Re: pci_map_sg() does not coalesce adjacent physical memory? x86

[Andrew Morton <akpm@linux-foundation.org>]

(cc's added)

On Mon, 17 Nov 2008 22:54:33 +0100 "Leon Woestenberg" <leon.woestenberg@gmail.com> wrote:

> Hello,
> 
> pci_map_sg() does not coalesce the scattergather list for me on x86.

In which kernel version(s)?

> Is this expected? Documentation mentions that coalescing is typically
> done by pci_map_sg().
> 
> Manually traversing scatterlists that describe large user space
> allocations I found that on my system 25% reduction of list length can
> be achieved.
> 
> 
> static int sgm_map_to_table(struct sg_mapping_t *sgm)
> {
>         int i, j = 0;
>         dma_addr_t addr = sg_dma_address(&sgm->sgl[0]);
>         unsigned int len = sg_dma_len(&sgm->sgl[0]);
>         dma_addr_t cont_addr = addr;
>         unsigned int cont_len = len;
>         for (i = 0; i < sgm->mapped_pages - 1; i++) {
>                 dma_addr_t next = sg_dma_address(&sgm->sgl[i + 1]);
>                 len = sg_dma_len(&sgm->sgl[i]);
>                 printk(KERN_DEBUG "%04d: addr=0x%08x length=0x%08x\n",
> i, addr, len);
>                 /* page i + 1 is non-contiguous with page i? */
>                 if (next != addr + len) {
>                         /* TODO create entry here (we could overwrite i) */
>                         printk(KERN_DEBUG "%4d: cont_addr=0x%08x
> cont_len=0x%08x\n",
>                                 j++, cont_addr, cont_len);
>                         cont_addr = next;
>                         cont_len = 0;
>                 }
>                 /* add page i + 1 to current contiguous block */
>                 cont_len += len;
>                 /* goto page i + 1 */
>                 addr = next;
>         }
>         /* TODO create entry here  (we could overwrite i) */
>         printk(KERN_DEBUG "%04d: addr=0x%08x length=0x%08x\n", i, addr, len);
>         printk(KERN_DEBUG "%4d: cont_addr=0x%08x length=0x%08x\n",
> j++, cont_addr, cont_len);
> }
>