[RFC PATCH 6/6] drm/msm/dpu: Disallow unallocated (DSC) resources to be returned

[Dmitry Baryshkov]

On 14/12/2022 01:22, Marijn Suijten wrote:
> 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];
>   	}
>  

These two chunks should come as two separate patches, each having it's 
own Fixes tag.

-- 
With best wishes
Dmitry