[PATCH v3] drm/msm/dpu: Disallow unallocated resources to be returned

[Marijn Suijten]

In the event that the topology requests resources that have not been
created by the system (because they are typically not represented in
dpu_mdss_cfg ^1), the resource(s) in global_state (in this case DSC
blocks, until their allocation/assignment is being sanity-checked in
"drm/msm/dpu: Reject topologies for which no DSC blocks are available")
remain NULL but will still be returned out of
dpu_rm_get_assigned_resources, where the caller expects to get an array
containing num_blks valid pointers (but instead gets these NULLs).

To prevent this from happening, where null-pointer dereferences
typically result in a hard-to-debug platform lockup, num_blks shouldn't
increase past NULL blocks and will print an error and break instead.
After all, max_blks represents the static size of the maximum number of
blocks whereas the actual amount varies per platform.

^1: which can happen after a git rebase ended up moving additions to
_dpu_cfg to a different struct which has the same patch context.

Fixes: bb00a452d6f7 ("drm/msm/dpu: Refactor resource manager")
Signed-off-by: Marijn Suijten <marijn.suijten at somainline.org>
---

Changes since v2:
- Dropped all 7 other patches that were queued for -next;
- Reworded error message to clarify that the requested resource should
  have already been allocated, rather than sounding like
  dpu_rm_get_assigned_resources is (re)allocating/(re)assigning
  resources here;
- This patch is now (implicitly!) based after "drm/msm/dpu: Reject
  topologies for which no DSC blocks are available", which should make
  it impossible to reach this condition, making it more of a safeguard
  in case of future code changes and/or hidden issues: and is more
  clearly conveyed in the patch message as well.

v2: https://lore.kernel.org/linux-arm-msm/20221221231943.1961117-5-marijn.suijten@somainline.org/

 drivers/gpu/drm/msm/disp/dpu1/dpu_rm.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_rm.c b/drivers/gpu/drm/msm/disp/dpu1/dpu_rm.c
index 73b3442e7467..7ada957adbbb 100644
--- a/drivers/gpu/drm/msm/disp/dpu1/dpu_rm.c
+++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_rm.c
@@ -660,6 +660,11 @@ int dpu_rm_get_assigned_resources(struct dpu_rm *rm,
 				  blks_size, enc_id);
 			break;
 		}
+		if (!hw_blks[i]) {
+			DPU_ERROR("Allocated resource %d unavailable to assign to enc %d\n",
+				  type, enc_id);
+			break;
+		}
 		blks[num_blks++] = hw_blks[i];
 	}
 
-- 
2.39.0