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

[Lyude Paul]

Just an FYI, my plan for some of this eDP backlight code is to move as much of
it into helpers as possible since we need to implement the same interface in
nouveau. We probably can figure out some other solution for handling this quirk
if this isn't possible, but could we maybe use the panel's OUI here and add a
quirk to drm_dp_helper.c instead?

On Thu, 2020-09-17 at 11:09 -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.
> 
> 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@xxxxxxxxxxxx>
> ---
> 
>  .../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;
-- 
Sincerely,
      Lyude Paul (she/her)
      Software Engineer at Red Hat