編寫簡單的 DMA 驅動程序

Question

我想在我正在處理的驅動程序中使用 DMA。 最終目標是確保數據在物理 RAM 中，而不是隱藏在緩存中。 為此，我試圖在將它與我當前的項目合並之前實現一個簡單的測試驅動程序。

據我了解，我可以設置一個 DMA 掩碼並分配一個一致的緩沖區，然后簡單地寫入虛擬地址。 然后我的其他設備可以從物理地址讀取。 如果有人能確認這是否確實如此，那就太好了。

不幸的是，分配失敗了，我無法破譯系統日志來解釋為什么會這樣。 我可能在結構設備上做錯了。 dma_alloc_coherent中的設備引用是做什么用的？

這是我目前的嘗試。 這是一個修改后的虛擬字符設備：

#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.");

這是我使用的 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 上運行這里是內核日志：

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

您需要為dma_addr_t值分配空間。

- 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);

這是一個在線示例，查看dma_alloc_coherent()調用者的源代碼應該可以確認這一點。

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API 是這種方式，因為“C”不允許多個返回值。 在 C++ 中，它可能是一個參考。 對於大多數對“C”有廣泛經驗的內核開發人員來說，這似乎是第二天性。

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我是怎么推斷出來的？ 這有幫助，

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

發出了warn調用，因此某些參數似乎是錯誤的。