Re: [PATCH] drm/msm: use orig_nents to iterate over scatterlist with per-process tables

From: Robin Murphy
Date: Tue Apr 05 2022 - 18:32:33 EST

On 2022-03-28 13:55, Jonathan Marek wrote:
This matches the implementation of iommu_map_sgtable() used for the
non-per-process page tables path.

This works around the dma_map_sgtable() call (used to invalidate cache)
overwriting sgt->nents with 1 (which is probably a separate issue).

FWIW that's expected behaviour. The sgtable APIs use nents to keep track of the number of DMA segments, while orig_nents represents the physical segments. IIUC you're not actually using the DMA mapping, just relying on its side-effects, so it's still the physical segments that you care about for your private IOMMU mapping here.

Fixes: b145c6e65eb0 ("drm/msm: Add support to create a local pagetable")
Signed-off-by: Jonathan Marek <jonathan@xxxxxxxx>
drivers/gpu/drm/msm/msm_iommu.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/msm/msm_iommu.c b/drivers/gpu/drm/msm/msm_iommu.c
index bcaddbba564df..22935ef26a3a1 100644
--- a/drivers/gpu/drm/msm/msm_iommu.c
+++ b/drivers/gpu/drm/msm/msm_iommu.c
@@ -58,7 +58,7 @@ static int msm_iommu_pagetable_map(struct msm_mmu *mmu, u64 iova,
u64 addr = iova;
unsigned int i;
- for_each_sg(sgt->sgl, sg, sgt->nents, i) {
+ for_each_sg(sgt->sgl, sg, sgt->orig_nents, i) {

Even better would be to use for_each_sgtable_sg(), which exists primarily for the sake of avoiding this exact confusion.


size_t size = sg->length;
phys_addr_t phys = sg_phys(sg);