Re: [RFC v2 1/8] drm/i915/dp: Program source OUI on eDP panels

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

On Wed, 16 Sep 2020, Lyude Paul <lyude@redhat.com> wrote:
> Since we're about to start adding support for Intel's magic HDR
> backlight interface over DPCD, we need to ensure we're properly
> programming this field so that Intel specific sink services are exposed.
> Otherwise, 0x300-0x3ff will just read zeroes.
>
> We also take care not to reprogram the source OUI if it already matches
> what we expect. This is just to be careful so that we don't accidentally
> take the panel out of any backlight control modes we found it in.
>
> v2:
> * Add careful parameter to intel_edp_init_source_oui() to avoid
>   re-writing the source OUI if it's already been set during driver
>   initialization
>
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> Cc: thaytan@noraisin.net
> Cc: Vasily Khoruzhick <anarsoul@gmail.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dp.c | 33 +++++++++++++++++++++++++
>  1 file changed, 33 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> index 4bd10456ad188..7db2b6a3cd52e 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> @@ -3424,6 +3424,29 @@ void intel_dp_sink_set_decompression_state(struct intel_dp *intel_dp,
>  			    enable ? "enable" : "disable");
>  }
>  
> +static void
> +intel_edp_init_source_oui(struct intel_dp *intel_dp, bool careful)
> +{
> +	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
> +	u8 oui[] = { 0x00, 0xaa, 0x01 };

Nitpick, could be static const.

> +	u8 buf[3] = { 0 };
> +
> +	/*
> +	 * During driver init, we want to be careful and avoid changing the source OUI if it's
> +	 * already set to what we want, so as to avoid clearing any state by accident
> +	 */

Did you actually observe any ill behaviour with unconditionally writing
the source OUI?

I mean it's easy to add the "careful" mode afterwards if there are
concrete issues, but it'll be hard to remove because it'll be a
functional change potentially causing regressions.

> +	if (careful) {
> +		if (drm_dp_dpcd_read(&intel_dp->aux, DP_SOURCE_OUI, buf, sizeof(buf)) < 0)
> +			drm_err(&i915->drm, "Failed to read source OUI\n");
> +
> +		if (memcmp(oui, buf, sizeof(oui)) == 0)
> +			return;
> +	}
> +
> +	if (drm_dp_dpcd_write(&intel_dp->aux, DP_SOURCE_OUI, oui, sizeof(oui)) < 0)
> +		drm_err(&i915->drm, "Failed to write source OUI\n");
> +}
> +
>  /* If the sink supports it, try to set the power state appropriately */
>  void intel_dp_sink_dpms(struct intel_dp *intel_dp, int mode)
>  {
> @@ -3443,6 +3466,10 @@ void intel_dp_sink_dpms(struct intel_dp *intel_dp, int mode)
>  	} else {
>  		struct intel_lspcon *lspcon = dp_to_lspcon(intel_dp);
>  
> +		/* Write the source OUI as early as possible */
> +		if (intel_dp_is_edp(intel_dp))
> +			intel_edp_init_source_oui(intel_dp, false);
> +

This hunk conflicts, will need a rebase.

BR,
Jani.


>  		/*
>  		 * When turning on, we need to retry for 1ms to give the sink
>  		 * time to wake up.
> @@ -4607,6 +4634,12 @@ intel_edp_init_dpcd(struct intel_dp *intel_dp)
>  	if (INTEL_GEN(dev_priv) >= 10 || IS_GEMINILAKE(dev_priv))
>  		intel_dp_get_dsc_sink_cap(intel_dp);
>  
> +	/*
> +	 * If needed, program our source OUI so we can make various Intel-specific AUX services
> +	 * available (such as HDR backlight controls)
> +	 */
> +	intel_edp_init_source_oui(intel_dp, true);
> +
>  	return true;
>  }

-- 
Jani Nikula, Intel Open Source Graphics Center
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