[Intel-gfx] [PATCH v3 1/3] drm/i915/gt: BUG_ON unexpected NULL at scatterlist walking

[Ramalingam C <ramalingam.c@intel.com>]

While locating the start of ccs scatterlist in smem scatterlist, that has
to be the size of lmem obj size + corresponding ccs data size, report bug
if scatterlist terminate before that length.

v2:
  s/GEM_BUG_ON/BUG_ON with more commenting [Matt]
v3:
  Converted GEM_BUG_ON into BUG_ON with more documentation [Matt]

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com> (v1)
---
 drivers/gpu/drm/i915/gt/intel_migrate.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_migrate.c b/drivers/gpu/drm/i915/gt/intel_migrate.c
index 9d552f30b627..168d17b6f48a 100644
--- a/drivers/gpu/drm/i915/gt/intel_migrate.c
+++ b/drivers/gpu/drm/i915/gt/intel_migrate.c
@@ -687,6 +687,16 @@ static void get_ccs_sg_sgt(struct sgt_dma *it, u32 bytes_to_cpy)
 		bytes_to_cpy -= len;
 
 		it->sg = __sg_next(it->sg);
+
+		/*
+		 * On Flat-CCS capable platform when we back the lmem pages with
+		 * smem pages we add extra pages at the end of the smem
+		 * scatterlist, to store the ccs data corresponding to the lmem
+		 * pages. get_ccs_sg_sgt() is called to get the pointer for the
+		 * start of the extra pages added at the end of smem scatterlist.
+		 * So scatterlist can't end at or before bytes_to_cpy.
+		 */
+		BUG_ON(!it->sg);
 		it->dma = sg_dma_address(it->sg);
 		it->max = it->dma + sg_dma_len(it->sg);
 	} while (bytes_to_cpy);
@@ -748,8 +758,10 @@ intel_context_migrate_copy(struct intel_context *ce,
 		 * Need to fix it.
 		 */
 		ccs_bytes_to_cpy = src_sz != dst_sz ? GET_CCS_BYTES(i915, bytes_to_cpy) : 0;
-		if (ccs_bytes_to_cpy)
+		if (ccs_bytes_to_cpy) {
+			WARN_ON(abs(src_sz - dst_sz) < ccs_bytes_to_cpy);
 			get_ccs_sg_sgt(&it_ccs, bytes_to_cpy);
+		}
 	}
 
 	src_offset = 0;
-- 
2.20.1