linux-kernel - pci_map_sg() does not coalesce adjacent physical memory? x86

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Message-ID: <c384c5ea0811171354u5d6f5be6v857f92b1e75769c7@mail.gmail.com>
Date:	Mon, 17 Nov 2008 22:54:33 +0100
From:	"Leon Woestenberg" <leon.woestenberg@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Subject: pci_map_sg() does not coalesce adjacent physical memory? x86

Hello,

pci_map_sg() does not coalesce the scattergather list for me on x86.
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);
}

Regards,
-- 
Leon
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