Nouveau - Nov 2021 - [PATCH v5 0/5] drm/dp, drm/i915: Finish basic PWM support for VESA backlight helpers

When I originally moved all of the VESA backlight code in i915 into DRM
helpers, one of the things I didn't have the hardware or time for
testing was machines that used a combination of PWM and DPCD in order to
control their backlights. This has since then caused some breakages and
resulted in us disabling DPCD backlight support on such machines. This
works fine, unless you have a machine that actually needs this
functionality for backlight controls to work at all. Additionally, we
will need to support PWM for when we start adding support for VESA's
product (as in the product of multiplication) control mode for better
brightness ranges.

So - let's finally finish up implementing basic support for these types
of backlights to solve these problems in our DP helpers, along with
implementing support for this in i915. And since digging into this issue
solved the last questions we really had about probing backlights in i915
for the most part, let's update some of the comments around that as
well!

Lyude Paul (5):
  drm/i915: Add support for panels with VESA backlights with PWM
    enable/disable
  drm/nouveau/kms/nv50-: Explicitly check DPCD backlights for aux
    enable/brightness
  drm/dp: Don't read back backlight mode in drm_edp_backlight_enable()
  drm/dp, drm/i915: Add support for VESA backlights using PWM for
    brightness control
  drm/i915: Clarify probing order in intel_dp_aux_init_backlight_funcs()

 drivers/gpu/drm/drm_dp_helper.c               | 108 ++++++++++--------
 .../drm/i915/display/intel_dp_aux_backlight.c |  81 ++++++++++---
 drivers/gpu/drm/nouveau/nouveau_backlight.c   |   5 +-
 include/drm/drm_dp_helper.h                   |   7 +-
 4 files changed, 132 insertions(+), 69 deletions(-)

-- 
2.31.1

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 at 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")
Reviewed-by: Ville Syrj?l? <ville.syrjala at linux.intel.com>
Cc: <stable at 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

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.

Reviewed-by: Karol Herbst <kherbst at redhat.com>
Signed-off-by: Lyude Paul <lyude at 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

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 just not reading back the current backlight mode on
initial enable. I don't think there should really be any downsides to this,
and this will ensure we don't leave any unsupported functionality enabled.

This should fix at least one (but not all) of the issues seen with DPCD
backlight support on fi-bdw-samus

v5:
* Just avoid reading back DPCD register - Doug Anderson

Signed-off-by: Lyude Paul <lyude at redhat.com>
Fixes: 867cf9cd73c3 ("drm/dp: Extract i915's eDP backlight code into
DRM helpers")
---
 drivers/gpu/drm/drm_dp_helper.c | 40 ++++++++++-----------------------
 1 file changed, 12 insertions(+), 28 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index ada0a1ff262d..af2aad2f4725 100644
--- a/drivers/gpu/drm/drm_dp_helper.c
+++ b/drivers/gpu/drm/drm_dp_helper.c
@@ -3363,27 +3363,13 @@ 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 = DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD;
 
-	ret = drm_dp_dpcd_readb(aux, DP_EDP_BACKLIGHT_MODE_SET_REGISTER,
&dpcd_buf);
-	if (ret != 1) {
-		drm_dbg_kms(aux->drm_dev,
-			    "%s: Failed to read backlight mode: %d\n", aux->name, ret);
-		return ret < 0 ? ret : -EIO;
-	}
-
-	new_dpcd_buf = dpcd_buf;
-
-	if ((dpcd_buf & DP_EDP_BACKLIGHT_CONTROL_MODE_MASK) !=
DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD) {
-		new_dpcd_buf &= ~DP_EDP_BACKLIGHT_CONTROL_MODE_MASK;
-		new_dpcd_buf |= DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD;
-
-		if (bl->pwmgen_bit_count) {
-			ret = drm_dp_dpcd_writeb(aux, DP_EDP_PWMGEN_BIT_COUNT,
bl->pwmgen_bit_count);
-			if (ret != 1)
-				drm_dbg_kms(aux->drm_dev, "%s: Failed to write aux pwmgen bit
count: %d\n",
-					    aux->name, ret);
-		}
+	if (bl->pwmgen_bit_count) {
+		ret = drm_dp_dpcd_writeb(aux, DP_EDP_PWMGEN_BIT_COUNT,
bl->pwmgen_bit_count);
+		if (ret != 1)
+			drm_dbg_kms(aux->drm_dev, "%s: Failed to write aux pwmgen bit count:
%d\n",
+				    aux->name, ret);
 	}
 
 	if (bl->pwm_freq_pre_divider) {
@@ -3393,16 +3379,14 @@ int drm_edp_backlight_enable(struct drm_dp_aux *aux,
const struct drm_edp_backli
 				    "%s: Failed to write aux backlight frequency: %d\n",
 				    aux->name, ret);
 		else
-			new_dpcd_buf |= DP_EDP_BACKLIGHT_FREQ_AUX_SET_ENABLE;
+			dpcd_buf |= DP_EDP_BACKLIGHT_FREQ_AUX_SET_ENABLE;
 	}
 
-	if (new_dpcd_buf != dpcd_buf) {
-		ret = drm_dp_dpcd_writeb(aux, DP_EDP_BACKLIGHT_MODE_SET_REGISTER,
new_dpcd_buf);
-		if (ret != 1) {
-			drm_dbg_kms(aux->drm_dev, "%s: Failed to write aux backlight mode:
%d\n",
-				    aux->name, ret);
-			return ret < 0 ? ret : -EIO;
-		}
+	ret = drm_dp_dpcd_writeb(aux, DP_EDP_BACKLIGHT_MODE_SET_REGISTER, dpcd_buf);
+	if (ret != 1) {
+		drm_dbg_kms(aux->drm_dev, "%s: Failed to write aux backlight mode:
%d\n",
+			    aux->name, ret);
+		return ret < 0 ? ret : -EIO;
 	}
 
 	ret = drm_edp_backlight_set_level(aux, bl, level);
-- 
2.31.1

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 at redhat.com>
Reviewed-by: Doug Anderson <dianders at chromium.org>
Cc: Rajeev Nandan <rajeevny at codeaurora.org>
Cc: Satadru Pramanik <satadru at gmail.com>
---
 drivers/gpu/drm/drm_dp_helper.c               | 72 +++++++++++++------
 .../drm/i915/display/intel_dp_aux_backlight.c | 44 +++++++++---
 include/drm/drm_dp_helper.h                   |  7 +-
 3 files changed, 89 insertions(+), 34 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
index af2aad2f4725..23f9073bc473 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 +3367,12 @@ int drm_edp_backlight_enable(struct drm_dp_aux *aux,
const struct drm_edp_backli
 			     const u16 level)
 {
 	int ret;
-	u8 dpcd_buf = DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD;
+	u8 dpcd_buf;
+
+	if (bl->aux_set)
+		dpcd_buf = DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD;
+	else
+		dpcd_buf = DP_EDP_BACKLIGHT_CONTROL_MODE_PWM;
 
 	if (bl->pwmgen_bit_count) {
 		ret = drm_dp_dpcd_writeb(aux, DP_EDP_PWMGEN_BIT_COUNT,
bl->pwmgen_bit_count);
@@ -3405,12 +3414,13 @@ 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.
  */
@@ -3434,6 +3444,9 @@ 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",
@@ -3519,7 +3532,7 @@ 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;
@@ -3534,6 +3547,9 @@ 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;
 
@@ -3564,7 +3580,7 @@ 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,
@@ -3584,24 +3600,38 @@ 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

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 at redhat.com>
Acked-by: Ville Syrj?l? <ville.syrjala at 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