Re: [PATCH] i915: Introduce quirk for shifting eDP brightness.

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Thu, Sep 17, 2020 at 11:09:06AM -0600, Kevin Chowski wrote:
> We have observed that Google Pixelbook's backlight hardware is
> interpretting these backlight bits from the most-significant side of the
> 16 bit word (if DP_EDP_PWMGEN_BIT_COUNT < 16), whereas the driver code
> assumes the peripheral cares about the least-significant bits.

The spec seems to agree with the msb interpretation. So looks like
the current code is just broken?

> 
> Testing was done from within Chrome OS's build environment when the
> patch is backported to 5.4 (the version we are newly targeting for the
> Pixelbook); for the record:
>    $ emerge-eve-kernelnext sys-kernel/chromeos-kernel-5_4 && \
>       ./update_kernel.sh --remote=$IP
> 
> I used `/sys/kernel/debug/dri/0/eDP-1/i915_dpcd` on my laptop to verify
> that the registers were being set according to what the actual hardware
> expects; I also observe that the backlight is noticeably brighter with
> this patch.
> 
> Signed-off-by: Kevin Chowski <chowski@chromium.org>
> ---
> 
>  .../drm/i915/display/intel_dp_aux_backlight.c | 34 +++++++++++++++++++
>  drivers/gpu/drm/i915/display/intel_quirks.c   | 13 +++++++
>  drivers/gpu/drm/i915/i915_drv.h               |  1 +
>  3 files changed, 48 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> index acbd7eb66cbe3..99c98f217356d 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> @@ -91,6 +91,23 @@ static u32 intel_dp_aux_get_backlight(struct intel_connector *connector)
>  	if (intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT)
>  		level = (read_val[0] << 8 | read_val[1]);
>  
> +	if (i915->quirks & QUIRK_SHIFT_EDP_BACKLIGHT_BRIGHTNESS) {
> +		if (!drm_dp_dpcd_readb(&intel_dp->aux, DP_EDP_PWMGEN_BIT_COUNT,
> +						&read_val[0])) {
> +			DRM_DEBUG_KMS("Failed to read DPCD register 0x%x\n",
> +					DP_EDP_PWMGEN_BIT_COUNT);
> +			return 0;
> +		}
> +		// Only bits 4:0 are used, 7:5 are reserved.
> +		read_val[0] = read_val[0] & 0x1F;
> +		if (read_val[0] > 16) {
> +			DRM_DEBUG_KMS("Invalid DP_EDP_PWNGEN_BIT_COUNT 0x%X, expected at most 16\n",
> +						read_val[0]);
> +			return 0;
> +		}
> +		level >>= 16 - read_val[0];
> +	}
> +
>  	return level;
>  }
>  
> @@ -106,6 +123,23 @@ intel_dp_aux_set_backlight(const struct drm_connector_state *conn_state, u32 lev
>  	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
>  	u8 vals[2] = { 0x0 };
>  
> +	if (i915->quirks & QUIRK_SHIFT_EDP_BACKLIGHT_BRIGHTNESS) {
> +		if (!drm_dp_dpcd_readb(&intel_dp->aux, DP_EDP_PWMGEN_BIT_COUNT,
> +						&vals[0])) {
> +			DRM_DEBUG_KMS("Failed to write aux backlight level: Failed to read DPCD register 0x%x\n",
> +					  DP_EDP_PWMGEN_BIT_COUNT);
> +			return;
> +		}
> +		// Only bits 4:0 are used, 7:5 are reserved.
> +		vals[0] = vals[0] & 0x1F;
> +		if (vals[0] > 16) {
> +			DRM_DEBUG_KMS("Failed to write aux backlight level: Invalid DP_EDP_PWNGEN_BIT_COUNT 0x%X, expected at most 16\n",
> +						vals[0]);
> +			return;
> +		}
> +		level <<= (16 - vals[0]) & 0xFFFF;
> +	}
> +
>  	vals[0] = level;
>  
>  	/* Write the MSB and/or LSB */
> diff --git a/drivers/gpu/drm/i915/display/intel_quirks.c b/drivers/gpu/drm/i915/display/intel_quirks.c
> index 46beb155d835f..63b27d49b2864 100644
> --- a/drivers/gpu/drm/i915/display/intel_quirks.c
> +++ b/drivers/gpu/drm/i915/display/intel_quirks.c
> @@ -53,6 +53,16 @@ static void quirk_increase_ddi_disabled_time(struct drm_i915_private *i915)
>  	drm_info(&i915->drm, "Applying Increase DDI Disabled quirk\n");
>  }
>  
> +/*
> + * Some eDP backlight hardware uses the most-significant bits of the brightness
> + * register, so brightness values must be shifted first.
> + */
> +static void quirk_shift_edp_backlight_brightness(struct drm_i915_private *i915)
> +{
> +	i915->quirks |= QUIRK_SHIFT_EDP_BACKLIGHT_BRIGHTNESS;
> +	DRM_INFO("Applying shift eDP backlight brightness quirk\n");
> +}
> +
>  struct intel_quirk {
>  	int device;
>  	int subsystem_vendor;
> @@ -156,6 +166,9 @@ static struct intel_quirk intel_quirks[] = {
>  	/* ASRock ITX*/
>  	{ 0x3185, 0x1849, 0x2212, quirk_increase_ddi_disabled_time },
>  	{ 0x3184, 0x1849, 0x2212, quirk_increase_ddi_disabled_time },
> +
> +	/* Google Pixelbook */
> +	{ 0x591E, 0x8086, 0x2212, quirk_shift_edp_backlight_brightness },
>  };
>  
>  void intel_init_quirks(struct drm_i915_private *i915)
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index e4f7f6518945b..cc93bede4fab8 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -525,6 +525,7 @@ struct i915_psr {
>  #define QUIRK_PIN_SWIZZLED_PAGES (1<<5)
>  #define QUIRK_INCREASE_T12_DELAY (1<<6)
>  #define QUIRK_INCREASE_DDI_DISABLED_TIME (1<<7)
> +#define QUIRK_SHIFT_EDP_BACKLIGHT_BRIGHTNESS (1<<8)
>  
>  struct intel_fbdev;
>  struct intel_fbc_work;
> -- 
> 2.28.0.618.gf4bc123cb7-goog

-- 
Ville Syrjälä
Intel