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 15:34 -0700, Dhinakaran Pandiyan wrote:
> 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.
Ignore this, the original patch was reading 16B. Thanks for clarifying
Matt.

> 
> > 
> >  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
> "=="
> 

I believe this is what Jani suggested.
if (memcmp(old_dpcd, new_dpcd, sizeof(new_dpcd)) {
	DRM_DEBUG_KMS();
	memcpy(old_dpcd, new_dpcd, sizeof(new_dpcd);	
}

We lose the information about which specific fields in the 6 bytes
changed, but that's okay IMO.

> > 
> > {
> > +			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_P
> > RE
> > 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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