scatterlist的相关概念与实例分析

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_LENGTHunsigned 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;} elsereturn 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memsetmemset(table, 0, sizeof(*table));//sg 条目数量不能为0if (nents == 0)return -EINVAL;
#ifdef CONFIG_ARCH_NO_SG_CHAINif (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} elsesg_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);elsetable->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ENDif (!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_SGBUG_ON(!virt_addr_valid(buf));
#endifsg_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_SGBUG_ON(sg_is_chain(sg));
#endifreturn (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;elseprev->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;
}