Re: [PATCH] drm/i915: implement EXTENDED_RECEIVER_CAPABILITY_FIELD_PRESENT

[Manasi Navare <manasi.d.navare@intel.com>]

Thanks Jani for backmerging the drm patch.

On Wed, Sep 26, 2018 at 12:42:16PM +0300, Jani Nikula wrote:
> On Thu, 13 Sep 2018, 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, remove incorrect check,
> > split into drm include changes into separate patch, commit message,
> > verbose debugging statements during overwrite.
> > v3: white space fixes
> > v4: make path dependent on DPCD revision > 1.2
> > v5: split into function, removed DPCD rev check
> > v6: add debugging prints for early exit conditions
> >
> > Signed-off-by: Matt Atwood <matthew.s.atwood@intel.com>
> 
> So I did the backmerge to get the DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT
> to dinq, and was about to merge... but I gotta nitpick here.
> 
> > ---
> >  drivers/gpu/drm/i915/intel_dp.c | 58 +++++++++++++++++++++++++++++++++
> >  1 file changed, 58 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> > index dde92e4af5d3..1190635e4135 100644
> > --- a/drivers/gpu/drm/i915/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/intel_dp.c
> > @@ -3731,6 +3731,62 @@ intel_dp_link_down(struct intel_encoder *encoder,
> >  	}
> >  }
> >  
> > +static void
> > +intel_dp_extended_receiver_capabilities(struct intel_dp *intel_dp)
> > +{
> > +	/*
> > +	 * Prior to DP1.3 the bit represented by
> > +	 * DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT was reserved.
> > +	 * if it is set DP_DPCD_REV at 0000h could be at a value less than
> > +	 * the true capability of the panel. The only way to check is to
> > +	 * then compare 0000h and 2200h.
> > +	 */
> > +	if (intel_dp->dpcd[DP_TRAINING_AUX_RD_INTERVAL] &
> > +	    DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT) {
> 
> Reverse the condition, early exit, and make the rest of the function so
> much easier to read by decreasing indent.
> 

Yes that makes sense, so just exit early if DP_EXTENDED_RECEIVER_CAP_FIELD_PRESENT is not set

> > +		uint8_t dpcd_ext[6];
> 
> u8
> 
> > +
> > +		DRM_DEBUG_KMS("DPCD: Extended Receiver Capability Field Present, accessing 02200h through 022FFh\n");
> 
> Logging the offsets is excessive.
> 
> > +
> > +		if (drm_dp_dpcd_read(&intel_dp->aux, DP_DP13_DPCD_REV,
> > +				     &dpcd_ext, sizeof(dpcd_ext)) < 0) {
> 
> != sizeof(dpcd_ext) is more accurate.
>

+1 for this change.
 
> > +			DRM_ERROR("DPCD failed read at extended capabilities\n");
> > +			return;
> > +		}
> > +
> > +		if (intel_dp->dpcd[DP_DPCD_REV] > dpcd_ext[DP_DPCD_REV]) {
> > +			DRM_DEBUG_KMS("DPCD extended DPCD rev less than base DPCD rev\n");
> > +			return;
> > +		}
> > +
> > +		if (memcmp(&intel_dp->dpcd[DP_DPCD_REV], &dpcd_ext[DP_DPCD_REV],
> > +			   sizeof(u8))) {
> > +			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));
> > +		}
> 
> I presume earlier versions of the patch had memcmp/memcpy on the whole
> dpcd_ext, but if for some reason not present in the changelog you really
> need to do this piecemeal, please do not use memcmp/memcpy on
> sizeof(u8)! Ditto below.
> 
> BR,
> Jani.
> 
> > +		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));

The other feedback here from Ville was that, the value of DP_MX_LINK_RATE set here
should be validated against the valid values in drm_dp_bw_code_to_link_rate() otherwise if the faulty panels set it to
a bad value, the driver will default to lowest rate of RBR/1.62Gbps

Manasi

> > +		}
> > +		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_PRESENT]);
> > +			memcpy(&intel_dp->dpcd[DP_DOWNSTREAMPORT_PRESENT],
> > +			       &dpcd_ext[DP_DOWNSTREAMPORT_PRESENT],
> > +			       sizeof(u8));
> > +		}
> > +	}
> > +}
> > +
> > +
> >  bool
> >  intel_dp_read_dpcd(struct intel_dp *intel_dp)
> >  {
> > @@ -3738,6 +3794,8 @@ intel_dp_read_dpcd(struct intel_dp *intel_dp)
> >  			     sizeof(intel_dp->dpcd)) < 0)
> >  		return false; /* aux transfer failed */
> >  
> > +	intel_dp_extended_receiver_capabilities(intel_dp);
> > +
> >  	DRM_DEBUG_KMS("DPCD: %*ph\n", (int) sizeof(intel_dp->dpcd), intel_dp->dpcd);
> >  
> >  	return intel_dp->dpcd[DP_DPCD_REV] != 0;
> 
> -- 
> Jani Nikula, Intel Open Source Graphics Center
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