DMA(二) - DMA接口函数

DMA接口函数

arch/arm/mm/dma-mapping.c 主要的作用是将虚拟地址映射到物理地址。
一致性DMA映射，分配指定大小的内存，并将物理地址映射到虚拟地址
dma_alloc_coherent
dma_mmap_coherent
dma_free_coherent
dma_alloc_writecombine
dma_mmap_writecombine
dma_free_writecombine
dma_alloc_coherent和dma_alloc_writecombine的区别是dma_alloc_coherent（cache disable，buffer disable），dma_alloc_writecombine（cache disable，buffer enable）

流式DMA映射，使用这个函数前需要事先分配连续物理内存，这个函数只是物理地址映射到虚拟地址
dma_map_single
dma_unmap_single

DMA池

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DMA cache一致性问题

DMA如果使用cache，那么一定要考虑cache的一致性。解决DMA导致的一致性的方法最简单的就是禁止DMA目标地址范围内的cache功能。但是这样就会牺牲性能。

从dma_alloc_coherent和dma_alloc_writecombie源码可以看出，
Dma_alloc_coherent pgprot_noncached 禁止了页表项中的C(Cacheable)域和B(Bufferable)域
此时由于关闭了cache/buffer，性能自然比较低
dma_alloc_writecombie pgprot_writecombine 只禁止了C （Cacheable） 域

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DMA接口函数使用实例

完整代码位置：dma映射函数

#include <linux/module.h>
#include <linux/init.h>
#include <linux/pci.h>
#include <linux/slab.h>
#include <linux/dma-mapping.h>
#include <linux/dmapool.h>
#include <linux/device.h>

// int direction = PCI_DMA_TODEVICE ;
// int direction = PCI_DMA_FROMDEVICE ;
static int direction = PCI_DMA_BIDIRECTIONAL;
//int direction = PCI_DMA_NONE;

static char *kbuf;
static dma_addr_t handle;
static size_t size = (10 * PAGE_SIZE);
static struct dma_pool *mypool;
static size_t pool_size = 1024;
static size_t pool_align = 8;

static void my_release(struct device *dev)
{
	pr_info("releasing DMA device\n");
}

static struct device dev = {
	.release = my_release
};

static void output(char *kbuf, dma_addr_t handle, size_t size, char *string)
{
	unsigned long diff;
	diff = (unsigned long)kbuf - handle;
	pr_info("kbuf=%12p, handle=%12p, size = %d\n", kbuf,
		(unsigned long *)handle, (int)size);
	pr_info("(kbuf-handle)= %12p, %12lu, PAGE_OFFSET=%12lu, compare=%lu\n",
		(void *)diff, diff, PAGE_OFFSET, diff - PAGE_OFFSET);
	strcpy(kbuf, string);
	pr_info("string written was, %s\n", kbuf);
}

static int __init my_init(void)
{
	dev_set_name(&dev, "my0");
	device_register(&dev);

	/* dma_alloc_coherent method */

	pr_info("Loading DMA allocation test module\n");
	pr_info("\nTesting dma_alloc_coherent()..........\n\n");
	kbuf = dma_alloc_coherent(NULL, size, &handle, GFP_KERNEL);
	output(kbuf, handle, size, "This is the dma_alloc_coherent() string");
	dma_free_coherent(NULL, size, kbuf, handle);

	/* dma_map/unmap_single */

	pr_info("\nTesting dma_map_single()................\n\n");
	kbuf = kmalloc(size, GFP_KERNEL);
	handle = dma_map_single(&dev, kbuf, size, direction);
	output(kbuf, handle, size, "This is the dma_map_single() string");
	dma_unmap_single(&dev, handle, size, direction);
	kfree(kbuf);

	/* dma_pool method */

	pr_info("\nTesting dma_pool_alloc()..........\n\n");
	mypool = dma_pool_create("mypool", NULL, pool_size, pool_align, 0);
	kbuf = dma_pool_alloc(mypool, GFP_KERNEL, &handle);
	output(kbuf, handle, size, "This is the dma_pool_alloc() string");
	dma_pool_free(mypool, kbuf, handle);
	dma_pool_destroy(mypool);

	device_unregister(&dev);

	return 0;
}

static void __exit my_exit(void)
{
	pr_info("Module Unloading\n");
}

module_init(my_init);
module_exit(my_exit);

MODULE_AUTHOR("Jerry Cooperstein");
MODULE_DESCRIPTION("LDD:2.0 s_23/lab1_dma.c");
MODULE_LICENSE("GPL v2");