Re: [RFC PATCH] fix problems with NETIF_F_HIGHDMA in networking drivers

[Robert Hancock <hancockrwd@gmail.com>]

On Sat, Feb 27, 2010 at 3:53 AM, David Miller <davem@davemloft.net> wrote:
> From: Robert Hancock <hancockrwd@gmail.com>
> 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?