DMA pools的使用

本文主要是介绍DMA pools的使用，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！

链接：https://blog.csdn.net/tiantao2012/article/details/60140732

前面我们讲申请大于一个page size的时候用dma_alloc_coherent，如果小于一个page size，则可以使用DMA-coherent buffers.
在使用DMA-coherent buffers 之前首先要调用来create buffer.
struct dma_pool *
dma_pool_create(const char *name, struct device *dev,
size_t size, size_t align, size_t alloc);
然后就可以调用dma_pool_alloc 从前面已经create的pool中申请buffer
void *dma_pool_alloc(struct dma_pool *pool, gfp_t gfp_flags,
dma_addr_t *dma_handle);
而
void *dma_pool_zalloc(struct dma_pool *pool, gfp_t mem_flags,
dma_addr_t *handle)
和dma_pool_alloc 类似，会把申请的memory清零.
buffer 使用完后调用dma_pool_free 来释放buffer
最后调用dma_pool_destroy 来销毁之前申请的pool。
在dma_pool_create 中可以指定align ，如果没有指定align，最小size是1个byte.
if (align == 0)
align = 1;
else if (align & (align - 1))
return NULL;

申请的size 最小是4个byte
if (size == 0)
return NULL;
else if (size < 4)
size = 4;
申请的size 要align，也可以看出调用最大只能申请一个page的size.
if ((size % align) != 0)
size = ALIGN(size, align);

allocation = max_t(size_t, size, PAGE_SIZE);

边界检查，边界必须小于size，且必须是2的指数.
if (!boundary)
boundary = allocation;
else if ((boundary < size) || (boundary & (boundary - 1)))
return NULL;
原来也是通过kmalloc 申请，然后给retval 赋值.
retval = kmalloc_node(sizeof(*retval), GFP_KERNEL, dev_to_node(dev));
if (!retval)
return retval;

strlcpy(retval->name, name, sizeof(retval->name));retval->dev = dev;INIT_LIST_HEAD(&retval->page_list);
spin_lock_init(&retval->lock);
retval->size = size;
retval->boundary = boundary;
retval->allocation = allocation;INIT_LIST_HEAD(&retval->pools);

如果dev->dma_pools 为null，则说明是第一次申请
if (list_empty(&dev->dma_pools))
empty = true;
list_add(&retval->pools, &dev->dma_pools);

如果是第一次申请，则创建sysfs attribute

if (empty) {int err;err = device_create_file(dev, &dev_attr_pools);if (err) {mutex_lock(&pools_lock);list_del(&retval->pools);mutex_unlock(&pools_lock);mutex_unlock(&pools_reg_lock);kfree(retval);return NULL;}
}

创建sysfs attribute，主要是调用下面的函数.
show_pools(struct device *dev, struct device_attribute *attr, char *buf)
{
unsigned temp;
unsigned size;
char *next;
struct dma_page *page;
struct dma_pool *pool;

next = buf;
size = PAGE_SIZE;temp = scnprintf(next, size, "poolinfo - 0.1\n");
size -= temp;
next += temp;mutex_lock(&pools_lock);
list_for_each_entry(pool, &dev->dma_pools, pools) {unsigned pages = 0;unsigned blocks = 0;spin_lock_irq(&pool->lock);list_for_each_entry(page, &pool->page_list, page_list) {pages++;blocks += page->in_use;}spin_unlock_irq(&pool->lock);/* per-pool info, no real statistics yet */temp = scnprintf(next, size, "%-16s %4u %4Zu %4Zu %2u\n",pool->name, blocks,pages * (pool->allocation / pool->size),pool->size, pages);size -= temp;next += temp;
}
mutex_unlock(&pools_lock);return PAGE_SIZE - size;

}

static DEVICE_ATTR(pools, S_IRUGO, show_pools, NULL);

调用dma_pool_alloc 从已经申请好的pool中查找memroy。
首先查找释放有足够的memory 来申请.
list_for_each_entry(page, &pool->page_list, page_list) {
if (page->offset < pool->allocation)
goto ready;
}
如果没有的话，调用pool_alloc_page 申请buffer，然后添加到原来的pool中
page = pool_alloc_page(pool, mem_flags & (~__GFP_ZERO));
if (!page)
return NULL;

spin_lock_irqsave(&pool->lock, flags);list_add(&page->page_list, &pool->page_list);

所以调用dma_pool_alloc 肯定会找到pool来申请，如果没有就申请memory再添加到原来的pool中，也算是一种lazy allocation。
page->in_use++;
offset = page->offset;
page->offset = *(int *)(page->vaddr + offset);
retval = offset + page->vaddr;
*handle = offset + page->dma;
计算后，最后返回retval，这个就是物理地址。
需要注意的是pool_alloc_page 函数中首先通过kmalloc 来申请，如果failed 就调用dma_alloc_coherent 来申请。
static struct dma_page *pool_alloc_page(struct dma_pool *pool, gfp_t mem_flags)
{
struct dma_page *page;

page = kmalloc(sizeof(*page), mem_flags);
if (!page)return NULL;
page->vaddr = dma_alloc_coherent(pool->dev, pool->allocation,&page->dma, mem_flags);
if (page->vaddr) {

#ifdef DMAPOOL_DEBUG
memset(page->vaddr, POOL_POISON_FREED, pool->allocation);
#endif
pool_initialise_page(pool, page);
page->in_use = 0;
page->offset = 0;
} else {
kfree(page);
page = NULL;
}
return page;
}
如果怀疑dma memory没人踩坏可以打开DMAPOOL_DEBUG，复制到现象是就会打印下面的log
WARNING: at /data2/repos/linux-2.6-iommu/lib/dma-debug.c:448
check_unmap+0x203/0x490()
Hardware name:
forcedeth 0000:00:08.0: DMA-API: device driver frees DMA memory with wrong
function [device address=0x00000000640444be] [size=66 bytes] [mapped as
single] [unmapped as page]
Modules linked in: nfsd exportfs bridge stp llc r8169
Pid: 0, comm: swapper Tainted: G W 2.6.28-dmatest-09289-g8bb99c0 #1
Call Trace:
[] warn_slowpath+0xf2/0x130
[] _spin_unlock+0x10/0x30
[] usb_hcd_link_urb_to_ep+0x75/0xc0
[] _spin_unlock_irqrestore+0x12/0x40
[] ohci_urb_enqueue+0x19f/0x7c0
[] queue_work+0x56/0x60
[] enqueue_task_fair+0x20/0x50
[] usb_hcd_submit_urb+0x379/0xbc0
[] cpumask_next_and+0x23/0x40
[] find_busiest_group+0x207/0x8a0
[] _spin_lock_irqsave+0x1f/0x50
[] check_unmap+0x203/0x490
[] debug_dma_unmap_page+0x49/0x50
[] nv_tx_done_optimized+0xc6/0x2c0
[] nv_nic_irq_optimized+0x73/0x2b0
[] handle_IRQ_event+0x34/0x70
[] handle_edge_irq+0xc9/0x150
[] do_IRQ+0xcb/0x1c0
[] ret_from_intr+0x0/0xa