概念
scatterlist
scatterlist用来描述一块内存，sg_table一般用于将物理不同大小的物理内存链接起来，一次性送给DMA控制器搬运
struct scatterlist {
unsigned long page_link; //指示该内存块所在的页面
unsigned int offset; //指示该内存块在页面中的偏移（起始位置）
unsigned int length; //该内存块的长度
dma_addr_t dma_address; //该内存块实际的物理起始地址
#ifdef CONFIG_NEED_SG_DMA_LENGTH
unsigned int dma_length; //相应的长度信息
#endif
};page_link：
(1).对于chain sg 来说，记录下一个 SG 数组的首地址，并且用bit[0] 和 bit[1] 来表示是chain sg 还是 end sg；
(2).对于 end sg 来说，只有bit[1] 为1，其他无意义；
(3).对于普通 sg 来说，记录的是关联的内存页块的地址；
sg_table
既然链接起物理内存，那么就需要多个sg；内核给了个sg_table和一系列api便于操作sg；
struct sg_table {
struct scatterlist *sgl; /* the list */
unsigned int nents; //实际的内存块映射数量
unsigned int orig_nents; ///内存块映射的数量
};sg_alloc_table一次可以分配page size / sizeof(scatterlist)个scatterlist结构体；如果超过这个数，就需要再通过sg_alloc_table分配scatterlist，并且通过sg_chain()来连接上一个sg_table和新的sg_table
sg_alloc_table
sg_kmalloc用以批量分配 sg 的内存;G_MAX_SINGLE_ALLOC：系统规定了每次sg_kmalloc的最大个数为4096/32 = 128个
int sg_alloc_table(struct sg_table *table, unsigned int nents, gfp_t gfp_mask)
{
int ret;
ret = __sg_alloc_table(table, nents, SG_MAX_SINGLE_ALLOC,
NULL, 0, gfp_mask, sg_kmalloc);
if (unlikely(ret))
__sg_free_table(table, SG_MAX_SINGLE_ALLOC, 0, sg_kfree);
return ret;
}
EXPORT_SYMBOL(sg_alloc_table);当申请的时候按照 SG_MAX_SINGLE_ALLOC，那么是一次性申请 4K 内存，系统直接调用 __get_free_page() 从buddy 中分配当没有达到 4K 内存，则通过kmalloc_array()申请 ；
static struct scatterlist *sg_kmalloc(unsigned int nents, gfp_t gfp_mask)
{
if (nents == SG_MAX_SINGLE_ALLOC) {
/*
* Kmemleak doesn't track page allocations as they are not
* commonly used (in a raw form) for kernel data structures.
* As we chain together a list of pages and then a normal
* kmalloc (tracked by kmemleak), in order to for that last
* allocation not to become decoupled (and thus a
* false-positive) we need to inform kmemleak of all the
* intermediate allocations.
*/
void *ptr = (void *) __get_free_page(gfp_mask);
kmemleak_alloc(ptr, PAGE_SIZE, 1, gfp_mask);
return ptr;
} else
return kmalloc_array(nents, sizeof(struct scatterlist),
gfp_mask);
}根据nents决定需不需要再次调用sg_kmalloc分配struct scatterlist数组，并返回首个scatterlist的地址，为什么叫数组，因为是在一个页面里面分配的，是连续的
int __sg_alloc_table(struct sg_table *table, unsigned int nents,
unsigned int max_ents, struct scatterlist *first_chunk,
unsigned int nents_first_chunk, gfp_t gfp_mask,
sg_alloc_fn *alloc_fn)
{
struct scatterlist *sg, *prv;
unsigned int left;
unsigned curr_max_ents = nents_first_chunk ?: max_ents;
unsigned prv_max_ents;
//准备初始化 sg_table，先memset
memset(table, 0, sizeof(*table));
//sg 条目数量不能为0
if (nents == 0)
return -EINVAL;
#ifdef CONFIG_ARCH_NO_SG_CHAIN
if (WARN_ON_ONCE(nents > max_ents))
return -EINVAL;
#endif
//初始化还没有申请的sg数目
left = nents;
prv = NULL;
do {
unsigned int sg_size, alloc_size = left;
//确定此次需要申请的sg 个数
//申请的sg超过最大值，将分多次分配
if (alloc_size > curr_max_ents) {
alloc_size = curr_max_ents;
sg_size = alloc_size - 1; //申请的sg数组中，最后一个作为一个chain，不作为有效sg
} else
sg_size = alloc_size;
//还剩余多少sg没有申请
left -= sg_size;
if (first_chunk) {
sg = first_chunk;
first_chunk = NULL;
} else {
sg = alloc_fn(alloc_size, gfp_mask); //调用sg分配的回调函数
}
if (unlikely(!sg)) {
/*
* Adjust entry count to reflect that the last
* entry of the previous table won't be used for
* linkage. Without this, sg_kfree() may get
* confused.
*/
if (prv)
table->nents = ++table->orig_nents;
return -ENOMEM;
}
/*
* 初始化此次申请的sg 数组，这些sg 在物理上是连续的，所以可以直接memset
* 另外，还会调用sg_mark_end() 初始化最后一个sg为 end sg
*/
sg_init_table(sg, alloc_size);
//更新sg_table->nents，初始化时 nents和orig_nents相同
table->nents = table->orig_nents += sg_size;
/*
* 当再次进入循环时，说明需要的nents是大于max_nents的，那么上一次申请肯定是按照最大值
* 申请.
* 第一次申请时，会将sg数组放入sg_table的sgl
* 当再进入循环时，需要连接新建的sg数组，所以要将prv的最后一个sg设为CHAIN
*/
if (prv)
sg_chain(prv, prv_max_ents, sg);
else
table->sgl = sg;
//如果没剩余sg需要分配了，将推出循环，此时将最新分配的sg数组的最后一个sg设为END
if (!left)
sg_mark_end(&sg[sg_size - 1]);
prv = sg;
prv_max_ents = curr_max_ents; //能进入下一个循环的话，上一个sg数组肯定按最大值申请的
curr_max_ents = max_ents;
} while (left);
return 0;
}
EXPORT_SYMBOL(__sg_alloc_table);用以配置铰链 sg，offset 和 length 为0，通过该函数将当前的sg数组与下一个sg数组通过chain sg捆绑在一起。
static inline void sg_chain(struct scatterlist *prv, unsigned int prv_nents,
struct scatterlist *sgl)
{
/*
* offset and length are unused for chain entry. Clear them.
*/
prv[prv_nents - 1].offset = 0;
prv[prv_nents - 1].length = 0;
/*
* Set lowest bit to indicate a link pointer, and make sure to clear
* the termination bit if it happens to be set.
*/
prv[prv_nents - 1].page_link = ((unsigned long) sgl | SG_CHAIN)
& ~SG_END;
}sg跟buffer
常用api
sg_set_page函数用sg_assign_page以将当前sg与某个内存页进行关联；并设置大小和偏移
static inline void sg_set_page(struct scatterlist *sg, struct page *page,
unsigned int len, unsigned int offset)
{
sg_assign_page(sg, page);
sg->offset = offset;
sg->length = len;
}sg_set_buf传入buf,然后用sg_set_page将sg与这个buf的page关联
static inline void sg_set_buf(struct scatterlist *sg, const void *buf,
unsigned int buflen)
{
#ifdef CONFIG_DEBUG_SG
BUG_ON(!virt_addr_valid(buf));
#endif
sg_set_page(sg, virt_to_page(buf), buflen, offset_in_page(buf));
}只初始化一个sg
void sg_init_table(struct scatterlist *sgl, unsigned int nents)
{
memset(sgl, 0, sizeof(*sgl) * nents);
sg_init_marker(sgl, nents);
}
EXPORT_SYMBOL(sg_init_table);
void sg_init_one(struct scatterlist *sg, const void *buf, unsigned int buflen)
{
sg_init_table(sg, 1);
sg_set_buf(sg, buf, buflen);
}
EXPORT_SYMBOL(sg_init_one);示例
int mmc_io_rw_extended(struct mmc_card *card, int write, unsigned fn,
unsigned addr, int incr_addr, u8 *buf, unsigned blocks, unsigned blksz)
{
struct mmc_request mrq = {NULL};
struct mmc_command cmd = {0};
struct mmc_data data = {0};
struct scatterlist sg, *sg_ptr;
struct sg_table sgtable;
unsigned int nents, left_size, i;
unsigned int seg_size = card->host->max_seg_size;
......
data.blksz = blksz;
/* Code in host drivers/fwk assumes that "blocks" always is >=1 */
data.blocks = blocks ? blocks : 1;
data.flags = write ? MMC_DATA_WRITE : MMC_DATA_READ;
left_size = data.blksz * data.blocks;
nents = (left_size - 1) / seg_size + 1;
if (nents > 1) {
if (sg_alloc_table(&sgtable, nents, GFP_KERNEL))
return -ENOMEM;
data.sg = sgtable.sgl;
data.sg_len = nents;
for_each_sg(data.sg, sg_ptr, data.sg_len, i) {
sg_set_page(sg_ptr, virt_to_page(buf + (i * seg_size)),
min(seg_size, left_size),
offset_in_page(buf + (i * seg_size)));
left_size = left_size - seg_size;
}
} else {
data.sg = &sg;
data.sg_len = 1;
sg_init_one(&sg, buf, left_size);
}
......
}sg跟DMA
常用api
判断当前sg是否为chain
#define sg_is_chain(sg) ((sg)->page_link & SG_CHAIN) 判断当前sg是否为last
#define sg_is_last(sg) ((sg)->page_link & SG_END)chain sg用来获取下一个指向的sg数组
#define sg_chain_ptr(sg) \
((struct scatterlist *) ((sg)->page_link & ~(SG_CHAIN | SG_END)))获取下一个sg,可能在下一个sg_table里
struct scatterlist *sg_next(struct scatterlist *sg)
{
if (sg_is_last(sg))
return NULL;
sg++;
if (unlikely(sg_is_chain(sg)))
sg = sg_chain_ptr(sg);
return sg;
}
EXPORT_SYMBOL(sg_next);遍历sg
#define for_each_sg(sglist, sg, nr, __i) \
for (__i = 0, sg = (sglist); __i < (nr); __i++, sg = sg_next(sg))获取sg关联的页块地址
static inline struct page *sg_page(struct scatterlist *sg)
{
#ifdef CONFIG_DEBUG_SG
BUG_ON(sg_is_chain(sg));
#endif
return (struct page *)((sg)->page_link & ~(SG_CHAIN | SG_END));
}示例
这是个支持sg的dma控制器；mmp_pdma_desc_hw用来dma描述符描述一个buf的信息，通过sg_dma_address将sg的总线物理地址，作为dma描述符的传输地址(源地址/目的地址)，用来发送数据到设备，或者从设备接收数据
mmp_pdma_prep_slave_sg将下一个描述符的地址，给到上一个描述符的--下一个描述符地址的成员，以实现DMA控制器自动遍历描述符，来传输sg的多个数据块。
struct mmp_pdma_desc_hw {
u32 ddadr; /* Points to the next descriptor + flags */
u32 dsadr; /* DSADR value for the current transfer */
u32 dtadr; /* DTADR value for the current transfer */
u32 dcmd; /* DCMD value for the current transfer */
} __aligned(32);
mmp_pdma_prep_slave_sg(struct dma_chan *dchan, struct scatterlist *sgl,
unsigned int sg_len, enum dma_transfer_direction dir,
unsigned long flags, void *context)
{
struct mmp_pdma_chan *chan = to_mmp_pdma_chan(dchan);
struct mmp_pdma_desc_sw *first = NULL, *prev = NULL, *new = NULL;
size_t len, avail;
struct scatterlist *sg;
dma_addr_t addr;
int i;
if ((sgl == NULL) || (sg_len == 0))
return NULL;
chan->byte_align = true;
mmp_pdma_config_write(dchan, &chan->slave_config, dir);
for_each_sg(sgl, sg, sg_len, i) {
addr = sg_dma_address(sg);
avail = sg_dma_len(sg);
do {
len = min_t(size_t, avail, PDMA_MAX_DESC_BYTES);
if (addr & 0x7)
chan->byte_align = true;
/* allocate and populate the descriptor */
new = mmp_pdma_alloc_descriptor(chan);
if (!new) {
dev_err(chan->dev, "no memory for desc\n");
goto fail;
}
new->desc.dcmd = chan->dcmd | (DCMD_LENGTH & len);
if (dir == DMA_MEM_TO_DEV) {
new->desc.dsadr = addr;
new->desc.dtadr = chan->dev_addr;
} else {
new->desc.dsadr = chan->dev_addr;
new->desc.dtadr = addr;
}
if (!first)
first = new;
else
prev->desc.ddadr = new->async_tx.phys; //将下一个描述符的地址，给到上一个描述符的--下一个描述符地址的成员；以实现控制器自动遍历描述符，来传输sg的多个数据块
new->async_tx.cookie = 0;
async_tx_ack(&new->async_tx);
prev = new;
/* Insert the link descriptor to the LD ring */
list_add_tail(&new->node, &first->tx_list);
/* update metadata */
addr += len;
avail -= len;
} while (avail);
}
first->async_tx.cookie = -EBUSY;
first->async_tx.flags = flags;
/* last desc and fire IRQ */
new->desc.ddadr = DDADR_STOP;
new->desc.dcmd |= DCMD_ENDIRQEN;
chan->dir = dir;
chan->cyclic_first = NULL;
return &first->async_tx;
fail:
if (first)
mmp_pdma_free_desc_list(chan, &first->tx_list);
return NULL;
}