如何在Linux驱动中使用DMA？

DMA（直接内存访问，Direct Memory Access）是一种允许硬件设备直接访问系统内存的技术，无需通过CPU的干预。它主要用于提高数据传输效率，减少CPU的负担。

工作原理：

• 初始化：CPU配置DMA控制器，指定数据传输的源地址、目标地址和数据量。

• 启动传输：DMA控制器接管，直接在设备和内存之间传输数据。

• 完成通知：传输完成后，DMA控制器通知CPU。

实验环境

raspberry pi3b

Linux raspberrypi 6.1.21-v8+ #1642 SMP PREEMPT Mon Apr  3 17:24:16 BST 2023 aarch64 GNU/Linux

驱动代码

// simple.c

#include <linux/completion.h>
#include <linux/dma-mapping.h>
#include <linux/dmaengine.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/slab.h>

static void _dma_transfer_completed(void *param) {
    struct completion *cmp = (struct completion *)param;
    complete(cmp);
}

static int __init simple_init(void) {
    dma_cap_mask_t mask;
    struct dma_chan *chan;
    struct dma_async_tx_descriptor *chan_desc;
    dma_cookie_t cookie;
    dma_addr_t src_addr, dst_addr;
    unsigned char *src_buf, *dst_buf;
    struct completion cmp;
    int status;

    pr_info("simple_init\n");

    dma_cap_zero(mask);
    dma_cap_set(DMA_SLAVE | DMA_PRIVATE, mask);
    chan = dma_request_channel(mask, NULL, NULL);
    if (!chan) {
        pr_err("dma_request_channel error\n");
        return -ENODEV;
    }

    src_buf =
        dma_alloc_coherent(chan->device->dev, 1024, &src_addr, GFP_KERNEL);
    dst_buf =
        dma_alloc_coherent(chan->device->dev, 1024, &dst_addr, GFP_KERNEL);

    memset(src_buf, 0, 1024);
    memset(dst_buf, 1, 1024);

    pr_info("before src_buf[0] = %d, dst_buf[0] = %d\n", src_buf[0],
            dst_buf[0]);

    chan_desc = dmaengine_prep_dma_memcpy(chan, dst_addr, src_addr, 1024,
                                          DMA_MEM_TO_MEM);
    if (!chan_desc) {
        pr_err("dmaengine_prep_dma_memcpy error\n");
        status = -1;
        goto free;
    }

    init_completion(&cmp);

    chan_desc->callback = _dma_transfer_completed;
    chan_desc->callback_param = &cmp;

    cookie = dmaengine_submit(chan_desc);

    dma_async_issue_pending(chan);

    if (wait_for_completion_timeout(&cmp, msecs_to_jiffies(3000)) < 0) {
        pr_err("completion timeout 3000ms\n");
        status = -1;
        goto rel;
    }

    status = dma_async_is_tx_complete(chan, cookie, NULL, NULL);
    if (status == DMA_COMPLETE) {
        status = 0;

        pr_info("DMA transfer has completed\n");
        pr_info("before src_buf[0] = %d, dst_buf[0] = %d\n", src_buf[0],
                dst_buf[0]);
    } else {
        pr_err("dma_async_is_tx_complete error\n");
    }

rel:
    dmaengine_terminate_all(chan);

free:
    dma_free_coherent(chan->device->dev, 1024, src_buf, src_addr);
    dma_free_coherent(chan->device->dev, 1024, dst_buf, dst_addr);
    dma_release_channel(chan);

    return status;
}

static void __exit simple_exit(void) { pr_info("simple_exit\n"); }

module_init(simple_init);
module_exit(simple_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("heng30");
MODULE_VERSION("v0.0.1");
MODULE_DESCRIPTION("A simple kernel module");

编译脚本

#!/bin/sh

top-dir = $(shell pwd)
kernel-version = $(shell uname -r)
kernel-dir ?= /lib/modules/$(kernel-version)/build

obj-m += simple.o

all:
        make -C $(kernel-dir) modules M=$(top-dir)

clean:
        rm -f *.o *.ko *.mod *.mod.c *.order *.symvers *.dtbo
        make -C $(kernel-dir) clean m=$(top-dir)

测试

• 编译程序：make

• 安装驱动：insmod simple.ko

• 查看输出：dmesg

[194177.368278] simple_init
[194177.369413] before src_buf[0] = 0, dst_buf[0] = 1
[194177.396623] DMA transfer has completed
[194177.396853] before src_buf[0] = 0, dst_buf[0] = 0
[194197.014928] simple_exit

• 移除驱动：rmmod simple.ko

参考

• 如何使用qemu启动raspi3b进行嵌入式开发？