[PATCH v4 1/5] drm/i915: Add support for panels with VESA backlights with PWM enable/disable

[PATCH v4 1/5] drm/i915: Add support for panels with VESA backlights with PWM enable/disable

[Lyude Paul]

This simply adds proper support for panel backlights that can be controlled
via VESA's backlight control protocol, but which also require that we
enable and disable the backlight via PWM instead of via the DPCD interface.
We also enable this by default, in order to fix some people's backlights
that were broken by not having this enabled.

For reference, backlights that require this and use VESA's backlight
interface tend to be laptops with hybrid GPUs, but this very well may
change in the future.

v4:
* Make sure that we call intel_backlight_level_to_pwm() in
  intel_dp_aux_vesa_enable_backlight() - vsyrjala

Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://gitlab.freedesktop.org/drm/intel/-/issues/3680
Fixes: fe7d52bccab6 ("drm/i915/dp: Don't use DPCD backlights that need PWM enable/disable")
Cc: <stable@vger.kernel.org> # v5.12+
---
 .../drm/i915/display/intel_dp_aux_backlight.c | 27 ++++++++++++++-----
 1 file changed, 21 insertions(+), 6 deletions(-)

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 569d17b4d00f..f05b71c01b8e 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
@@ -293,6 +293,13 @@ intel_dp_aux_vesa_enable_backlight(const struct intel_crtc_state *crtc_state,
 	struct intel_panel *panel = &connector->panel;
 	struct intel_dp *intel_dp = enc_to_intel_dp(connector->encoder);
 
+	if (!panel->backlight.edp.vesa.info.aux_enable) {
+		u32 pwm_level = intel_backlight_invert_pwm_level(connector,
+								 panel->backlight.pwm_level_max);
+
+		panel->backlight.pwm_funcs->enable(crtc_state, conn_state, pwm_level);
+	}
+
 	drm_edp_backlight_enable(&intel_dp->aux, &panel->backlight.edp.vesa.info, level);
 }
 
@@ -304,6 +311,10 @@ static void intel_dp_aux_vesa_disable_backlight(const struct drm_connector_state
 	struct intel_dp *intel_dp = enc_to_intel_dp(connector->encoder);
 
 	drm_edp_backlight_disable(&intel_dp->aux, &panel->backlight.edp.vesa.info);
+
+	if (!panel->backlight.edp.vesa.info.aux_enable)
+		panel->backlight.pwm_funcs->disable(old_conn_state,
+						    intel_backlight_invert_pwm_level(connector, 0));
 }
 
 static int intel_dp_aux_vesa_setup_backlight(struct intel_connector *connector, enum pipe pipe)
@@ -321,6 +332,15 @@ static int intel_dp_aux_vesa_setup_backlight(struct intel_connector *connector,
 	if (ret < 0)
 		return ret;
 
+	if (!panel->backlight.edp.vesa.info.aux_enable) {
+		ret = panel->backlight.pwm_funcs->setup(connector, pipe);
+		if (ret < 0) {
+			drm_err(&i915->drm,
+				"Failed to setup PWM backlight controls for eDP backlight: %d\n",
+				ret);
+			return ret;
+		}
+	}
 	panel->backlight.max = panel->backlight.edp.vesa.info.max;
 	panel->backlight.min = 0;
 	if (current_mode == DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD) {
@@ -340,12 +360,7 @@ intel_dp_aux_supports_vesa_backlight(struct intel_connector *connector)
 	struct intel_dp *intel_dp = intel_attached_dp(connector);
 	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
 
-	/* TODO: We currently only support AUX only backlight configurations, not backlights which
-	 * require a mix of PWM and AUX controls to work. In the mean time, these machines typically
-	 * work just fine using normal PWM controls anyway.
-	 */
-	if ((intel_dp->edp_dpcd[1] & DP_EDP_BACKLIGHT_AUX_ENABLE_CAP) &&
-	    drm_edp_backlight_supported(intel_dp->edp_dpcd)) {
+	if (drm_edp_backlight_supported(intel_dp->edp_dpcd)) {
 		drm_dbg_kms(&i915->drm, "AUX Backlight Control Supported!\n");
 		return true;
 	}
-- 
2.31.1

Re: [PATCH v4 1/5] drm/i915: Add support for panels with VESA backlights with PWM enable/disable

[Ville Syrjälä]

On Tue, Oct 26, 2021 at 06:08:44PM -0400, Lyude Paul wrote:
> This simply adds proper support for panel backlights that can be controlled
> via VESA's backlight control protocol, but which also require that we
> enable and disable the backlight via PWM instead of via the DPCD interface.
> We also enable this by default, in order to fix some people's backlights
> that were broken by not having this enabled.
> 
> For reference, backlights that require this and use VESA's backlight
> interface tend to be laptops with hybrid GPUs, but this very well may
> change in the future.
> 
> v4:
> * Make sure that we call intel_backlight_level_to_pwm() in
>   intel_dp_aux_vesa_enable_backlight() - vsyrjala
> 
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> Link: https://gitlab.freedesktop.org/drm/intel/-/issues/3680
> Fixes: fe7d52bccab6 ("drm/i915/dp: Don't use DPCD backlights that need PWM enable/disable")
> Cc: <stable@vger.kernel.org> # v5.12+

Seems consistent enough.
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

> ---
>  .../drm/i915/display/intel_dp_aux_backlight.c | 27 ++++++++++++++-----
>  1 file changed, 21 insertions(+), 6 deletions(-)
> 
> 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 569d17b4d00f..f05b71c01b8e 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
> @@ -293,6 +293,13 @@ intel_dp_aux_vesa_enable_backlight(const struct intel_crtc_state *crtc_state,
>  	struct intel_panel *panel = &connector->panel;
>  	struct intel_dp *intel_dp = enc_to_intel_dp(connector->encoder);
>  
> +	if (!panel->backlight.edp.vesa.info.aux_enable) {
> +		u32 pwm_level = intel_backlight_invert_pwm_level(connector,
> +								 panel->backlight.pwm_level_max);
> +
> +		panel->backlight.pwm_funcs->enable(crtc_state, conn_state, pwm_level);
> +	}
> +
>  	drm_edp_backlight_enable(&intel_dp->aux, &panel->backlight.edp.vesa.info, level);
>  }
>  
> @@ -304,6 +311,10 @@ static void intel_dp_aux_vesa_disable_backlight(const struct drm_connector_state
>  	struct intel_dp *intel_dp = enc_to_intel_dp(connector->encoder);
>  
>  	drm_edp_backlight_disable(&intel_dp->aux, &panel->backlight.edp.vesa.info);
> +
> +	if (!panel->backlight.edp.vesa.info.aux_enable)
> +		panel->backlight.pwm_funcs->disable(old_conn_state,
> +						    intel_backlight_invert_pwm_level(connector, 0));
>  }
>  
>  static int intel_dp_aux_vesa_setup_backlight(struct intel_connector *connector, enum pipe pipe)
> @@ -321,6 +332,15 @@ static int intel_dp_aux_vesa_setup_backlight(struct intel_connector *connector,
>  	if (ret < 0)
>  		return ret;
>  
> +	if (!panel->backlight.edp.vesa.info.aux_enable) {
> +		ret = panel->backlight.pwm_funcs->setup(connector, pipe);
> +		if (ret < 0) {
> +			drm_err(&i915->drm,
> +				"Failed to setup PWM backlight controls for eDP backlight: %d\n",
> +				ret);
> +			return ret;
> +		}
> +	}
>  	panel->backlight.max = panel->backlight.edp.vesa.info.max;
>  	panel->backlight.min = 0;
>  	if (current_mode == DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD) {
> @@ -340,12 +360,7 @@ intel_dp_aux_supports_vesa_backlight(struct intel_connector *connector)
>  	struct intel_dp *intel_dp = intel_attached_dp(connector);
>  	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
>  
> -	/* TODO: We currently only support AUX only backlight configurations, not backlights which
> -	 * require a mix of PWM and AUX controls to work. In the mean time, these machines typically
> -	 * work just fine using normal PWM controls anyway.
> -	 */
> -	if ((intel_dp->edp_dpcd[1] & DP_EDP_BACKLIGHT_AUX_ENABLE_CAP) &&
> -	    drm_edp_backlight_supported(intel_dp->edp_dpcd)) {
> +	if (drm_edp_backlight_supported(intel_dp->edp_dpcd)) {
>  		drm_dbg_kms(&i915->drm, "AUX Backlight Control Supported!\n");
>  		return true;
>  	}
> -- 
> 2.31.1

-- 
Ville Syrjälä
Intel

[PATCH v4 2/5] drm/nouveau/kms/nv50-: Explicitly check DPCD backlights for aux enable/brightness

[Lyude Paul]

Since we don't support hybrid AUX/PWM backlights in nouveau right now,
let's add some explicit checks so that we don't break nouveau once we
enable support for these backlights in other drivers.

Signed-off-by: Lyude Paul <lyude@redhat.com>
---
 drivers/gpu/drm/nouveau/nouveau_backlight.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/nouveau/nouveau_backlight.c b/drivers/gpu/drm/nouveau/nouveau_backlight.c
index 1cbd71abc80a..ae2f2abc8f5a 100644
--- a/drivers/gpu/drm/nouveau/nouveau_backlight.c
+++ b/drivers/gpu/drm/nouveau/nouveau_backlight.c
@@ -308,7 +308,10 @@ nv50_backlight_init(struct nouveau_backlight *bl,
 		if (ret < 0)
 			return ret;
 
-		if (drm_edp_backlight_supported(edp_dpcd)) {
+		/* TODO: Add support for hybrid PWM/DPCD panels */
+		if (drm_edp_backlight_supported(edp_dpcd) &&
+		    (edp_dpcd[1] & DP_EDP_BACKLIGHT_AUX_ENABLE_CAP) &&
+		    (edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_AUX_SET_CAP)) {
 			NV_DEBUG(drm, "DPCD backlight controls supported on %s\n",
 				 nv_conn->base.name);
 
-- 
2.31.1

Re: [Nouveau] [PATCH v4 2/5] drm/nouveau/kms/nv50-: Explicitly check DPCD backlights for aux enable/brightness

[Karol Herbst]

On Wed, Oct 27, 2021 at 12:09 AM Lyude Paul <lyude@redhat.com> wrote:
>
> Since we don't support hybrid AUX/PWM backlights in nouveau right now,
> let's add some explicit checks so that we don't break nouveau once we
> enable support for these backlights in other drivers.
>
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> ---
>  drivers/gpu/drm/nouveau/nouveau_backlight.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/nouveau/nouveau_backlight.c b/drivers/gpu/drm/nouveau/nouveau_backlight.c
> index 1cbd71abc80a..ae2f2abc8f5a 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_backlight.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_backlight.c
> @@ -308,7 +308,10 @@ nv50_backlight_init(struct nouveau_backlight *bl,
>                 if (ret < 0)
>                         return ret;
>
> -               if (drm_edp_backlight_supported(edp_dpcd)) {
> +               /* TODO: Add support for hybrid PWM/DPCD panels */
> +               if (drm_edp_backlight_supported(edp_dpcd) &&
> +                   (edp_dpcd[1] & DP_EDP_BACKLIGHT_AUX_ENABLE_CAP) &&
> +                   (edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_AUX_SET_CAP)) {
>                         NV_DEBUG(drm, "DPCD backlight controls supported on %s\n",
>                                  nv_conn->base.name);
>
> --
> 2.31.1
>

Reviewed-by: Karol Herbst <kherbst@redhat.com>

[PATCH v4 3/5] drm/dp: Disable unsupported features in DP_EDP_BACKLIGHT_MODE_SET_REGISTER

[Lyude Paul]

As it turns out, apparently some machines will actually leave additional
backlight functionality like dynamic backlight control on before the OS
loads. Currently we don't take care to disable unsupported features when
writing back the backlight mode, which can lead to some rather strange
looking behavior when adjusting the backlight.

So, let's fix this by ensuring we only keep supported features enabled for
panel backlights - which should fix some of the issues we were seeing from
this on fi-bdw-samus.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: 867cf9cd73c3 ("drm/dp: Extract i915's eDP backlight code into DRM helpers")
---
 drivers/gpu/drm/drm_dp_helper.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index ada0a1ff262d..8f2032a955cf 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -3372,7 +3372,9 @@ int drm_edp_backlight_enable(struct drm_dp_aux *aux, const struct drm_edp_backli
 		return ret < 0 ? ret : -EIO;
 	}
 
-	new_dpcd_buf = dpcd_buf;
+	/* Disable any backlight functionality we don't support that might be on */
+	new_dpcd_buf = dpcd_buf & (DP_EDP_BACKLIGHT_CONTROL_MODE_MASK |
+				   DP_EDP_BACKLIGHT_FREQ_AUX_SET_ENABLE);
 
 	if ((dpcd_buf & DP_EDP_BACKLIGHT_CONTROL_MODE_MASK) != DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD) {
 		new_dpcd_buf &= ~DP_EDP_BACKLIGHT_CONTROL_MODE_MASK;
@@ -3394,6 +3396,8 @@ int drm_edp_backlight_enable(struct drm_dp_aux *aux, const struct drm_edp_backli
 				    aux->name, ret);
 		else
 			new_dpcd_buf |= DP_EDP_BACKLIGHT_FREQ_AUX_SET_ENABLE;
+	} else {
+		new_dpcd_buf &= ~DP_EDP_BACKLIGHT_FREQ_AUX_SET_ENABLE;
 	}
 
 	if (new_dpcd_buf != dpcd_buf) {
-- 
2.31.1

Re: [PATCH v4 3/5] drm/dp: Disable unsupported features in DP_EDP_BACKLIGHT_MODE_SET_REGISTER

[Doug Anderson]

Hi,

On Tue, Oct 26, 2021 at 3:09 PM Lyude Paul <lyude@redhat.com> wrote:
>
> As it turns out, apparently some machines will actually leave additional
> backlight functionality like dynamic backlight control on before the OS
> loads. Currently we don't take care to disable unsupported features when
> writing back the backlight mode, which can lead to some rather strange
> looking behavior when adjusting the backlight.
>
> So, let's fix this by ensuring we only keep supported features enabled for
> panel backlights - which should fix some of the issues we were seeing from
> this on fi-bdw-samus.
>
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> Fixes: 867cf9cd73c3 ("drm/dp: Extract i915's eDP backlight code into DRM helpers")
> ---
>  drivers/gpu/drm/drm_dp_helper.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
> index ada0a1ff262d..8f2032a955cf 100644
> --- a/drivers/gpu/drm/drm_dp_helper.c
> +++ b/drivers/gpu/drm/drm_dp_helper.c
> @@ -3372,7 +3372,9 @@ int drm_edp_backlight_enable(struct drm_dp_aux *aux, const struct drm_edp_backli
>                 return ret < 0 ? ret : -EIO;
>         }
>
> -       new_dpcd_buf = dpcd_buf;
> +       /* Disable any backlight functionality we don't support that might be on */
> +       new_dpcd_buf = dpcd_buf & (DP_EDP_BACKLIGHT_CONTROL_MODE_MASK |
> +                                  DP_EDP_BACKLIGHT_FREQ_AUX_SET_ENABLE);

My first thought when reading the above was: if we're masking so much
stuff out, why do we bother reading the old value back out at all?

I guess the two places you use the old value for are:

1. You avoid setting the "DP_EDP_PWMGEN_BIT_COUNT" if the backlight
was already configured for DPCD mode.

2. You avoid writing the register if you didn't change it.

I would actually argue that use #1 is probably a bug. If you're
worried about the firmware leaving the backlight configured in a
strange way, it could very well have left the backlight configured in
DPCD mode but set a different "bit count" than you want, right? Maybe
you should just always set the bit count?

Use #2 is fine, but does it buy you anything? Are writes to the DCPD
bus somehow more expensive than reads? ...or maybe you're expecting
that a display will glitch / act badly if you write the same value
that's already there?


So I guess my instinct here is that you should avoid reading all
together and just program the value you want.

-Doug

Re: [PATCH v4 3/5] drm/dp: Disable unsupported features in DP_EDP_BACKLIGHT_MODE_SET_REGISTER

[Lyude Paul]

On Thu, 2021-10-28 at 11:27 -0700, Doug Anderson wrote:
> Hi,
> 
> On Tue, Oct 26, 2021 at 3:09 PM Lyude Paul <lyude@redhat.com> wrote:
> > 
> > As it turns out, apparently some machines will actually leave additional
> > backlight functionality like dynamic backlight control on before the OS
> > loads. Currently we don't take care to disable unsupported features when
> > writing back the backlight mode, which can lead to some rather strange
> > looking behavior when adjusting the backlight.
> > 
> > So, let's fix this by ensuring we only keep supported features enabled for
> > panel backlights - which should fix some of the issues we were seeing from
> > this on fi-bdw-samus.
> > 
> > Signed-off-by: Lyude Paul <lyude@redhat.com>
> > Fixes: 867cf9cd73c3 ("drm/dp: Extract i915's eDP backlight code into DRM
> > helpers")
> > ---
> >  drivers/gpu/drm/drm_dp_helper.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_dp_helper.c
> > b/drivers/gpu/drm/drm_dp_helper.c
> > index ada0a1ff262d..8f2032a955cf 100644
> > --- a/drivers/gpu/drm/drm_dp_helper.c
> > +++ b/drivers/gpu/drm/drm_dp_helper.c
> > @@ -3372,7 +3372,9 @@ int drm_edp_backlight_enable(struct drm_dp_aux *aux,
> > const struct drm_edp_backli
> >                 return ret < 0 ? ret : -EIO;
> >         }
> > 
> > -       new_dpcd_buf = dpcd_buf;
> > +       /* Disable any backlight functionality we don't support that might
> > be on */
> > +       new_dpcd_buf = dpcd_buf & (DP_EDP_BACKLIGHT_CONTROL_MODE_MASK |
> > +                                  DP_EDP_BACKLIGHT_FREQ_AUX_SET_ENABLE);
> 
> My first thought when reading the above was: if we're masking so much
> stuff out, why do we bother reading the old value back out at all?
> 
> I guess the two places you use the old value for are:
> 
> 1. You avoid setting the "DP_EDP_PWMGEN_BIT_COUNT" if the backlight
> was already configured for DPCD mode.
> 
> 2. You avoid writing the register if you didn't change it.
> 
> I would actually argue that use #1 is probably a bug. If you're
> worried about the firmware leaving the backlight configured in a
> strange way, it could very well have left the backlight configured in
> DPCD mode but set a different "bit count" than you want, right? Maybe
> you should just always set the bit count?
> 
> Use #2 is fine, but does it buy you anything? Are writes to the DCPD
> bus somehow more expensive than reads? ...or maybe you're expecting
> that a display will glitch / act badly if you write the same value
> that's already there?
> 
> 
> So I guess my instinct here is that you should avoid reading all
> together and just program the value you want.

Good point, will respin this in a little bit

> 
> -Doug
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

[PATCH v4 4/5] drm/dp, drm/i915: Add support for VESA backlights using PWM for brightness control

[Lyude Paul]

Now that we've added support to i915 for controlling panel backlights that
need PWM to be enabled/disabled, let's finalize this and add support for
controlling brightness levels via PWM as well. This should hopefully put us
towards the path of supporting _ALL_ backlights via VESA's DPCD interface
which would allow us to finally start trusting the DPCD again.

Note however that we still don't enable using this by default on i915 when
it's not needed, primarily because I haven't yet had a chance to confirm if
it's safe to do this on the one machine in Intel's CI that had an issue
with this: samus-fi-bdw. I have done basic testing of this on other
machines though, by manually patching i915 to force it into PWM-only mode
on some of my laptops.

v2:
* Correct documentation (thanks Doug!)
* Get rid of backlight caps

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Cc: Rajeev Nandan <rajeevny@codeaurora.org>
Cc: Satadru Pramanik <satadru@gmail.com>
---
 drivers/gpu/drm/drm_dp_helper.c               | 76 +++++++++++++------
 .../drm/i915/display/intel_dp_aux_backlight.c | 44 ++++++++---
 include/drm/drm_dp_helper.h                   |  7 +-
 3 files changed, 91 insertions(+), 36 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index 8f2032a955cf..b91b092e10f0 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -3290,6 +3290,10 @@ int drm_edp_backlight_set_level(struct drm_dp_aux *aux, const struct drm_edp_bac
 	int ret;
 	u8 buf[2] = { 0 };
 
+	/* The panel uses the PWM for controlling brightness levels */
+	if (!bl->aux_set)
+		return 0;
+
 	if (bl->lsb_reg_used) {
 		buf[0] = (level & 0xff00) >> 8;
 		buf[1] = (level & 0x00ff);
@@ -3316,7 +3320,7 @@ drm_edp_backlight_set_enable(struct drm_dp_aux *aux, const struct drm_edp_backli
 	int ret;
 	u8 buf;
 
-	/* The panel uses something other then DPCD for enabling its backlight */
+	/* This panel uses the EDP_BL_PWR GPIO for enablement */
 	if (!bl->aux_enable)
 		return 0;
 
@@ -3351,11 +3355,11 @@ drm_edp_backlight_set_enable(struct drm_dp_aux *aux, const struct drm_edp_backli
  * restoring any important backlight state such as the given backlight level, the brightness byte
  * count, backlight frequency, etc.
  *
- * Note that certain panels, while supporting brightness level controls over DPCD, may not support
- * having their backlights enabled via the standard %DP_EDP_DISPLAY_CONTROL_REGISTER. On such panels
- * &drm_edp_backlight_info.aux_enable will be set to %false, this function will skip the step of
- * programming the %DP_EDP_DISPLAY_CONTROL_REGISTER, and the driver must perform the required
- * implementation specific step for enabling the backlight after calling this function.
+ * Note that certain panels do not support being enabled or disabled via DPCD, but instead require
+ * that the driver handle enabling/disabling the panel through implementation-specific means using
+ * the EDP_BL_PWR GPIO. For such panels, &drm_edp_backlight_info.aux_enable will be set to %false,
+ * this function becomes a no-op, and the driver is expected to handle powering the panel on using
+ * the EDP_BL_PWR GPIO.
  *
  * Returns: %0 on success, negative error code on failure.
  */
@@ -3363,7 +3367,7 @@ int drm_edp_backlight_enable(struct drm_dp_aux *aux, const struct drm_edp_backli
 			     const u16 level)
 {
 	int ret;
-	u8 dpcd_buf, new_dpcd_buf;
+	u8 dpcd_buf, new_dpcd_buf, new_mode;
 
 	ret = drm_dp_dpcd_readb(aux, DP_EDP_BACKLIGHT_MODE_SET_REGISTER, &dpcd_buf);
 	if (ret != 1) {
@@ -3376,9 +3380,14 @@ int drm_edp_backlight_enable(struct drm_dp_aux *aux, const struct drm_edp_backli
 	new_dpcd_buf = dpcd_buf & (DP_EDP_BACKLIGHT_CONTROL_MODE_MASK |
 				   DP_EDP_BACKLIGHT_FREQ_AUX_SET_ENABLE);
 
-	if ((dpcd_buf & DP_EDP_BACKLIGHT_CONTROL_MODE_MASK) != DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD) {
+	if (bl->aux_set)
+		new_mode = DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD;
+	else
+		new_mode = DP_EDP_BACKLIGHT_CONTROL_MODE_PWM;
+
+	if ((dpcd_buf & DP_EDP_BACKLIGHT_CONTROL_MODE_MASK) != new_mode) {
 		new_dpcd_buf &= ~DP_EDP_BACKLIGHT_CONTROL_MODE_MASK;
-		new_dpcd_buf |= DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD;
+		new_dpcd_buf |= new_mode;
 
 		if (bl->pwmgen_bit_count) {
 			ret = drm_dp_dpcd_writeb(aux, DP_EDP_PWMGEN_BIT_COUNT, bl->pwmgen_bit_count);
@@ -3425,12 +3434,13 @@ EXPORT_SYMBOL(drm_edp_backlight_enable);
  * @aux: The DP AUX channel to use
  * @bl: Backlight capability info from drm_edp_backlight_init()
  *
- * This function handles disabling DPCD backlight controls on a panel over AUX. Note that some
- * panels have backlights that are enabled/disabled by other means, despite having their brightness
- * values controlled through DPCD. On such panels &drm_edp_backlight_info.aux_enable will be set to
- * %false, this function will become a no-op (and we will skip updating
- * %DP_EDP_DISPLAY_CONTROL_REGISTER), and the driver must take care to perform it's own
- * implementation specific step for disabling the backlight.
+ * This function handles disabling DPCD backlight controls on a panel over AUX.
+ *
+ * Note that certain panels do not support being enabled or disabled via DPCD, but instead require
+ * that the driver handle enabling/disabling the panel through implementation-specific means using
+ * the EDP_BL_PWR GPIO. For such panels, &drm_edp_backlight_info.aux_enable will be set to %false,
+ * this function becomes a no-op, and the driver is expected to handle powering the panel off using
+ * the EDP_BL_PWR GPIO.
  *
  * Returns: %0 on success or no-op, negative error code on failure.
  */
@@ -3454,6 +3464,9 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
 	int ret;
 	u8 pn, pn_min, pn_max;
 
+	if (!bl->aux_set)
+		return 0;
+
 	ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT, &pn);
 	if (ret != 1) {
 		drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap: %d\n",
@@ -3539,7 +3552,7 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
 }
 
 static inline int
-drm_edp_backlight_probe_level(struct drm_dp_aux *aux, struct drm_edp_backlight_info *bl,
+drm_edp_backlight_probe_state(struct drm_dp_aux *aux, struct drm_edp_backlight_info *bl,
 			      u8 *current_mode)
 {
 	int ret;
@@ -3554,6 +3567,9 @@ drm_edp_backlight_probe_level(struct drm_dp_aux *aux, struct drm_edp_backlight_i
 	}
 
 	*current_mode = (mode_reg & DP_EDP_BACKLIGHT_CONTROL_MODE_MASK);
+	if (!bl->aux_set)
+		return 0;
+
 	if (*current_mode == DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD) {
 		int size = 1 + bl->lsb_reg_used;
 
@@ -3584,7 +3600,7 @@ drm_edp_backlight_probe_level(struct drm_dp_aux *aux, struct drm_edp_backlight_i
  * @bl: The &drm_edp_backlight_info struct to fill out with information on the backlight
  * @driver_pwm_freq_hz: Optional PWM frequency from the driver in hz
  * @edp_dpcd: A cached copy of the eDP DPCD
- * @current_level: Where to store the probed brightness level
+ * @current_level: Where to store the probed brightness level, if any
  * @current_mode: Where to store the currently set backlight control mode
  *
  * Initializes a &drm_edp_backlight_info struct by probing @aux for it's backlight capabilities,
@@ -3604,24 +3620,38 @@ drm_edp_backlight_init(struct drm_dp_aux *aux, struct drm_edp_backlight_info *bl
 
 	if (edp_dpcd[1] & DP_EDP_BACKLIGHT_AUX_ENABLE_CAP)
 		bl->aux_enable = true;
+	if (edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_AUX_SET_CAP)
+		bl->aux_set = true;
 	if (edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT)
 		bl->lsb_reg_used = true;
 
+	/* Sanity check caps */
+	if (!bl->aux_set && !(edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_PWM_PIN_CAP)) {
+		drm_dbg_kms(aux->drm_dev,
+			    "%s: Panel supports neither AUX or PWM brightness control? Aborting\n",
+			    aux->name);
+		return -EINVAL;
+	}
+
 	ret = drm_edp_backlight_probe_max(aux, bl, driver_pwm_freq_hz, edp_dpcd);
 	if (ret < 0)
 		return ret;
 
-	ret = drm_edp_backlight_probe_level(aux, bl, current_mode);
+	ret = drm_edp_backlight_probe_state(aux, bl, current_mode);
 	if (ret < 0)
 		return ret;
 	*current_level = ret;
 
 	drm_dbg_kms(aux->drm_dev,
-		    "%s: Found backlight level=%d/%d pwm_freq_pre_divider=%d mode=%x\n",
-		    aux->name, *current_level, bl->max, bl->pwm_freq_pre_divider, *current_mode);
-	drm_dbg_kms(aux->drm_dev,
-		    "%s: Backlight caps: pwmgen_bit_count=%d lsb_reg_used=%d aux_enable=%d\n",
-		    aux->name, bl->pwmgen_bit_count, bl->lsb_reg_used, bl->aux_enable);
+		    "%s: Found backlight: aux_set=%d aux_enable=%d mode=%d\n",
+		    aux->name, bl->aux_set, bl->aux_enable, *current_mode);
+	if (bl->aux_set) {
+		drm_dbg_kms(aux->drm_dev,
+			    "%s: Backlight caps: level=%d/%d pwm_freq_pre_divider=%d lsb_reg_used=%d\n",
+			    aux->name, *current_level, bl->max, bl->pwm_freq_pre_divider,
+			    bl->lsb_reg_used);
+	}
+
 	return 0;
 }
 EXPORT_SYMBOL(drm_edp_backlight_init);
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 f05b71c01b8e..96fe3eaba44a 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
@@ -282,6 +282,12 @@ intel_dp_aux_vesa_set_backlight(const struct drm_connector_state *conn_state, u3
 	struct intel_panel *panel = &connector->panel;
 	struct intel_dp *intel_dp = enc_to_intel_dp(connector->encoder);
 
+	if (!panel->backlight.edp.vesa.info.aux_set) {
+		const u32 pwm_level = intel_backlight_level_to_pwm(connector, level);
+
+		intel_backlight_set_pwm_level(conn_state, pwm_level);
+	}
+
 	drm_edp_backlight_set_level(&intel_dp->aux, &panel->backlight.edp.vesa.info, level);
 }
 
@@ -294,8 +300,13 @@ intel_dp_aux_vesa_enable_backlight(const struct intel_crtc_state *crtc_state,
 	struct intel_dp *intel_dp = enc_to_intel_dp(connector->encoder);
 
 	if (!panel->backlight.edp.vesa.info.aux_enable) {
-		u32 pwm_level = intel_backlight_invert_pwm_level(connector,
-								 panel->backlight.pwm_level_max);
+		u32 pwm_level;
+
+		if (!panel->backlight.edp.vesa.info.aux_set)
+			pwm_level = intel_backlight_level_to_pwm(connector, level);
+		else
+			pwm_level = intel_backlight_invert_pwm_level(connector,
+								     panel->backlight.pwm_level_max);
 
 		panel->backlight.pwm_funcs->enable(crtc_state, conn_state, pwm_level);
 	}
@@ -332,7 +343,7 @@ static int intel_dp_aux_vesa_setup_backlight(struct intel_connector *connector,
 	if (ret < 0)
 		return ret;
 
-	if (!panel->backlight.edp.vesa.info.aux_enable) {
+	if (!panel->backlight.edp.vesa.info.aux_set || !panel->backlight.edp.vesa.info.aux_enable) {
 		ret = panel->backlight.pwm_funcs->setup(connector, pipe);
 		if (ret < 0) {
 			drm_err(&i915->drm,
@@ -341,14 +352,27 @@ static int intel_dp_aux_vesa_setup_backlight(struct intel_connector *connector,
 			return ret;
 		}
 	}
-	panel->backlight.max = panel->backlight.edp.vesa.info.max;
-	panel->backlight.min = 0;
-	if (current_mode == DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD) {
-		panel->backlight.level = current_level;
-		panel->backlight.enabled = panel->backlight.level != 0;
+
+	if (panel->backlight.edp.vesa.info.aux_set) {
+		panel->backlight.max = panel->backlight.edp.vesa.info.max;
+		panel->backlight.min = 0;
+		if (current_mode == DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD) {
+			panel->backlight.level = current_level;
+			panel->backlight.enabled = panel->backlight.level != 0;
+		} else {
+			panel->backlight.level = panel->backlight.max;
+			panel->backlight.enabled = false;
+		}
 	} else {
-		panel->backlight.level = panel->backlight.max;
-		panel->backlight.enabled = false;
+		panel->backlight.max = panel->backlight.pwm_level_max;
+		panel->backlight.min = panel->backlight.pwm_level_min;
+		if (current_mode == DP_EDP_BACKLIGHT_CONTROL_MODE_PWM) {
+			panel->backlight.level = panel->backlight.pwm_funcs->get(connector, pipe);
+			panel->backlight.enabled = panel->backlight.pwm_enabled;
+		} else {
+			panel->backlight.level = panel->backlight.max;
+			panel->backlight.enabled = false;
+		}
 	}
 
 	return 0;
diff --git a/include/drm/drm_dp_helper.h b/include/drm/drm_dp_helper.h
index afdf7f4183f9..8b2ed4199284 100644
--- a/include/drm/drm_dp_helper.h
+++ b/include/drm/drm_dp_helper.h
@@ -1868,7 +1868,7 @@ drm_dp_sink_can_do_video_without_timing_msa(const u8 dpcd[DP_RECEIVER_CAP_SIZE])
  *
  * Note that currently this function will return %false for panels which support various DPCD
  * backlight features but which require the brightness be set through PWM, and don't support setting
- * the brightness level via the DPCD. This is a TODO.
+ * the brightness level via the DPCD.
  *
  * Returns: %True if @edp_dpcd indicates that VESA backlight controls are supported, %false
  * otherwise
@@ -1876,8 +1876,7 @@ drm_dp_sink_can_do_video_without_timing_msa(const u8 dpcd[DP_RECEIVER_CAP_SIZE])
 static inline bool
 drm_edp_backlight_supported(const u8 edp_dpcd[EDP_DISPLAY_CTL_CAP_SIZE])
 {
-	return (edp_dpcd[1] & DP_EDP_TCON_BACKLIGHT_ADJUSTMENT_CAP) &&
-		(edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_AUX_SET_CAP);
+	return !!(edp_dpcd[1] & DP_EDP_TCON_BACKLIGHT_ADJUSTMENT_CAP);
 }
 
 /*
@@ -2238,6 +2237,7 @@ drm_dp_has_quirk(const struct drm_dp_desc *desc, enum drm_dp_quirk quirk)
  * @max: The maximum backlight level that may be set
  * @lsb_reg_used: Do we also write values to the DP_EDP_BACKLIGHT_BRIGHTNESS_LSB register?
  * @aux_enable: Does the panel support the AUX enable cap?
+ * @aux_set: Does the panel support setting the brightness through AUX?
  *
  * This structure contains various data about an eDP backlight, which can be populated by using
  * drm_edp_backlight_init().
@@ -2249,6 +2249,7 @@ struct drm_edp_backlight_info {
 
 	bool lsb_reg_used : 1;
 	bool aux_enable : 1;
+	bool aux_set : 1;
 };
 
 int
-- 
2.31.1

[PATCH v4 5/5] drm/i915: Clarify probing order in intel_dp_aux_init_backlight_funcs()

[Lyude Paul]

Hooray! We've managed to hit enough bugs upstream that I've been able to
come up with a pretty solid explanation for how backlight controls are
actually supposed to be detected and used these days. As well, having the
rest of the PWM bits in VESA's backlight interface implemented seems to
have fixed all of the problematic brightness controls laptop panels that
we've hit so far.

So, let's actually document this instead of just calling the laptop panels
liars. As well, I would like to formally apologize to all of the laptop
panels I called liars. I'm sorry laptop panels, hopefully you can all
forgive me and we can move past this~

Signed-off-by: Lyude Paul <lyude@redhat.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
---
 .../drm/i915/display/intel_dp_aux_backlight.c    | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

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 96fe3eaba44a..8b9c925c4c16 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
@@ -456,11 +456,17 @@ int intel_dp_aux_init_backlight_funcs(struct intel_connector *connector)
 	}
 
 	/*
-	 * A lot of eDP panels in the wild will report supporting both the
-	 * Intel proprietary backlight control interface, and the VESA
-	 * backlight control interface. Many of these panels are liars though,
-	 * and will only work with the Intel interface. So, always probe for
-	 * that first.
+	 * Since Intel has their own backlight control interface, the majority of machines out there
+	 * using DPCD backlight controls with Intel GPUs will be using this interface as opposed to
+	 * the VESA interface. However, other GPUs (such as Nvidia's) will always use the VESA
+	 * interface. This means that there's quite a number of panels out there that will advertise
+	 * support for both interfaces, primarily systems with Intel/Nvidia hybrid GPU setups.
+	 *
+	 * There's a catch to this though: on many panels that advertise support for both
+	 * interfaces, the VESA backlight interface will stop working once we've programmed the
+	 * panel with Intel's OUI - which is also required for us to be able to detect Intel's
+	 * backlight interface at all. This means that the only sensible way for us to detect both
+	 * interfaces is to probe for Intel's first, and VESA's second.
 	 */
 	if (try_intel_interface && intel_dp_aux_supports_hdr_backlight(connector)) {
 		drm_dbg_kms(dev, "Using Intel proprietary eDP backlight controls\n");
-- 
2.31.1