Re: [PATCH] drm/panfrost: DMA map all pages shared with the GPU

From: Robin Murphy
Date: Mon Oct 14 2019 - 11:46:45 EST

On 14/10/2019 16:16, Steven Price wrote:
Pages shared with the GPU are (often) not cache coherent with the CPU so
cache maintenance is required to flush the CPU's caches. This was
already done when mapping pages on fault, but wasn't previously done
when mapping a freshly allocated page.

Fix this by moving the call to dma_map_sg() into mmu_map_sg() ensuring
that it is always called when pages are mapped onto the GPU. Since
mmu_map_sg() can now fail the code also now has to handle an error

Not performing this cache maintenance can cause errors in the GPU output
(CPU caches are later flushed and overwrite the GPU output). In theory
it also allows the GPU (and by extension user space) to observe the
memory contents prior to sanitization.

For the non-heap case, aren't the pages already supposed to be mapped by drm_gem_shmem_get_pages_sgt()?

(Hmm, maybe I should try hooking up the GPU SMMU on my Juno to serve as a cheeky DMA-API-mishap detector...)


Fixes: f3ba91228e8e ("drm/panfrost: Add initial panfrost driver")
Signed-off-by: Steven Price <steven.price@xxxxxxx>
drivers/gpu/drm/panfrost/panfrost_mmu.c | 20 ++++++++++++--------
1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/panfrost/panfrost_mmu.c b/drivers/gpu/drm/panfrost/panfrost_mmu.c
index bdd990568476..0495e48c238d 100644
--- a/drivers/gpu/drm/panfrost/panfrost_mmu.c
+++ b/drivers/gpu/drm/panfrost/panfrost_mmu.c
@@ -248,6 +248,9 @@ static int mmu_map_sg(struct panfrost_device *pfdev, struct panfrost_mmu *mmu,
struct io_pgtable_ops *ops = mmu->pgtbl_ops;
u64 start_iova = iova;
+ if (!dma_map_sg(pfdev->dev, sgt->sgl, sgt->nents, DMA_BIDIRECTIONAL))
+ return -EINVAL;
for_each_sg(sgt->sgl, sgl, sgt->nents, count) {
unsigned long paddr = sg_dma_address(sgl);
size_t len = sg_dma_len(sgl);
@@ -275,6 +278,7 @@ int panfrost_mmu_map(struct panfrost_gem_object *bo)
struct panfrost_device *pfdev = to_panfrost_device(obj->dev);
struct sg_table *sgt;
+ int ret;
if (WARN_ON(bo->is_mapped))
return 0;
@@ -286,10 +290,12 @@ int panfrost_mmu_map(struct panfrost_gem_object *bo)
if (WARN_ON(IS_ERR(sgt)))
return PTR_ERR(sgt);
- mmu_map_sg(pfdev, bo->mmu, bo->node.start << PAGE_SHIFT, prot, sgt);
- bo->is_mapped = true;
+ ret = mmu_map_sg(pfdev, bo->mmu, bo->node.start << PAGE_SHIFT,
+ prot, sgt);
+ if (ret == 0)
+ bo->is_mapped = true;
- return 0;
+ return ret;
void panfrost_mmu_unmap(struct panfrost_gem_object *bo)
@@ -503,12 +509,10 @@ int panfrost_mmu_map_fault_addr(struct panfrost_device *pfdev, int as, u64 addr)
if (ret)
goto err_pages;
- if (!dma_map_sg(pfdev->dev, sgt->sgl, sgt->nents, DMA_BIDIRECTIONAL)) {
- ret = -EINVAL;
+ ret = mmu_map_sg(pfdev, bo->mmu, addr,
+ if (ret)
goto err_map;
- }
- mmu_map_sg(pfdev, bo->mmu, addr, IOMMU_WRITE | IOMMU_READ | IOMMU_NOEXEC, sgt);
bo->is_mapped = true;