On Sat, Feb 27, 2010 at 3:53 AM, David Miller <davem@xxxxxxxxxxxxx> wrote:
> From: Robert Hancock <hancockrwd@xxxxxxxxx>
> Date: Fri, 26 Feb 2010 21:08:04 -0600
>
>> That seems like a reasonable approach to me. Only question is how to
>> implement the check for DMA_64BIT. Can we just check page_to_phys on
>> each of the pages in the skb to see if it's > 0xffffffff ? Are there
>> any architectures where it's more complicated than that?
>
> On almost every platform it's "more complicated than that".
>
> This is the whole issue. What matters is the final DMA address and
> since we have IOMMUs and the like, it is absolutely not tenable to
> solve this by checking physical address attributes.
Yeah, physical address isn't quite right. There is a precedent for
such a check in the block layer though - look at
blk_queue_bounce_limit in block/blk-settings.c:
void blk_queue_bounce_limit(struct request_queue *q, u64 dma_mask)
{
unsigned long b_pfn = dma_mask >> PAGE_SHIFT;
int dma = 0;
q->bounce_gfp = GFP_NOIO;
#if BITS_PER_LONG == 64
/*
* Assume anything <= 4GB can be handled by IOMMU. Actually
* some IOMMUs can handle everything, but I don't know of a
* way to test this here.
*/
if (b_pfn < (min_t(u64, 0xffffffffUL, BLK_BOUNCE_HIGH) >> PAGE_SHIFT))
dma = 1;
q->limits.bounce_pfn = max_low_pfn;
#else
if (b_pfn < blk_max_low_pfn)
dma = 1;
q->limits.bounce_pfn = b_pfn;
#endif
if (dma) {
init_emergency_isa_pool();
q->bounce_gfp = GFP_NOIO | GFP_DMA;
q->limits.bounce_pfn = b_pfn;
}
}
and then in mm/bounce.c:
static void __blk_queue_bounce(struct request_queue *q, struct bio **bio_orig,
mempool_t *pool)
{
struct page *page;
struct bio *bio = NULL;
int i, rw = bio_data_dir(*bio_orig);
struct bio_vec *to, *from;
bio_for_each_segment(from, *bio_orig, i) {
page = from->bv_page;
/*
* is destination page below bounce pfn?
*/
if (page_to_pfn(page) <= queue_bounce_pfn(q))
continue;
Following that logic then, it appears that page_to_pfn(page) >
(0xffffffff >> PAGE_SHIFT) should tell us what we want to know for the
DMA_64BIT flag.. or am I missing something?
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