udmabuf

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User space mappable dma buffer device driver for Linux.

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User space mappable dma buffer device driver for Linux.

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, 6.1, 6.6, 6.12, 6.18 (the author tested on 3.18, 4.4, 4.8, 4.12, 4.14, 4.19, 5.4, 5.10, 6.1, 6.6, 6.12, 6.18).
CPU: ARM Cortex-A9 (Xilinx ZYNQ / Altera CycloneV SoC)
CPU: ARM64 Cortex-A53 (Xilinx ZYNQ UltraScale+ MPSoC)
CPU: RISC-V (Microchip Polarfire SoC FPGA)
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

Categoly	udmabuf	u-dma-buf
module name	udmabuf.ko	u-dma-buf.ko
source file	udmabuf.c	u-dma-buf.c
sys class name	/sys/class/udmabuf/	/sys/class/u-dma-buf/
DT compatible prop.	"ikwzm,udmabuf-0.10.a"	"ikwzm,u-dma-buf"

Usage

Compile

Makefile

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

Parameter Name	Description	Default Value
ARCH	Architecture Name	`$(shell uname -m \| sed -e s/arm./arm/ -e s/aarch64./arm64/ -e s/riscv.*/riscv/ )`
KERNEL_SRC	Kernel Source Directory	`/lib/modules/$(shell uname -r)/build`
CONFIG_U_DMA_BUF_DEBUG	Enable debug report	y
CONFIG_U_DMA_BUF_QUIRK_MMAP	Enable quirk-mmap	y
CONFIG_U_DMA_BUF_IN_KERNEL_FUNCTIONS	Enable in-kernel functions	y
CONFIG_U_DMA_BUF_IOCTL	Enable ioctl	y
CONFIG_U_DMA_BUF_EXPORT	Enable PRIME DMA-BUFS export	y

Cross Compile

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

bash

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:

bash

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.

bash

shell$ mkdir /drivers/staging/u-dma-buf

Copy files to Linux Kernel Source Tree.

bash

shell$ cp Kconfig Makefile u-dma-buf.c /drivers/staging/u-dma-buf

Add u-dma-buf to Kconfig

bash

shell$ diff /drivers/staging/Kconfig
  :
+source "drivers/staging/u-dma-buf/Kconfig"
+

Add u-dma-buf to Makefile

makefile

shell$ diff /drivers/staging/Makefile
  :
+obj-$(CONFIG_U_DMA_BUF) += u-dma-buf/

Set CONFIG_U_DMA_BUF

For make menuconfig, set the following:

bash

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

If you write it directly in defconfig:

bash

shell$ diff /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).

bash

zynq$ insmod u-dma-buf.ko udmabuf0=1048576
u-dma-buf udmabuf0: driver version = 5.5.0
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.

rules

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

The module can be uninstalled by the rmmod command.

bash

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:

Parameter Name	Type	Default	Description
udmabuf[0-7]	ulong	0	u-dma-buf[0-7] buffer size
info_enable	int	1	install/uninstall infomation enable
dma_mask_bit	int	32	dma mask bit size
udmabuf[0-7]_bind	charp	""	u-dma-buf[0-7] bind device name
bind	charp	""	bind device name
quirk_mmap_mode	int	2 or 3	quirk mmap mode(1:off,2:on,3:auto,4:page)

`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. **

`udmabuf[0-7]_bind`

This parameter specifies the parent device of u-dma-buf[x] (x is a number from 0 to 7). If this parameter is an empty string (default value) and the bind parameter described below is an empty string (default value), u-dma-buf[x] is created as a new platform device. If this parameter is an empty string (default value) and the bind parameter described below is not an empty string, u-dma-buf[x] is created as a child device of the device indicated by the bind parameter. If this parameter is not an empty string, u-dma-buf[x] is created as a child device of the device indicated by this parameter.

The format of the string specified in this parameter is "/".

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

The `` 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 of udmabuf0, do the following

…

`bind`

This parameter specifies the parent device of u-dma-buf[0-7]. If the preceding udmabuf[x]_bind is an empty string (default value) and this parameter is an empty string (default value), u-dma-buf[x] is created as a new platform device. If the preceding udmabuf[x]_bind is an empty string (default value) and this parameter is an not empty string, u-dma-buf[x] is created as a child device of the device indicated by this bind parameter.

The format of the string specified in this parameter is "/".

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

The `` specifies the name of the device under bus management.

`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 this parameter is 4, quirk-mmap is used in quirk-mmap-page mode.

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.

devicetree

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

bash

zynq$ insmod u-dma-buf.ko
u-dma-buf udmabuf0: driver version = 5.5.0
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
quirk-mmap-page
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.

devicetree

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

If you want to specify a buffer size of 4GiB or more, specify a 64bit value as follows. A 64-bit val

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PublishedAug 1, 2026

UpdatedSep 17, 2026

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udmabuf

About

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

Overview

Introduction of u-dma-buf

Architecture of u-dma-buf

Supported platforms

Note: udmabuf to u-dma-buf

Why u-dma-buf instead of udmabuf

Changes from udmabuf to u-dma-buf

Usage

Compile

Makefile

Cross Compile

Self Compile

Build in Linux Source Tree

Make directory in Linux Kernel Source Tree.

Copy files to Linux Kernel Source Tree.

Add u-dma-buf to Kconfig

Add u-dma-buf to Makefile

Set CONFIG_U_DMA_BUF

Install

Installation with the insmod

Installation with the Debian package

Configuration via the module parameters

udmabuf[0-7]

info_enable

dma_mask_bit

udmabuf[0-7]_bind

bind

quirk_mmap_mode

Configuration via the device tree file

compatible

size

Issues· 0 open

> Tags

> Discussion

> Details

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`udmabuf[0-7]`

`info_enable`

`dma_mask_bit`

`udmabuf[0-7]_bind`

`bind`

`quirk_mmap_mode`

`compatible`

`size`