, "udmabuf@0" reserves physical memory from the CMA area specified by "image_buf0".

The memory-region property is optional. When the memory-region property is not specified, u-dma-buf allocates the DMA buffer from the CMA area allocated to the Linux kernel.

Configuration via the /dev/u-dma-buf-mgr

Since u-dma-buf v4.0, u-dma-buf devices can be create or delete using u-dma-buf-mgr. See https://github.com/ikwzm/u-dma-buf-mgr for more information.

Device file

When u-dma-buf is loaded into the kernel, the following device files are created. <device-name> is a placeholder for the device name described in the previous section.

• /dev/<device-name>
• /sys/class/u-dma-buf/<device-name>/phys_addr
• /sys/class/u-dma-buf/<device-name>/size
• /sys/class/u-dma-buf/<device-name>/sync_mode
• /sys/class/u-dma-buf/<device-name>/sync_offset
• /sys/class/u-dma-buf/<device-name>/sync_size
• /sys/class/u-dma-buf/<device-name>/sync_direction
• /sys/class/u-dma-buf/<device-name>/sync_owner
• /sys/class/u-dma-buf/<device-name>/sync_for_cpu
• /sys/class/u-dma-buf/<device-name>/sync_for_device
• /sys/class/u-dma-buf/<device-name>/dma_coherent

/dev/<device-name>

/dev/<device-name> is used when mmap()-ed to the user space or accessed via read()/write().

    if ((fd  = open("/dev/udmabuf0", O_RDWR)) != -1) {
        buf = mmap(NULL, buf_size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
        /* Do some read/write access to buf */
        close(fd);
    }

The device file can be directly read/written by specifying the device as the target of dd in the shell.

zynq$ dd if=/dev/urandom of=/dev/udmabuf0 bs=4096 count=1024
1024+0 records in
1024+0 records out
4194304 bytes (4.2 MB) copied, 3.07516 s, 1.4 MB/s

zynq$dd if=/dev/udmabuf4 of=random.bin
8192+0 records in
8192+0 records out
4194304 bytes (4.2 MB) copied, 0.173866 s, 24.1 MB/s

phys_addr

The physical address of a DMA buffer can be retrieved by reading /sys/class/u-dma-buf/<device-name>/phys_addr.

    unsigned char  attr[1024];
    unsigned long  phys_addr;
    if ((fd  = open("/sys/class/u-dma-buf/udmabuf0/phys_addr", O_RDONLY)) != -1) {
        read(fd, attr, 1024);
        sscanf(attr, "%x", &phys_addr);
        close(fd);
    }

size

The size of a DMA buffer can be retrieved by reading /sys/class/u-dma-buf/<device-name>/size.

    unsigned char  attr[1024];
    unsigned int   buf_size;
    if ((fd  = open("/sys/class/u-dma-buf/udmabuf0/size", O_RDONLY)) != -1) {
        read(fd, attr, 1024);
        sscanf(attr, "%d", &buf_size);
        close(fd);
    }

sync_mode

The device file /sys/class/u-dma-buf/<device-name>/sync_mode is used to configure the behavior when u-dma-buf is opened with the O_SYNC flag.

    unsigned char  attr[1024];
    unsigned long  sync_mode = 2;
    if ((fd  = open("/sys/class/u-dma-buf/udmabuf0/sync_mode", O_WRONLY)) != -1) {
        sprintf(attr, "%d", sync_mode);
        write(fd, attr, strlen(attr));
        close(fd);
    }

Details on O_SYNC and cache management will be described in the next section.

sync_offset

The device file /sys/class/u-dma-buf/<device-name>/sync_offset is used to specify the start address of a memory block of which cache is manually managed.

    unsigned char  attr[1024];
    unsigned long  sync_offset = 0x00000000;
    if ((fd  = open("/sys/class/u-dma-buf/udmabuf0/sync_offset", O_WRONLY)) != -1) {
        sprintf(attr, "%d", sync_offset); /* or sprintf(attr, "0x%x", sync_offset); */
        write(fd, attr, strlen(attr));
        close(fd);
    }

Details of manual cache management is described in the next section.

sync_size

The device file /sys/class/u-dma-buf/<device-name>/sync_size is used to specify the size of a memory block of which cache is manually managed.

    unsigned char  attr[1024];
    unsigned long  sync_size = 1024;
    if ((fd  = open("/sys/class/u-dma-buf/udmabuf0/sync_size", O_WRONLY)) != -1) {
        sprintf(attr, "%d", sync_size); /* or sprintf(attr, "0x%x", sync_size); */
        write(fd, attr, strlen(attr));
        close(fd);
    }

Details of manual cache management is described in the next section.

sync_direction

The device file /sys/class/u-dma-buf/<device-name>/sync_direction is used to set the direction of DMA transfer to/from the DMA buffer of which cache is manually managed.

• 0: sets DMA_BIDIRECTIONAL
• 1: sets DMA_TO_DEVICE
• 2: sets DMA_FROM_DEVICE

    unsigned char  attr[1024];
    unsigned long  sync_direction = 1;
    if ((fd  = open("/sys/class/u-dma-buf/udmabuf0/sync_direction", O_WRONLY)) != -1) {
        sprintf(attr, "%d", sync_direction);
        write(fd, attr, strlen(attr));
        close(fd);
    }

Details of manual cache management is described in the next section.

dma_coherent

The device file /sys/class/u-dma-buf/<device-name>/dma_coherent can read whether the coherency of DMA buffer and CPU cache can be guaranteed by hardware. It is able to specify whether or not it is able to guarantee by hardware with the dma-coherent property in the device tree, but this device file is read-only.

If this value is 1, the coherency of DMA buffer and CPU cache can be guaranteed by hardware. If this value is 0, the coherency of DMA buffer and CPU cache can be not guaranteed by hardware.

    unsigned char  attr[1024];
    int dma_coherent;
    if ((fd  = open("/sys/class/u-dma-buf/udmabuf0/dma_coherent", O_RDONLY)) != -1) {
        read(fd, attr, 1024);
        sscanf(attr, "%x", &dma_coherent);
        close(fd);
    }

sync_owner

The device file /sys/class/u-dma-buf/<device-name>/sync_owner reports the owner of the memory block in the manual cache management mode. If this value is 1, the buffer is owned by the device. If this value is 0, the buffer is owned by the cpu.

    unsigned char  attr[1024];
    int sync_owner;
    if ((fd  = open("/sys/class/u-dma-buf/udmabuf0/sync_owner", O_RDONLY)) != -1) {
        read(fd, attr, 1024);
        sscanf(attr, "%x", &sync_owner);
        close(fd);
    }

Details of manual cache management is described in the next section.

sync_for_cpu

In the manual cache management mode, CPU can be the owner of the buffer by writing non-zero to the device file /sys/class/u-dma-buf/<device-name>/sync_for_cpu. This device file is write only.

If '1' is written to device file, if sync_direction is 2(=DMA_FROM_DEVICE) or 0(=DMA_BIDIRECTIONAL), the write to the device file invalidates a cache specified by sync_offset and sync_size.

    unsigned char  attr[1024];
    unsigned long  sync_for_cpu = 1;
    if ((fd  = open("/sys/class/u-dma-buf/udmabuf0/sync_for_cpu", O_WRONLY)) != -1) {
        sprintf(attr, "%d", sync_for_cpu);
        write(fd, attr, strlen(attr));
        close(fd);
    }

The value written to this device file can include sync_offset, sync_size, and sync_direction.

    unsigned char  attr[1024];
    unsigned long  sync_offset    = 0;
    unsigned long  sync_size      = 0x10000;
    unsigned int   sync_direction = 0;
    unsigned long  sync_for_cpu   = 1;
    if ((fd  = open("/sys/class/u-dma-buf/udmabuf0/sync_for_cpu", O_WRONLY)) != -1) {
        sprintf(attr, "0x%08X%08X", (sync_offset & 0xFFFFFFFF), (sync_size & 0xFFFFFFF0) | (sync_direction << 2) | sync_for_cpu);
        write(fd, attr, strlen(attr));
        close(fd);
    }

The sync_offset/sync_size/sync_direction specified by sync_for_cpu is temporary and does not affect the sync_offset or sync_size or sync_direction device files.

Details of manual cache management is described in the next section.

sync_for_device

In the manual cache management mode, DEVICE can be the owner of the buffer by writing non-zero to the device file /sys/class/u-dma-buf/<device-name>/sync_for_device. This device file is write only.

If '1' is written to device file, if sync_direction is 1(=DMA_TO_DEVICE) or 0(=DMA_BIDIRECTIONAL), the write to the device file flushes a cache specified by sync_offset and sync_size (i.e. the cached data, if any, will be updated with data on DDR memory).

    unsigned char  attr[1024];
    unsigned long  sync_for_device = 1;
    if ((fd  = open("/sys/class/u-dma-buf/udmabuf0/sync_for_device", O_WRONLY)) != -1) {
        sprintf(attr, "%d", sync_for_device);
        write(fd, attr, strlen(attr));
        close(fd);
    }

The value written to this device file can include sync_offset, sync_size, and sync_direction.

    unsigned char  attr[1024];
    unsigned long  sync_offset     = 0;
    unsigned long  sync_size       = 0x10000;
    unsigned int   sync_direction  = 0;
    unsigned long  sync_for_device = 1;
    if ((fd  = open("/sys/class/u-dma-buf/udmabuf0/sync_for_device", O_WRONLY)) != -1) {
        sprintf(attr, "0x%08X%08X", (sync_offset & 0xFFFFFFFF), (sync_size & 0xFFFFFFF0) | (sync_direction << 2) | sync_for_device);
        write(fd, attr, strlen(attr));
        close(fd);
    }

The sync_offset/sync_size/sync_direction specified by sync_for_device is temporary and does not affect the sync_offset or sync_size or sync_direction device files.

Details of manual cache management is described in the next section.

Coherency of data on DMA buffer and CPU cache

CPU usually accesses to a DMA buffer on the main memory using cache, and a hardware accelerator logic accesses to data stored in the DMA buffer on the main memory. In this situation, coherency between data stored on CPU cache and them on the main memory should be considered carefully.

When the coherency is maintained by hardware

When hardware assures the coherency, CPU cache can be turned on without additional treatment. For example, ZYNQ provides ACP (Accelerator Coherency Port), and the coherency is maintained by hardware as long as the accelerator accesses to the main memory via this port.

In this case, accesses from CPU to the main memory can be fast by using CPU cache as usual. To enable CPU cache on the DMA buffer allocated by u-dma-buf, open u-dma-buf without specifying the O_SYNC flag.

    /* To enable CPU cache on the DMA buffer, */
    /* open u-dma-buf without specifying the `O_SYNC` flag. */
    if ((fd  = open("/dev/udmabuf0", O_RDWR)) != -1) {
        buf = mmap(NULL, buf_size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
        /* Read/write access to the buffer */
        close(fd);
    }

The manual management of cache, described in the following section, will not be necessary when hardware maintains the coherency.

If the dma-coherent property is specified in the device tree, specify that coherency can be guaranteed with hardware. In this case, the cache control described in "2. Manual cache management with the CPU cache still being enabled" described later is not performed.

When hardware does not maintain the coherency

To maintain coherency of data between CPU and the main memory, another coherency mechanism is necessary. u-dma-buf supports two different ways of coherency maintenance; one is to disable CPU cache, and the other is to involve manual cache flush/invalidation with CPU cache being enabled.

1. Disabling CPU cache

To disable CPU cache of allocated DMA buffer, specify the O_SYNC flag when opening u-dma-buf.

    /* To disable CPU cache on the DMA buffer, */
    /* open u-dma-buf with the `O_SYNC` flag. */
    if ((fd  = open("/dev/udmabuf0", O_RDWR | O_SYNC)) != -1) {
        buf = mmap(NULL, buf_size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
        /* Read/write access to the buffer */
        close(fd);
    }

As listed below, sync_mode can be used to configure the cache behavior when the O_SYNC flag is present in open():

• sync_mode=0: CPU cache is enabled regardless of the O_SYNC flag presence.
• sync_mode=1: If O_SYNC is specified, CPU cache is disabled. If O_SYNC is not specified, CPU cache is enabled.
• sync_mode=2: If O_SYNC is specified, CPU cache is disabled but CPU uses write-combine when writing data to DMA buffer improves performance by combining multiple write accesses. If O_SYNC is not specified, CPU cache is enabled.
• sync_mode=3: If O_SYNC is specified, DMA coherency mode is used. If O_SYNC is not specified, CPU cache is enabled.
• sync_mode=4: CPU cache is enabled regardless of the O_SYNC flag presence.
• sync_mode=5: CPU cache is disabled regardless of the O_SYNC flag presence.
• sync_mode=6: CPU uses write-combine to write data to DMA buffer regardless of O_SYNC presence.
• sync_mode=7: DMA coherency mode is used regardless of O_SYNC presence.

As a practical example, the execution times of a sample program listed below were measured under several test conditions as presented in the table.

int check_buf(unsigned char* buf, unsigned int size)
{
    int m = 256;
    int n = 10;
    int i, k;
    int error_count = 0;
    while(--n > 0) {
      for(i = 0; i < size; i = i + m) {
        m = (i+256 < size) ? 256 : (size-i);
        for(k = 0; k < m; k++) {
          buf[i+k] = (k & 0xFF);
        }
        for(k = 0; k < m; k++) {
          if (buf[i+k] != (k & 0xFF)) {
            error_count++;
          }
        }
      }
    }
    return error_count;
}
int clear_buf(unsigned char* buf, unsigned int size)
{
    int n = 100;
    int error_count = 0;
    while(--n > 0) {
      memset((void*)buf, 0, size);
    }
    return error_count;
}

Table-1　The execution time of the sample program checkbuf

Table-2　The execution time of the sample program clearbuf

Note: on using O_SYNC flag on ARM64

For v2.1.1 or earier, udmabuf used pgprot_writecombine() on ARM64 and sync_mode=1(noncached). The reason is that a bus error occurred in memset() in udmabuf_test.c when using pgprot_noncached().

However, as reported in #28, when using pgprot_writecombine() on ARM64, it was found that there was a problem with cache coherency.

Therefore, since v2.1.2, when sync_mode = 1, it was changed to use pgprot_noncached(). This is because cache coherency issues are very difficult to understand and difficult to debug. Rather than worrying about the cache coherency problem, we decided that it was easier to understand when the bus error occurred.

This change requires alignment attention when using O_SYNC cache control on ARM64. You probably won't be able to use memset().

If a problem occurs, either cache coherency is maintained by hardware, or use a method described below that manually cache management with CPU cache still being enabled.

2. Manual cache management with the CPU cache still being enabled

As explained above, by opening u-dma-buf without specifying the O_SYNC flag, CPU cache can be left turned on. However, for ARM or ARM64, this is only possible if quirk-mmap is enabled. quirk-mmap will be discussed in detail later.

    /* To enable CPU cache on the DMA buffer, */
    /* open u-dma-buf without specifying the `O_SYNC` flag. */
    if ((fd  = open("/dev/udmabuf0", O_RDWR)) != -1) {
        buf = mmap(NULL, buf_size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
        /* Read/write access to the buffer */
        close(fd);
    }

To manually manage cache coherency, users need to follow the

1. Specify a memory area shared between CPU and accelerator via sync_offset and sync_size device files. sync_offset accepts an offset from the start address of the allocated buffer in units of bytes. The size of the shared memory area should be set to sync_size in units of bytes.
2. Data transfer direction should be set to sync_direction. If the accelerator performs only read accesses to the memory area, sync_direction should be set to 1(=DMA_TO_DEVICE), and to 2(=DMA_FROM_DEVICE) if only write accesses.
3. If the accelerator reads and writes data from/to the memory area, sync_direction should be set to 0(=DMA_BIDIRECTIONAL).

Following the above configuration, sync_for_cpu and/or sync_for_device should be used to set the owner of the buffer specified by the above-mentioned offset and the size.

When CPU accesses to the buffer, '1' should be written to sync_for_cpu to set CPU as the owner. Upon the write to sync_for_cpu, CPU cache is invalidated if sync_direction is 2(=DMA_FROM_DEVICE) or 0(=DMA_BIDIRECTIONAL). Once CPU is becomes the owner of the buffer, the accelerator cannot access the buffer.

On the other hand, when the accelerator needs to access the buffer, '1' should be written to sync_for_device to change ownership of the buffer to the accelerator. Upon the write to sync_for_device, the CPU cache of the specified memory area is flushed using data on the main memory.

However, if the dma-coherent property is specified in the device tree, CPU cache is not invalidated and flushed.

Note: What is quirk-mmap?

The Linux Kernel mainline turns off caching when doing mmap() for architectures such as ARM and ARM64 where cache aliasing problems can occur.

However, u-dma-buf provides quirk-mmap to enable caching in cases where the above architecture does not cause cache alias problems. The quirk-mmap is u-dma-buf's own mmap mechanism and does not utilize the dma_mmap_coherent() provided by the dma-mapping API in the linux kernel. This may cause problems in some cases, so please be careful when using it.

Example using u-dma-buf with Python

The programming language "Python" provides an extension called "NumPy". This section explains how to do the same operation as "ndarry" by mapping the DMA buffer allocated in the kernel with memmap of "NumPy" with u-dma-buf.

Udmabuf Class

import numpy as np

class Udmabuf:
    """A simple u-dma-buf class"""
    def __init__(self, name):
        self.name           = name
        self.device_name    = '/dev/%s'                 % self.name
        self.class_path     = '/sys/class/u-dma-buf/%s' % self.name
        self.phys_addr      = self.get_value('phys_addr', 16)
        self.buf_size       = self.get_value('size')
        self.sync_offset    = None
        self.sync_size      = None
        self.sync_direction = None

    def memmap(self, dtype, shape):
        self.item_size = np.dtype(dtype).itemsize
        self.array     = np.memmap(self.device_name, dtype=dtype, mode='r+', shape=shape)
        return self.array

    def get_value(self, name, radix=10):
        value = None
        for line in open(self.class_path + '/' + name):
            value = int(line, radix)
            break
        return value
    def set_value(self, name, value):
        f = open(self.class_path + '/' + name, 'w')
        f.write(str(value))
        f.close

    def set_sync_area(self, direction=None, offset=None, size=None):
        if offset is None:
            self.sync_offset    = self.get_value('sync_offset')
        else:
            self.set_value('sync_offset', offset)
            self.sync_offset    = offset
        if size   is None:
            self.sync_size      = self.get_value('sync_size')
        else:
            self.set_value('sync_size', size)
            self.sync_size      = size
        if direction is None:
            self.sync_direction = self.get_value('sync_direction')
        else:
            self.set_value('sync_direction', direction)
            self.sync_direction = direction

    def set_sync_to_device(self, offset=None, size=None):
        self.set_sync_area(1, offset, size)

    def set_sync_to_cpu(self, offset=None, size=None):
        self.set_sync_area(2, offset, size)

    def set_sync_to_bidirectional(self, offset=None, size=None):
        self.set_sync_area(3, offset, size)

    def sync_for_cpu(self):
        self.set_value('sync_for_cpu', 1)

    def sync_for_device(self):
        self.set_value('sync_for_device', 1)

udmabuf_test.py

from udmabuf import Udmabuf
import numpy as np
import time
def test_1(a):
    for i in range (0,9):
        a *= 0
        a += 0x31
if __name__ == '__main__':
    udmabuf      = Udmabuf('udmabuf0')
    test_dtype   = np.uint8
    test_size    = udmabuf.buf_size//(np.dtype(test_dtype).itemsize)
    udmabuf.memmap(dtype=test_dtype, shape=(test_size))
    comparison   = np.zeros(test_size, dtype=test_dtype)
    print ("test_size  : %d" % test_size)
    start        = time.time()
    test_1(udmabuf.array)
    elapsed_time = time.time() - start
    print ("udmabuf0   : elapsed_time:{0}".format(elapsed_time) + "[sec]")
    start        = time.time()
    test_1(comparison)
    elapsed_time = time.time() - start
    print ("comparison : elapsed_time:{0}".format(elapsed_time) + "[sec]")
    if np.array_equal(udmabuf.array, comparison):
        print ("udmabuf0 == comparison : OK")
    else:
        print ("udmabuf0 != comparison : NG")

Execution result

Install u-dma-buf. In this example, 8MiB DMA buffer is reserved as "udmabuf0".

zynq# insmod u-dma-buf.ko udmabuf0=8388608
[ 1183.911189] u-dma-buf udmabuf0: driver version = 4.4.1
[ 1183.921238] u-dma-buf udmabuf0: major number   = 240
[ 1183.931275] u-dma-buf udmabuf0: minor number   = 0
[ 1183.936063] u-dma-buf udmabuf0: phys address   = 0x0000000041600000
[ 1183.942328] u-dma-buf udmabuf0: buffer size    = 8388608
[ 1183.947641] u-dma-buf u-dma-buf.0: driver installed.

Executing the script in the previous section gives the following results.

zynq# python3 udmabuf_test.py
test_size  : 8388608
udmabuf0   : elapsed_time:0.11204075813293457[sec]
comparison : elapsed_time:0.11488151550292969[sec]
udmabuf0 == comparison : OK

The execution time for "udmabuf0"(buffer area secured in the kernel) and the same operation with ndarray (comparison) were almost the same. That is, it seems that "udmabuf0" is also effective CPU cache.

I confirmed the contents of "udmabuf0" after running this script.

zynq# dd if=/dev/udmabuf0 of=udmabuf0.bin bs=8388608
1+0 records in
1+0 records out
8388608 bytes (8.4 MB) copied, 0.151531 s, 55.4 MB/s
shell# 
shell# od -t x1 udmabuf0.bin
0000000 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31
*
40000000

After executing the script, it was confirmed that the result of the execution remains in the buffer. Just to be sure, let's check that NumPy can read it.

zynq# python
Python 2.7.9 (default, Aug 13 2016, 17:56:53)
[GCC 4.9.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import numpy as np
>>> a = np.memmap('/dev/udmabuf0', dtype=np.uint8, mode='r+', shape=(8388608))
>>> a
memmap([49, 49, 49, ..., 49, 49, 49], dtype=uint8)
>>> a.itemsize
1
>>> a.size
8388608
>>>