Re: [PATCH] drm/amd/display: use a more accurate check in dm_helpers_dp_read_dpcd()

[Ville Syrjälä]

On Thu, Mar 09, 2023 at 04:30:27PM -0500, 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.
> Also, we should WARN_ON() here since this condition is only ever met, if
> there is an issue worth investigating. So, compare the return value of
> drm_dp_dpcd_read() to size and WARN_ON() if they aren't equal.
> 
> Signed-off-by: Hamza Mahfooz <hamza.mahfooz@xxxxxxx>
> ---
>  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..ed2ed7b1d869 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 !WARN_ON(drm_dp_dpcd_read(&aconnector->dm_dp_aux.aux, address,
> +					 data, size) != size);

Just FYI there are devices out there that violate the DP spec and reads
from specific DPCD registers simply fail instead of returning the
expected 0.

>  }
>  
>  bool dm_helpers_dp_write_dpcd(
> -- 
> 2.39.2

-- 
Ville Syrjälä
Intel