linux-kernel - Re: [PATCH 6.1.y] drm/amd/display: Don't refer to dc_sink in is_dsc_need_re

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Message-ID: <82d8db95-2f62-4124-aff8-424252f505df@oracle.com>
Date: Mon, 9 Dec 2024 11:43:32 +0530
From: Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>
To: jianqi.ren.cn@...driver.com, wayne.lin@....com, gregkh@...uxfoundation.org
Cc: stable@...r.kernel.org, harry.wentland@....com, sunpeng.li@....com,
        Rodrigo.Siqueira@....com, alexander.deucher@....com,
        christian.koenig@....com, airlied@...il.com, daniel@...ll.ch,
        Jerry.Zuo@....com, zaeem.mohamed@....com,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, Vegard Nossum <vegard.nossum@...cle.com>
Subject: Re: [PATCH 6.1.y] drm/amd/display: Don't refer to dc_sink in
 is_dsc_need_re_compute

Hi Jianqi,

On 09/12/24 12:06, jianqi.ren.cn@...driver.com wrote:
> From: Wayne Lin <wayne.lin@....com>
> 
> [ Upstream commit fcf6a49d79923a234844b8efe830a61f3f0584e4 ]
> 
> [Why]
> When unplug one of monitors connected after mst hub, encounter null pointer dereference.
> 
> It's due to dc_sink get released immediately in early_unregister() or detect_ctx(). When
> commit new state which directly referring to info stored in dc_sink will cause null pointer
> dereference.
> 
> [how]
> Remove redundant checking condition. Relevant condition should already be covered by checking
> if dsc_aux is null or not. Also reset dsc_aux to NULL when the connector is disconnected.
> 
> Reviewed-by: Jerry Zuo <jerry.zuo@....com>
> Acked-by: Zaeem Mohamed <zaeem.mohamed@....com>
> Signed-off-by: Wayne Lin <wayne.lin@....com>
> Tested-by: Daniel Wheeler <daniel.wheeler@....com>
> Signed-off-by: Alex Deucher <alexander.deucher@....com>
> Signed-off-by: Jianqi Ren <jianqi.ren.cn@...driver.com>
> ---
>   drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c
> index 1acef5f3838f..a1619f4569cf 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c
> @@ -183,6 +183,8 @@ amdgpu_dm_mst_connector_early_unregister(struct drm_connector *connector)
>   		dc_sink_release(dc_sink);
>   		aconnector->dc_sink = NULL;
>   		aconnector->edid = NULL;
> +		aconnector->dsc_aux = NULL;
> +		port->passthrough_aux = NULL;
>   	}
>   
>   	aconnector->mst_status = MST_STATUS_DEFAULT;
> @@ -487,6 +489,8 @@ dm_dp_mst_detect(struct drm_connector *connector,
>   		dc_sink_release(aconnector->dc_sink);
>   		aconnector->dc_sink = NULL;
>   		aconnector->edid = NULL;
> +		aconnector->dsc_aux = NULL;
> +		port->passthrough_aux = NULL;
>   
>   		amdgpu_dm_set_mst_status(&aconnector->mst_status,
>   			MST_REMOTE_EDID | MST_ALLOCATE_NEW_PAYLOAD | MST_CLEAR_ALLOCATED_PAYLOAD,

This backport doesn't look right to me, can you please clarify these.

1.  I think it is worth documenting in commit message just before your 
SOB on why you don't need this hunk:

                         MST_REMOTE_EDID | MST_ALLOCATE_NEW_PAYLOAD | 
MST_CLEAR_ALLOCATED_PAYLOAD,
@@ -1238,14 +1242,6 @@ static bool is_dsc_need_re_compute(
                 if (!aconnector || !aconnector->dsc_aux)
                         continue;

-               /*
-                *      check if cached virtual MST DSC caps are 
available and DSC is supported
-                *      as per specifications in their Virtual DPCD 
registers.
-               */
-               if 
(!(aconnector->dc_sink->dsc_caps.dsc_dec_caps.is_dsc_supported ||
- 
aconnector->dc_link->dpcd_caps.dsc_caps.dsc_basic_caps.fields.dsc_support.DSC_PASSTHROUGH_SUPPORT))
-                       continue;
-
                 stream_on_link[new_stream_on_link_num] = aconnector;
                 new_stream_on_link_num++;


which is part of upstream, I understand that this is not in 6.1.y code, 
if so is it even affected ?

2. Also commit message says:

""
Remove redundant checking condition. Relevant condition should already 
be covered by checking if dsc_aux is null or not. Also reset dsc_aux to 
NULL when the connector is disconnected.
""

but the if(!aconnector->dsc_aux) is not in 6.1.y, so something is 
missing. Maybe 6.1.y is not affected or we need more backports to 6.1.y 
to make them clean cherry-picks ?

Thanks,
Harshit