VCN_INFO_TABLE_MAX_NUM_INSTANCES vs AMDGPU_MAX_VCN_INSTANCES

[Christian König]

Am 16.05.22 um 19:49 schrieb Ernst Sjöstrand:
> Den mån 16 maj 2022 kl 17:13 skrev Alex Deucher <alexdeucher at gmail.com>:
>
>     On Sun, May 15, 2022 at 11:46 AM Ernst Sjöstrand
>     <ernstp at gmail.com> wrote:
>     >
>     > smatch found this problem on amd-staging-drm-next:
>     >
>     > drivers/gpu/drm/amd/amdgpu/amdgpu_discovery.c:1443
>     amdgpu_discovery_get_vcn_info() error: buffer overflow
>     'adev->vcn.vcn_codec_disable_mask' 2 <= 3
>     >
>     > This is caused by:
>     > #define AMDGPU_MAX_VCN_INSTANCES 2
>     > #define VCN_INFO_TABLE_MAX_NUM_INSTANCES 4
>     >
>     > Can we just drop VCN_INFO_TABLE_MAX_NUM_INSTANCES completely and
>     use AMDGPU_MAX_VCN_INSTANCES everywhere instead (and bump it to 4)?
>
>     We should be able to bump AMDGPU_MAX_VCN_INSTANCES to 4 (although it
>     would waste some memory in the places it is used at this point).
>     VCN_INFO_TABLE_MAX_NUM_INSTANCES is part of a firmware structure so we
>     can't change that without breaking the firmware structure.
>
>     Alex
>
>
> It would be nice to get rid of this pattern and make sure it doesn't 
> happen again when the VCN info table is raised to 5.
> It's very similar to the HWIP_MAX_INSTANCE issue.

No, as Alex explained that distinction is intentional.

The firmware definition is 4 for future extensions, that doesn't mean 
that this is currently used.

There is currently simply no need to set AMDGPU_MAX_VCN_INSTANCES to 
more than 2.

Regards,
Christian.

>
> //E
>
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