[RFC PATCH 6/6] drm/msm/dpu: Disallow unallocated (DSC) 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) 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.

In the specific case of DSC initial resource allocation should behave
more like LMs and CTLs where NULL resources are skipped.  The current
hardcoded mapping of DSC blocks should be loosened separately as DPU
5.0.0 introduced a crossbar where DSC blocks can be "somewhat" freely
bound to any PP and CTL, but that hardcoding currently means that we
will return an error when the topology reserves a DSC that isn't
available, instead of looking for the next free one.

^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.

Signed-off-by: Marijn Suijten <marijn.suijten at somainline.org>
---
 drivers/gpu/drm/msm/disp/dpu1/dpu_rm.c | 10 ++++++++++
 1 file changed, 10 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..dcbf03d2940a 100644
--- a/drivers/gpu/drm/msm/disp/dpu1/dpu_rm.c
+++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_rm.c
@@ -496,6 +496,11 @@ static int _dpu_rm_reserve_dsc(struct dpu_rm *rm,
 
 	/* check if DSC required are allocated or not */
 	for (i = 0; i < num_dsc; i++) {
+		if (!rm->dsc_blks[i]) {
+			DPU_ERROR("DSC %d does not exist\n", i);
+			return -EIO;
+		}
+
 		if (global_state->dsc_to_enc_id[i]) {
 			DPU_ERROR("DSC %d is already allocated\n", i);
 			return -EIO;
@@ -660,6 +665,11 @@ int dpu_rm_get_assigned_resources(struct dpu_rm *rm,
 				  blks_size, enc_id);
 			break;
 		}
+		if (!hw_blks[i]) {
+			DPU_ERROR("No more resource %d available to assign to enc %d\n",
+				  type, enc_id);
+			break;
+		}
 		blks[num_blks++] = hw_blks[i];
 	}
 
-- 
2.38.1