[Intel-gfx] [PATCH 2/8] drm/i915/dp: Check error return during DPCD capability queries

[Ville Syrjälä]

On Wed, Apr 26, 2017 at 04:40:07PM +0300, Imre Deak wrote:
> The assumptions of these users of drm_dp_dpcd_readb() is that the passed
> in output buffer won't change in case of error, but this isn't
> guaranteed.

Hmm. We blindly copy as many bytes from the rxbuf into the user
provided buffer as the hardware told us to. And whether the transfer
actually is considered a success or not depends on what the first
received byte says. I don't recall what the DP spec really says about
replying with more than one byte on failure, but I guess we shouldn't
depend on it anyway.

We could actually make that guarantee if we moved txbuf+rxbuf up
into drm_dp_dpcd_access() and did the copy to the user buffer
only on succes. But that would require changing all the DP
capable drivers at the same time, so would require someone
motivated enough.

In the meantime
Reviewed-by: Ville Syrjälä <ville.syrjala at linux.intel.com>

> Fix this by treating any error as the lack of the given
> capability.
> 
> In case of DP_SINK_DEVICE_AUX_FRAME_SYNC_CAP an error would leave the
> buffer uninitialized even with the above assumption.
> 
> Signed-off-by: Imre Deak <imre.deak at intel.com>
> ---
>  drivers/gpu/drm/i915/intel_dp.c | 20 ++++++++++++--------
>  1 file changed, 12 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index 08834f7..4a6feb6 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -3088,7 +3088,8 @@ static bool intel_dp_get_y_cord_status(struct intel_dp *intel_dp)
>  {
>  	uint8_t psr_caps = 0;
>  
> -	drm_dp_dpcd_readb(&intel_dp->aux, DP_PSR_CAPS, &psr_caps);
> +	if (drm_dp_dpcd_readb(&intel_dp->aux, DP_PSR_CAPS, &psr_caps) != 1)
> +		return false;
>  	return psr_caps & DP_PSR2_SU_Y_COORDINATE_REQUIRED;
>  }
>  
> @@ -3096,9 +3097,9 @@ static bool intel_dp_get_colorimetry_status(struct intel_dp *intel_dp)
>  {
>  	uint8_t dprx = 0;
>  
> -	drm_dp_dpcd_readb(&intel_dp->aux,
> -			DP_DPRX_FEATURE_ENUMERATION_LIST,
> -			&dprx);
> +	if (drm_dp_dpcd_readb(&intel_dp->aux, DP_DPRX_FEATURE_ENUMERATION_LIST,
> +			      &dprx) != 1)
> +		return false;
>  	return dprx & DP_VSC_SDP_EXT_FOR_COLORIMETRY_SUPPORTED;
>  }
>  
> @@ -3106,7 +3107,9 @@ static bool intel_dp_get_alpm_status(struct intel_dp *intel_dp)
>  {
>  	uint8_t alpm_caps = 0;
>  
> -	drm_dp_dpcd_readb(&intel_dp->aux, DP_RECEIVER_ALPM_CAP, &alpm_caps);
> +	if (drm_dp_dpcd_readb(&intel_dp->aux, DP_RECEIVER_ALPM_CAP,
> +			      &alpm_caps) != 1)
> +		return false;
>  	return alpm_caps & DP_ALPM_CAP;
>  }
>  
> @@ -3679,9 +3682,10 @@ intel_edp_init_dpcd(struct intel_dp *intel_dp)
>  		uint8_t frame_sync_cap;
>  
>  		dev_priv->psr.sink_support = true;
> -		drm_dp_dpcd_readb(&intel_dp->aux,
> -				  DP_SINK_DEVICE_AUX_FRAME_SYNC_CAP,
> -				  &frame_sync_cap);
> +		if (drm_dp_dpcd_readb(&intel_dp->aux,
> +				      DP_SINK_DEVICE_AUX_FRAME_SYNC_CAP,
> +				      &frame_sync_cap) != 1)
> +			frame_sync_cap = 0;
>  		dev_priv->psr.aux_frame_sync = frame_sync_cap ? true : false;
>  		/* PSR2 needs frame sync as well */
>  		dev_priv->psr.psr2_support = dev_priv->psr.aux_frame_sync;
> -- 
> 2.5.0
> 
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-- 
Ville Syrjälä
Intel OTC