linux-kernel - Re: [PATCH v2] drm/amd/display: use a more accurate check in dm_helpers_dp_read

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Date:   Mon, 20 Mar 2023 16:21:55 -0400
From:   Harry Wentland <harry.wentland@....com>
To:     Hamza Mahfooz <hamza.mahfooz@....com>,
        amd-gfx@...ts.freedesktop.org
Cc:     Leo Li <sunpeng.li@....com>,
        Rodrigo Siqueira <Rodrigo.Siqueira@....com>,
        Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        "Pan, Xinhui" <Xinhui.Pan@....com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>, Lyude Paul <lyude@...hat.com>,
        Wayne Lin <Wayne.Lin@....com>,
        Qingqing Zhuo <qingqing.zhuo@....com>,
        Wenjing Liu <wenjing.liu@....com>,
        Sung Joon Kim <sungjoon.kim@....com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] drm/amd/display: use a more accurate check in
 dm_helpers_dp_read_dpcd()

On 3/10/23 13:42, Hamza Mahfooz wrote:
> We should be checking if drm_dp_dpcd_read() returns the size that we are
> asking it to read instead of just checking if it is greater than zero.
> So, compare the return value of drm_dp_dpcd_read() to the requested
> read size.
> 
> Signed-off-by: Hamza Mahfooz <hamza.mahfooz@....com>

I don't think we can ever hit a case where the return value is greater
than 0 but not equal to size as drm_dp_dpcd_access already checks for it.

Either way, the stricter check makes sense and is
Reviewed-by: Harry Wentland <harry.wentland@....com>

Harry

> ---
> v2: drop the WARN_ON().
> ---
>  drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
> index 8d598b322e5b..9c1e91c2179e 100644
> --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
> +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_helpers.c
> @@ -511,8 +511,8 @@ bool dm_helpers_dp_read_dpcd(
>  		return false;
>  	}
>  
> -	return drm_dp_dpcd_read(&aconnector->dm_dp_aux.aux, address,
> -			data, size) > 0;
> +	return drm_dp_dpcd_read(&aconnector->dm_dp_aux.aux, address, data,
> +				size) == size;
>  }
>  
>  bool dm_helpers_dp_write_dpcd(