Re: [1/2] drm/prime: Iterate SG DMA addresses separately

On 11/04/18 18:11, Robin Murphy wrote:

For dma_map_sg(), DMA API implementations are free to merge consecutive segments into a single DMA mapping if conditions are suitable, thus the resulting DMA addresses may be packed into fewer entries than ttm->sg->nents implies.

drm_prime_sg_to_page_addr_arrays() does not account for this, meaning its callers either have to reject the 0 < count < nents case or risk getting bogus addresses back later. Fortunately this is relatively easy to deal with having to rejig structures to also store the mapped count, since the total DMA length should still be equal to the total buffer length. All we need is a separate scatterlist cursor to iterate the DMA addresses separately from the CPU addresses.

Signed-off-by: Robin Murphy robin.murphy@arm.com

Off the back of Sinan's proposal for a workaround, I took a closer look and this jumped out - I have no hardware to test it, nor do I really know my way around this code, so I'm probably missing something, but at face value this seems like the only obvious problem, and worth fixing either way.

These patches are based on drm-next, and compile-tested (for arm64) only.

Robin.

drivers/gpu/drm/drm_prime.c | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c index 7856a9b3f8a8..db3dc8489afc 100644 --- a/drivers/gpu/drm/drm_prime.c +++ b/drivers/gpu/drm/drm_prime.c @@ -933,16 +933,18 @@ int drm_prime_sg_to_page_addr_arrays(struct sg_table *sgt, struct page **pages, dma_addr_t *addrs, int max_entries) { unsigned count;

• struct scatterlist *sg;

• struct scatterlist *sg, *dma_sg; struct page *page;

• u32 len, index;

• u32 len, dma_len, index; dma_addr_t addr;

  index = 0;

• dma_sg = sgt->sgl;

• dma_len = sg_dma_len(dma_sg);

• addr = sg_dma_address(dma_sg); for_each_sg(sgt->sgl, sg, sgt->nents, count) { len = sg->length; page = sg_page(sg);

• addr = sg_dma_address(sg);
  
  

  while (len > 0) { if (WARN_ON(index >= max_entries))

@@ -957,6 +959,12 @@ int drm_prime_sg_to_page_addr_arrays(struct sg_table *sgt, struct page **pages, len -= PAGE_SIZE;

+ dma_len -= PAGE_SIZE;

Ugh, somehow that bit got lost. Told you I'd have missed something, although I was rather assuming it would be something less obvious...

Robin.

	index++;
}

• if (dma_len == 0) {
  

• 	dma_sg = sg_next(dma_sg);
  

• 	dma_len = sg_dma_len(dma_sg);
  

• 	addr = sg_dma_address(dma_sg);
  

• }
  

  } return 0; }