gpu: nvgpu: protect missing sgt in gk20a_mem_phys

Return zero for missing sgt instead of attempting to dereference NULL.
Those NULL conditions should be almost nonexistent, and zero is not
normally used.

When reading gk20a_mem_phys() in gk20a_gr_get_chid_from_ctx() from an
isr, the mem desc may race with channel deletion and get suddendly
zeroed, even if the channel's in_use flag would be set. Plain zero
results in expected behaviour.

Change-Id: Id8ce37798d6fd3ceeb96a3f521c82569fccf30aa
Signed-off-by: Konsta Holtta <kholtta@nvidia.com>
Reviewed-on: http://git-master/r/729006
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>
Tested-by: Terje Bergstrom <tbergstrom@nvidia.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/drivers/gpu/nvgpu/gk20a/mm_gk20a.h b/drivers/gpu/nvgpu/gk20a/mm_gk20a.h
index 57f7a373..0ff11d09 100644
--- a/drivers/gpu/nvgpu/gk20a/mm_gk20a.h
+++ b/drivers/gpu/nvgpu/gk20a/mm_gk20a.h
@@ -458,7 +458,9 @@ void gk20a_gmmu_free_attr(struct gk20a *g,
 
 static inline phys_addr_t gk20a_mem_phys(struct mem_desc *mem)
 {
-        return sg_phys(mem->sgt->sgl);
+        /* the sgt may get null if this is accessed e.g. in an isr during
+         * channel deletion */
+        return mem->sgt ? sg_phys(mem->sgt->sgl) : 0;
 }
 
 u64 gk20a_locked_gmmu_map(struct vm_gk20a *vm,