Re: [Intel-gfx] [PATCH 2/2] drm/i915: implement EXTENDED_RECEIVER_CAPABILITY_FIELD_PRESENT

[Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>]

On Tue, 2018-07-17 at 14:49 -0700, matthew.s.atwood@intel.com wrote:
> From: Matt Atwood <matthew.s.atwood@intel.com>
> 
> According to DP spec (2.9.3.1 of DP 1.4) if
> EXTENDED_RECEIVER_CAPABILITY_FIELD_PRESENT is set the addresses in
> DPCD
> 02200h through 0220Fh shall contain the DPRX's true capability. These
> values will match 00000h through 0000Fh, except for DPCD_REV,
> MAX_LINK_RATE, DOWN_STREAM_PORT_PRESENT.
> 
> Read from DPCD once for all 3 values as this is an expensive
> operation.
> Spec mentions that all of address space 02200h through 0220Fh should
> contain the right information however currently only 3 values can
> differ.
> 
> There is no address space in the intel_dp->dpcd struct for addresses
> 02200h through 0220Fh, and since so much of the data is a identical,
> simply overwrite the values stored in 00000h through 0000Fh with the
> values that can be overwritten from addresses 02200h through 0220Fh.
> 
> This patch helps with backward compatibility for devices pre DP1.3.
> 
> v2: read only dpcd values which can be affected,

I still see 6 bytes read and 3 copied.

>  remove incorrect check,
> split into drm include changes into separate patch, commit message,
> verbose debugging statements during overwrite.
> 
> Signed-off-by: Matt Atwood <matthew.s.atwood@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_dp.c | 37
> +++++++++++++++++++++++++++++++++
>  1 file changed, 37 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/intel_dp.c
> b/drivers/gpu/drm/i915/intel_dp.c
> index dde92e4af5d3..364cf41a8b89 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -3738,6 +3738,43 @@ intel_dp_read_dpcd(struct intel_dp *intel_dp)
>  			     sizeof(intel_dp->dpcd)) < 0)
>  		return false; /* aux transfer failed */
>  
> +	if (intel_dp->dpcd[DP_TRAINING_AUX_RD_INTERVAL] &
> +	    DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT) {
> +		uint8_t dpcd_ext[6];
> +
> +		DRM_DEBUG_KMS("DPCD: Extended Receiver Capability
> Field Present, accessing 02200h through 022FFh\n");
> +
> +		if (drm_dp_dpcd_read(&intel_dp->aux,
> DP_DP13_DPCD_REV,
> +		    &dpcd_ext, sizeof(dpcd_ext)) < 0)
> +			return false; /* aux transfer failed */
> +
> +		if (memcmp(&intel_dp->dpcd[DP_DPCD_REV],
> &dpcd_ext[DP_DPCD_REV],
> +			   sizeof(u8))) 

Why use memcmp and memcmpy if it's just one byte? You could just use
"=="

> {
> +			DRM_DEBUG_KMS("DPCD: new value for DPCD
> Revision previous value %2x new value %2x\n",
> +				      intel_dp->dpcd[DP_DPCD_REV],
> +				      dpcd_ext[DP_DPCD_REV]);
> +			memcpy(&intel_dp->dpcd[DP_DPCD_REV],
> +			       &dpcd_ext[DP_DPCD_REV],
> +			       sizeof(u8));
> +		}
> +		if (memcmp(&intel_dp->dpcd[DP_MAX_LINK_RATE],
> +			   &dpcd_ext[DP_MAX_LINK_RATE], sizeof(u8)))
> {
> +			DRM_DEBUG_KMS("DPCD: new value for DPCD Max
> Link Rate previous value %2x new value %2x\n",
> +				      intel_dp-
> >dpcd[DP_MAX_LINK_RATE],
> +				      dpcd_ext[DP_MAX_LINK_RATE]);
> +			memcpy(&intel_dp->dpcd[DP_MAX_LINK_RATE],
> +			       &dpcd_ext[DP_MAX_LINK_RATE],
> sizeof(u8));
> +		}
> +		if (memcmp(&intel_dp-
> >dpcd[DP_DOWNSTREAMPORT_PRESENT],
> +			   &dpcd_ext[DP_DOWNSTREAMPORT_PRESENT],
> sizeof(u8))) {
> +			DRM_DEBUG_KMS("DPCD: new value for DPCD
> Downstream Port Present  previous value %2x new value %2x\n",
> +				      intel_dp-
> >dpcd[DP_DOWNSTREAMPORT_PRESENT],
> +				      dpcd_ext[DP_DOWNSTREAMPORT_PRE
> SENT]);
> +			memcpy(&intel_dp-
> >dpcd[DP_DOWNSTREAMPORT_PRESENT],
> +			       &dpcd_ext[DP_DOWNSTREAMPORT_PRESENT],
> +			       sizeof(u8));
> +		}
> +	}
>  	DRM_DEBUG_KMS("DPCD: %*ph\n", (int) sizeof(intel_dp->dpcd),
> intel_dp->dpcd);
>  
>  	return intel_dp->dpcd[DP_DPCD_REV] != 0;
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