编写简单的 DMA 驱动程序

Question

我想在我正在处理的驱动程序中使用 DMA。 最终目标是确保数据在物理 RAM 中，而不是隐藏在缓存中。 为此，我试图在将它与我当前的项目合并之前实现一个简单的测试驱动程序。

据我了解，我可以设置一个 DMA 掩码并分配一个一致的缓冲区，然后简单地写入虚拟地址。 然后我的其他设备可以从物理地址读取。 如果有人能确认这是否确实如此，那就太好了。

不幸的是，分配失败了，我无法破译系统日志来解释为什么会这样。 我可能在结构设备上做错了。 dma_alloc_coherent中的设备引用是做什么用的？

这是我目前的尝试。 这是一个修改后的虚拟字符设备：

#include <linux/module.h>
#include <linux/fs.h>
#include <linux/device.h>
#include <linux/cdev.h>
#include <linux/uaccess.h>
#include <linux/dma-mapping.h>

static char hello_world[]="Hello World\n";

static dev_t hello_dev_number;
static struct cdev *driver_object;
static struct class *hello_class;
static struct device *hello_dev;
int errorType;

void *virtAddr;
size_t size = 32;
dma_addr_t *physAddr;
int flag = GFP_KERNEL;

static ssize_t driver_read( struct file *instance, char __user *user, size_t count, loff_t *offset )
{
    unsigned long not_copied, to_copy;

    to_copy = min( count, strlen(hello_world)+1 );
    not_copied=copy_to_user(user,hello_world,to_copy);
    *offset += to_copy-not_copied;
    return to_copy-not_copied;
}

static struct file_operations fops = {
    .owner= THIS_MODULE,
    .read= driver_read,
};

static int __init mod_init( void )
{
    int debug;
    printk("starting insertion");
    if (alloc_chrdev_region(&hello_dev_number,0,1,"Hello")<0)
        return -EIO;
    driver_object = cdev_alloc();
    if (driver_object==NULL){
        errorType = EIO;
        goto free_device_number;
    }
    driver_object->owner = THIS_MODULE;
    driver_object->ops = &fops;
    if (cdev_add(driver_object,hello_dev_number,1)){
        errorType=EIO;
        goto free_cdev;
    }
    hello_class = class_create( THIS_MODULE, "Hello" );
    if (IS_ERR( hello_class )) {
        pr_err( "hello: no udev support\n");
        errorType=EIO;
        goto free_cdev;
    }
    hello_dev = device_create( hello_class, NULL, hello_dev_number, NULL, "%s", "hello" );
    if (IS_ERR( hello_dev )) {
        pr_err( "hello: device_create failed\n");
        errorType=EIO;
        goto free_class;
    }
    printk("dma is happening");
    debug = dma_set_mask_and_coherent(hello_dev, DMA_BIT_MASK(32));
    printk("dma mask returns: %d", debug);
    if(debug==0){
        printk("setting mask failed");
        errorType=EIO;
        goto free_dev;
    }
    printk("goodbye crule world!");
    virtAddr = dma_alloc_coherent(hello_dev, size, physAddr, flag);
    printk("I'm actually alive");
    if(virtAddr==NULL){
        printk("virtual address null, dma failed!");
        errorType=ENOMEM;
        goto free_dev;
    }
    printk("success");
    return 0;
free_dev:
    device_destroy( hello_class, hello_dev_number );
free_class:
    class_destroy( hello_class );
free_cdev:
    kobject_put( &driver_object->kobj );
free_device_number:
    unregister_chrdev_region( hello_dev_number, 1 );
    return -errorType;
}

static void __exit mod_exit( void )
{
    dma_free_coherent(hello_dev, size, virtAddr, *physAddr);
    device_destroy( hello_class, hello_dev_number );
    class_destroy( hello_class );
    cdev_del( driver_object );
    unregister_chrdev_region( hello_dev_number, 1 );
    return;
}

module_init( mod_init );
module_exit( mod_exit );

MODULE_AUTHOR("ME");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Testing DMA.");

这是我使用的 Makefile：

obj-m+=dma_test.o

all:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

在 ARMv7 AM335x Sitara CPU (Beaglebone Black)、Linux Kernel 5.4.106-ti-r27 上运行这里是内核日志：

Mar 14 02:38:23 beaglebone kernel: [  100.662640] starting insertion
Mar 14 02:38:23 beaglebone kernel: [  100.667068] dma is happening
Mar 14 02:38:23 beaglebone kernel: [  100.667087] dma mask returns: -5
Mar 14 02:38:23 beaglebone kernel: [  100.667092] goodbye crule world!
Mar 14 02:38:23 beaglebone kernel: [  100.667100] ------------[ cut here ]------------
Mar 14 02:38:23 beaglebone kernel: [  100.667128] WARNING: CPU: 0 PID: 2360 at kernel/dma/mapping.c:272 dma_alloc_attrs+0x118/0x128
Mar 14 02:38:23 beaglebone kernel: [  100.667134] Modules linked in: dma_test(O+) c_can_platform c_can can_dev evdev usb_f_acm u_serial usb_f_ecm usb_f_mass_storage usb_f_rndis u_ether libcomposite uio_pdrv_genirq(O) uio iptable_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 iptable_mangle iptable_filter dmatest(O) ip_tables x_tables icss_iep prueth_ecap spidev
Mar 14 02:38:23 beaglebone kernel: [  100.667216] CPU: 0 PID: 2360 Comm: insmod Tainted: G           O      5.4.106-ti-r27 #1buster
Mar 14 02:38:23 beaglebone kernel: [  100.667222] Hardware name: Generic AM33XX (Flattened Device Tree)
Mar 14 02:38:23 beaglebone kernel: [  100.667228] Backtrace: 
Mar 14 02:38:23 beaglebone kernel: [  100.667248] [<c0e37fd8>] (dump_backtrace) from [<c0e38390>] (show_stack+0x20/0x24)
Mar 14 02:38:23 beaglebone kernel: [  100.667259]  r7:600f0113 r6:c14e3154 r5:00000000 r4:c14e3154
Mar 14 02:38:23 beaglebone kernel: [  100.667274] [<c0e38370>] (show_stack) from [<c0e49af0>] (dump_stack+0xb8/0xcc)
Mar 14 02:38:23 beaglebone kernel: [  100.667289] [<c0e49a38>] (dump_stack) from [<c013c6cc>] (__warn+0xe0/0x108)
Mar 14 02:38:23 beaglebone kernel: [  100.667299]  r7:00000110 r6:00000009 r5:c01c7090 r4:c11194a8
Mar 14 02:38:23 beaglebone kernel: [  100.667309] [<c013c5ec>] (__warn) from [<c0e38c00>] (warn_slowpath_fmt+0x70/0xd8)
Mar 14 02:38:23 beaglebone kernel: [  100.667318]  r7:00000110 r6:c11194a8 r5:c1405fc8 r4:00000000
Mar 14 02:38:23 beaglebone kernel: [  100.667328] [<c0e38b94>] (warn_slowpath_fmt) from [<c01c7090>] (dma_alloc_attrs+0x118/0x128)
Mar 14 02:38:23 beaglebone kernel: [  100.667339]  r9:00000cc0 r8:00000000 r7:00000020 r6:dc78d600 r5:c1405fc8 r4:c0f01734
Mar 14 02:38:23 beaglebone kernel: [  100.667368] [<c01c6f78>] (dma_alloc_attrs) from [<bf00c1b4>] (mod_init+0x1b4/0x1000 [dma_test])
Mar 14 02:38:23 beaglebone kernel: [  100.667379]  r9:bf0e00cc r8:d29cff30 r7:dc78d600 r6:fffffffb r5:00000000 r4:bf0e0300
Mar 14 02:38:23 beaglebone kernel: [  100.667399] [<bf00c000>] (mod_init [dma_test]) from [<c0103268>] (do_one_initcall+0x50/0x2d0)
Mar 14 02:38:23 beaglebone kernel: [  100.667408]  r7:00000000 r6:bf0e01f0 r5:bf00c000 r4:c1405fc8
Mar 14 02:38:23 beaglebone kernel: [  100.667423] [<c0103218>] (do_one_initcall) from [<c01f0878>] (do_init_module+0x70/0x274)
Mar 14 02:38:23 beaglebone kernel: [  100.667433]  r8:d29cff30 r7:bf0e00c0 r6:bf0e01f0 r5:db20cf40 r4:bf0e00c0
Mar 14 02:38:23 beaglebone kernel: [  100.667444] [<c01f0808>] (do_init_module) from [<c01f2adc>] (load_module+0x1f64/0x2370)
Mar 14 02:38:23 beaglebone kernel: [  100.667452]  r6:bf0e01f0 r5:00000000 r4:bf0e01c0
Mar 14 02:38:23 beaglebone kernel: [  100.667463] [<c01f0b78>] (load_module) from [<c01f318c>] (sys_finit_module+0xc0/0x110)
Mar 14 02:38:23 beaglebone kernel: [  100.667474]  r10:0000017b r9:d29ce000 r8:c0101204 r7:0043c7e0 r6:00000003 r5:00000000
Mar 14 02:38:23 beaglebone kernel: [  100.667480]  r4:c1405fc8
Mar 14 02:38:23 beaglebone kernel: [  100.667491] [<c01f30cc>] (sys_finit_module) from [<c0101000>] (ret_fast_syscall+0x0/0x54)
Mar 14 02:38:23 beaglebone kernel: [  100.667499] Exception stack(0xd29cffa8 to 0xd29cfff0)
Mar 14 02:38:23 beaglebone kernel: [  100.667511] ffa0:                   3d6d5800 00000000 00000003 0043c7e0 00000000 bece2578
Mar 14 02:38:23 beaglebone kernel: [  100.667522] ffc0: 3d6d5800 00000000 00000000 0000017b 015347e0 00000000 bece26f8 00000000
Mar 14 02:38:23 beaglebone kernel: [  100.667531] ffe0: bece2528 bece2518 00434e41 b6cbbd92
Mar 14 02:38:23 beaglebone kernel: [  100.667540]  r7:0000017b r6:00000000 r5:00000000 r4:3d6d5800
Mar 14 02:38:23 beaglebone kernel: [  100.667547] ---[ end trace 870c2d2ad09e80fa ]---
Mar 14 02:38:23 beaglebone kernel: [  100.667565] Hello hello: coherent DMA mask is unset
Mar 14 02:38:23 beaglebone kernel: [  100.667571] I'm actually alive

Answer 1

您需要为dma_addr_t值分配空间。

- dma_addr_t *physAddr;
+ dma_addr_t physAddr;

- virtAddr = dma_alloc_coherent(hello_dev, size, physAddr, flag);
+ virtAddr = dma_alloc_coherent(hello_dev, size, &physAddr, flag);

-  dma_free_coherent(hello_dev, size, virtAddr, *physAddr);
+  dma_free_coherent(hello_dev, size, virtAddr, physAddr);

这是一个在线示例，查看dma_alloc_coherent()调用者的源代码应该可以确认这一点。

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API 是这种方式，因为“C”不允许多个返回值。 在 C++ 中，它可能是一个参考。 对于大多数对“C”有广泛经验的内核开发人员来说，这似乎是第二天性。

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我是怎么推断出来的？ 这有帮助，

[<c013c5ec>] (__warn) from [<c0e38c00>] (warn_slowpath_fmt+0x70/0xd8)
[<c01c7090>] (dma_alloc_attrs+0x118/0x128)
[<bf00c1b4>] (mod_init+0x1b4/0x1000 [dma_test])

发出了warn调用，因此某些参数似乎是错误的。