Re: [PATCH v7] drm/i915: implement EXTENDED_RECEIVER_CAPABILITY_FIELD_PRESENT

[Jani Nikula <jani.nikula@linux.intel.com>]

On Tue, 27 Nov 2018, Manasi Navare <manasi.d.navare@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
> v7 (From Manasi):
> * Memcpy, memcmp and debig logging based on sizeof(dpcd_ext) (Jani N)
> * Exit early (Jani N)
>
> Cc: Jani Nikula <jani.nikula@linux.intel.com>
> Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
> Signed-off-by: Matt Atwood <matthew.s.atwood@intel.com>
> Tested-by: Manasi Navare <manasi.d.navare@intel.com>
> Acked-by: Manasi Navare <manasi.d.navare@intel.com>
> Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_dp.c | 41 +++++++++++++++++++++++++++++++++
>  1 file changed, 41 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index 70ae3d57316b..a9eb14a4ab27 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -3802,6 +3802,45 @@ intel_dp_link_down(struct intel_encoder *encoder,
>  	}
>  }
>  
> +static void
> +intel_dp_extended_receiver_capabilities(struct intel_dp *intel_dp)
> +{
> +	u8 dpcd_ext[6];
> +
> +	/*
> +	 * 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))
> +		return;
> +
> +	DRM_DEBUG_KMS("DPCD: Reading extended receiver capabilities\n");

Superfluous debug logging.

> +
> +	if (drm_dp_dpcd_read(&intel_dp->aux, DP_DP13_DPCD_REV,
> +			     &dpcd_ext, sizeof(dpcd_ext)) != sizeof(dpcd_ext)) {
> +		DRM_ERROR("DPCD failed read at extended capabilities\n");

Most of our dpcd failures are logged using DRM_DEBUG_KMS. The ones that
log DRM_ERROR seem to be very recent additions deviating from the debug
loggin practice. There isn't much the user can do, really.

> +		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");

Okay, seems like a rare event.

> +		return;
> +	}
> +	if (!memcmp(intel_dp->dpcd, dpcd_ext, sizeof(dpcd_ext))) {
> +		DRM_DEBUG_KMS("Extended Receiver Cap DPCD match the base DPCD\n");

I don't think this debug logging is needed.

> +		return;
> +	}
> +
> +	DRM_DEBUG_KMS("Base DPCD: %*ph\n", (int)sizeof(dpcd_ext), intel_dp->dpcd);

Using sizeof(dpcd_ext) when printing something else is a red flag. You
could log the whole dpcd here.

	DRM_DEBUG_KMS("DPCD: %*ph (base)\n", (int) sizeof(intel_dp->dpcd), intel_dp->dpcd);

> +	DRM_DEBUG_KMS("Extended Receiver Cap DPCD: %*ph\n",
> +		      (int)sizeof(dpcd_ext), dpcd_ext);

The caller will log the *updated* DPCD right after this returns, you
don't need to log dpcd_ext.

> +	memcpy(intel_dp->dpcd, dpcd_ext, sizeof(dpcd_ext));
> +}
> +
> +

Superfluous newline.

>  bool
>  intel_dp_read_dpcd(struct intel_dp *intel_dp)
>  {
> @@ -3809,6 +3848,8 @@ 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);

One other alternative is to have
intel_dp_extended_receiver_capabilities() return true if the cap exists
and is different from current DPCD, and *all* DPCD logging would be done
here. Both the old and the new. Like so:

	DRM_DEBUG_KMS("DPCD: %*ph\n", (int) sizeof(intel_dp->dpcd), intel_dp->dpcd);
	if (intel_dp_extended_receiver_capabilities(intel_dp))
		DRM_DEBUG_KMS("DPCD: %*ph\n (ext)", (int) sizeof(intel_dp->dpcd), intel_dp->dpcd);

BR,
Jani.

>  
>  	return intel_dp->dpcd[DP_DPCD_REV] != 0;

-- 
Jani Nikula, Intel Open Source Graphics Center
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