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

[Robin Murphy]

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.

Robin.

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