Re: [PATCH 1/5] drm/i915: Fix eDP DPCD aux max backlight calculations

Re: [PATCH 1/5] drm/i915: Fix eDP DPCD aux max backlight calculations

[AceLan Kao]

This series of patches works well on a new platform which has a panel
only controls by DPCD aux brightness.

Tested-by: AceLan Kao <acelan.kao@canonical.com>

-- 
Chia-Lin Kao(AceLan)
http://blog.acelan.idv.tw/
E-Mail: acelan.kaoATcanonical.com (s/AT/@/)
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Re: [PATCH 1/5] drm/i915: Fix eDP DPCD aux max backlight calculations

[Jani Nikula]

On Thu, 12 Dec 2019, AceLan Kao <acelan@gmail.com> wrote:
> This series of patches works well on a new platform which has a panel
> only controls by DPCD aux brightness.
>
> Tested-by: AceLan Kao <acelan.kao@canonical.com>

Please fix your mail client to actually reply to the thread, and retain
all the recipients. As it is, your replies are disconnected from the
actual patches.

BR,
Jani.


-- 
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Re: [PATCH 1/5] drm/i915: Fix eDP DPCD aux max backlight calculations

[AceLan Kao]

I tried to reply the patch from the mailing list archive by clicking
the author name
https://lists.freedesktop.org/archives/dri-devel/2019-November/246278.html

mailto:dri-devel%40lists.freedesktop.org?Subject=Re%3A%20%5BPATCH%201/5%5D%20drm/i915%3A%20Fix%20eDP%20DPCD%20aux%20max%20backlight%20calculations&In-Reply-To=%3C20191122231616.2574-2-lyude%40redhat.com%3E

It looks like it contains the subject and in-reply-to, I thought it
should reply to the correct thread.
Other patches in this series seem work, but this one doesn't.

Jani Nikula <jani.nikula@linux.intel.com> 於 2019年12月13日 週五 下午11:22寫道：

>
> On Thu, 12 Dec 2019, AceLan Kao <acelan@gmail.com> wrote:
> > This series of patches works well on a new platform which has a panel
> > only controls by DPCD aux brightness.
> >
> > Tested-by: AceLan Kao <acelan.kao@canonical.com>
>
> Please fix your mail client to actually reply to the thread, and retain
> all the recipients. As it is, your replies are disconnected from the
> actual patches.
>
> BR,
> Jani.
>
>
> --
> Jani Nikula, Intel Open Source Graphics Center



--
Chia-Lin Kao(AceLan)
http://blog.acelan.idv.tw/
E-Mail: acelan.kaoATcanonical.com (s/AT/@/)
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Re: [PATCH 1/5] drm/i915: Fix eDP DPCD aux max backlight calculations

[Jani Nikula]

On Mon, 16 Dec 2019, AceLan Kao <acelan@gmail.com> wrote:
> I tried to reply the patch from the mailing list archive by clicking
> the author name
> https://lists.freedesktop.org/archives/dri-devel/2019-November/246278.html
>
> mailto:dri-devel%40lists.freedesktop.org?Subject=Re%3A%20%5BPATCH%201/5%5D%20drm/i915%3A%20Fix%20eDP%20DPCD%20aux%20max%20backlight%20calculations&In-Reply-To=%3C20191122231616.2574-2-lyude%40redhat.com%3E
>
> It looks like it contains the subject and in-reply-to, I thought it
> should reply to the correct thread.
> Other patches in this series seem work, but this one doesn't.

FWIW none of the replies had In-reply-to: header in place.

BR,
Jani.


-- 
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[PATCH 0/5] drm/i915: eDP DPCD aux backlight fixes

[Lyude Paul]

I recently got a ThinkPad X1 Extreme 2nd Generation for fixing some
issues on, and noticed that out of the box the backlight doesn't work.
Along the way of fixing that, I found a few issues with how i915 handles
DPCD AUX backlight control and fixed them. Now I've got working
backlight controls, hooray!

Note that this patch series also enables DPCD aux backlight control by
default based on what the VBT tells us.

Lyude Paul (5):
  drm/i915: Fix eDP DPCD aux max backlight calculations
  drm/i915: Assume 100% brightness when not in DPCD control mode
  drm/i915: Fix DPCD register order in intel_dp_aux_enable_backlight()
  drm/i915: Auto detect DPCD backlight support by default
  drm/i915: Force DPCD backlight mode on X1 Extreme 2nd Gen 4K AMOLED
    panel

 drivers/gpu/drm/drm_dp_helper.c               |   4 +
 .../drm/i915/display/intel_display_types.h    |   3 +
 .../drm/i915/display/intel_dp_aux_backlight.c | 167 ++++++++++++------
 drivers/gpu/drm/i915/i915_params.c            |   2 +-
 drivers/gpu/drm/i915/i915_params.h            |   2 +-
 include/drm/drm_dp_helper.h                   |   8 +
 6 files changed, 134 insertions(+), 52 deletions(-)

-- 
2.21.0

[PATCH 1/5] drm/i915: Fix eDP DPCD aux max backlight calculations

[Lyude Paul]

Max backlight value for the panel was being calculated using byte
count i.e. 0xffff if 2 bytes are supported for backlight brightness
and 0xff if 1 byte is supported. However, EDP_PWMGEN_BIT_COUNT
determines the number of active control bits used for the brightness
setting. Thus, even if the panel uses 2 byte setting, it might not use
all the control bits. Thus, max backlight should be set based on the
value of EDP_PWMGEN_BIT_COUNT instead of assuming 65535 or 255.

Additionally, EDP_PWMGEN_BIT_COUNT was being updated based on the VBT
frequency which results in a different max backlight value. Thus,
setting of EDP_PWMGEN_BIT_COUNT is moved to setup phase instead of
enable so that max backlight can be calculated correctly. Only the
frequency divider is set during the enable phase using the value of
EDP_PWMGEN_BIT_COUNT.

This is based off the original patch series from Furquan Shaikh
<furquan@google.com>:

https://patchwork.freedesktop.org/patch/317255/?series=62326&rev=3

Changes since original patch:
* Remove unused intel_dp variable in intel_dp_aux_setup_backlight()
* Fix checkpatch issues
* Make sure that we rewrite the pwmgen bit count whenever we bring the
  panel out of D3 mode

Cc: Furquan Shaikh <furquan@google.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
---
 .../drm/i915/display/intel_display_types.h    |   3 +
 .../drm/i915/display/intel_dp_aux_backlight.c | 139 ++++++++++++------
 2 files changed, 95 insertions(+), 47 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_display_types.h b/drivers/gpu/drm/i915/display/intel_display_types.h
index 83ea04149b77..2a8d8cae638e 100644
--- a/drivers/gpu/drm/i915/display/intel_display_types.h
+++ b/drivers/gpu/drm/i915/display/intel_display_types.h
@@ -214,6 +214,9 @@ struct intel_panel {
 		u8 controller;		/* bxt+ only */
 		struct pwm_device *pwm;
 
+		/* DPCD backlight */
+		u8 pwmgen_bit_count;
+
 		struct backlight_device *device;
 
 		/* Connector and platform specific backlight functions */
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 020422da2ae2..fad470553cf9 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
@@ -111,61 +111,28 @@ static bool intel_dp_aux_set_pwm_freq(struct intel_connector *connector)
 {
 	struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
 	struct intel_dp *intel_dp = enc_to_intel_dp(&connector->encoder->base);
-	int freq, fxp, fxp_min, fxp_max, fxp_actual, f = 1;
-	u8 pn, pn_min, pn_max;
+	const u8 pn = connector->panel.backlight.pwmgen_bit_count;
+	int freq, fxp, f, fxp_actual, fxp_min, fxp_max;
 
-	/* Find desired value of (F x P)
-	 * Note that, if F x P is out of supported range, the maximum value or
-	 * minimum value will applied automatically. So no need to check that.
-	 */
 	freq = dev_priv->vbt.backlight.pwm_freq_hz;
-	DRM_DEBUG_KMS("VBT defined backlight frequency %u Hz\n", freq);
 	if (!freq) {
 		DRM_DEBUG_KMS("Use panel default backlight frequency\n");
 		return false;
 	}
 
 	fxp = DIV_ROUND_CLOSEST(KHz(DP_EDP_BACKLIGHT_FREQ_BASE_KHZ), freq);
+	f = clamp(DIV_ROUND_CLOSEST(fxp, 1 << pn), 1, 255);
+	fxp_actual = f << pn;
 
-	/* Use highest possible value of Pn for more granularity of brightness
-	 * adjustment while satifying the conditions below.
-	 * - Pn is in the range of Pn_min and Pn_max
-	 * - F is in the range of 1 and 255
-	 * - FxP is within 25% of desired value.
-	 *   Note: 25% is arbitrary value and may need some tweak.
-	 */
-	if (drm_dp_dpcd_readb(&intel_dp->aux,
-			       DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &pn_min) != 1) {
-		DRM_DEBUG_KMS("Failed to read pwmgen bit count cap min\n");
-		return false;
-	}
-	if (drm_dp_dpcd_readb(&intel_dp->aux,
-			       DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &pn_max) != 1) {
-		DRM_DEBUG_KMS("Failed to read pwmgen bit count cap max\n");
-		return false;
-	}
-	pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
-	pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
-
+	/* Ensure frequency is within 25% of desired value */
 	fxp_min = DIV_ROUND_CLOSEST(fxp * 3, 4);
 	fxp_max = DIV_ROUND_CLOSEST(fxp * 5, 4);
-	if (fxp_min < (1 << pn_min) || (255 << pn_max) < fxp_max) {
-		DRM_DEBUG_KMS("VBT defined backlight frequency out of range\n");
-		return false;
-	}
-
-	for (pn = pn_max; pn >= pn_min; pn--) {
-		f = clamp(DIV_ROUND_CLOSEST(fxp, 1 << pn), 1, 255);
-		fxp_actual = f << pn;
-		if (fxp_min <= fxp_actual && fxp_actual <= fxp_max)
-			break;
-	}
 
-	if (drm_dp_dpcd_writeb(&intel_dp->aux,
-			       DP_EDP_PWMGEN_BIT_COUNT, pn) < 0) {
-		DRM_DEBUG_KMS("Failed to write aux pwmgen bit count\n");
+	if (fxp_min > fxp_actual || fxp_actual > fxp_max) {
+		DRM_DEBUG_KMS("Actual frequency out of range\n");
 		return false;
 	}
+
 	if (drm_dp_dpcd_writeb(&intel_dp->aux,
 			       DP_EDP_BACKLIGHT_FREQ_SET, (u8) f) < 0) {
 		DRM_DEBUG_KMS("Failed to write aux backlight freq\n");
@@ -178,6 +145,7 @@ static void intel_dp_aux_enable_backlight(const struct intel_crtc_state *crtc_st
 					  const struct drm_connector_state *conn_state)
 {
 	struct intel_connector *connector = to_intel_connector(conn_state->connector);
+	struct intel_panel *panel = &connector->panel;
 	struct intel_dp *intel_dp = enc_to_intel_dp(&connector->encoder->base);
 	u8 dpcd_buf, new_dpcd_buf, edp_backlight_mode;
 
@@ -197,6 +165,12 @@ static void intel_dp_aux_enable_backlight(const struct intel_crtc_state *crtc_st
 	case DP_EDP_BACKLIGHT_CONTROL_MODE_PRODUCT:
 		new_dpcd_buf &= ~DP_EDP_BACKLIGHT_CONTROL_MODE_MASK;
 		new_dpcd_buf |= DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD;
+
+		if (drm_dp_dpcd_writeb(&intel_dp->aux,
+				       DP_EDP_PWMGEN_BIT_COUNT,
+				       panel->backlight.pwmgen_bit_count) < 0)
+			DRM_DEBUG_KMS("Failed to write aux pwmgen bit count\n");
+
 		break;
 
 	/* Do nothing when it is already DPCD mode */
@@ -225,20 +199,91 @@ static void intel_dp_aux_disable_backlight(const struct drm_connector_state *old
 	set_aux_backlight_enable(enc_to_intel_dp(old_conn_state->best_encoder), false);
 }
 
+static u32 intel_dp_aux_calc_max_backlight(struct intel_connector *connector)
+{
+	struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
+	struct intel_dp *intel_dp = enc_to_intel_dp(&connector->encoder->base);
+	struct intel_panel *panel = &connector->panel;
+	u32 max_backlight = 0;
+	int freq, fxp, fxp_min, fxp_max, fxp_actual, f = 1;
+	u8 pn, pn_min, pn_max;
+
+	if (drm_dp_dpcd_readb(&intel_dp->aux, DP_EDP_PWMGEN_BIT_COUNT, &pn)) {
+		pn &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+		max_backlight = (1 << pn) - 1;
+	}
+
+	/* Find desired value of (F x P)
+	 * Note that, if F x P is out of supported range, the maximum value or
+	 * minimum value will applied automatically. So no need to check that.
+	 */
+	freq = dev_priv->vbt.backlight.pwm_freq_hz;
+	DRM_DEBUG_KMS("VBT defined backlight frequency %u Hz\n", freq);
+	if (!freq) {
+		DRM_DEBUG_KMS("Use panel default backlight frequency\n");
+		return max_backlight;
+	}
+
+	fxp = DIV_ROUND_CLOSEST(KHz(DP_EDP_BACKLIGHT_FREQ_BASE_KHZ), freq);
+
+	/* Use highest possible value of Pn for more granularity of brightness
+	 * adjustment while satifying the conditions below.
+	 * - Pn is in the range of Pn_min and Pn_max
+	 * - F is in the range of 1 and 255
+	 * - FxP is within 25% of desired value.
+	 *   Note: 25% is arbitrary value and may need some tweak.
+	 */
+	if (drm_dp_dpcd_readb(&intel_dp->aux,
+			      DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &pn_min) != 1) {
+		DRM_DEBUG_KMS("Failed to read pwmgen bit count cap min\n");
+		return max_backlight;
+	}
+	if (drm_dp_dpcd_readb(&intel_dp->aux,
+			      DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &pn_max) != 1) {
+		DRM_DEBUG_KMS("Failed to read pwmgen bit count cap max\n");
+		return max_backlight;
+	}
+	pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+	pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+
+	fxp_min = DIV_ROUND_CLOSEST(fxp * 3, 4);
+	fxp_max = DIV_ROUND_CLOSEST(fxp * 5, 4);
+	if (fxp_min < (1 << pn_min) || (255 << pn_max) < fxp_max) {
+		DRM_DEBUG_KMS("VBT defined backlight frequency out of range\n");
+		return max_backlight;
+	}
+
+	for (pn = pn_max; pn >= pn_min; pn--) {
+		f = clamp(DIV_ROUND_CLOSEST(fxp, 1 << pn), 1, 255);
+		fxp_actual = f << pn;
+		if (fxp_min <= fxp_actual && fxp_actual <= fxp_max)
+			break;
+	}
+
+	DRM_DEBUG_KMS("Using eDP pwmgen bit count of %d\n", pn);
+	if (drm_dp_dpcd_writeb(&intel_dp->aux,
+			       DP_EDP_PWMGEN_BIT_COUNT, pn) < 0) {
+		DRM_DEBUG_KMS("Failed to write aux pwmgen bit count\n");
+		return max_backlight;
+	}
+	panel->backlight.pwmgen_bit_count = pn;
+
+	max_backlight = (1 << pn) - 1;
+
+	return max_backlight;
+}
+
 static int intel_dp_aux_setup_backlight(struct intel_connector *connector,
 					enum pipe pipe)
 {
-	struct intel_dp *intel_dp = enc_to_intel_dp(&connector->encoder->base);
 	struct intel_panel *panel = &connector->panel;
 
-	if (intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT)
-		panel->backlight.max = 0xFFFF;
-	else
-		panel->backlight.max = 0xFF;
+	panel->backlight.max = intel_dp_aux_calc_max_backlight(connector);
+	if (!panel->backlight.max)
+		return -ENODEV;
 
 	panel->backlight.min = 0;
 	panel->backlight.level = intel_dp_aux_get_backlight(connector);
-
 	panel->backlight.enabled = panel->backlight.level != 0;
 
 	return 0;
-- 
2.21.0

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[PATCH 1/5] drm/i915: Fix eDP DPCD aux max backlight calculations

[Lyude Paul]

Max backlight value for the panel was being calculated using byte
count i.e. 0xffff if 2 bytes are supported for backlight brightness
and 0xff if 1 byte is supported. However, EDP_PWMGEN_BIT_COUNT
determines the number of active control bits used for the brightness
setting. Thus, even if the panel uses 2 byte setting, it might not use
all the control bits. Thus, max backlight should be set based on the
value of EDP_PWMGEN_BIT_COUNT instead of assuming 65535 or 255.

Additionally, EDP_PWMGEN_BIT_COUNT was being updated based on the VBT
frequency which results in a different max backlight value. Thus,
setting of EDP_PWMGEN_BIT_COUNT is moved to setup phase instead of
enable so that max backlight can be calculated correctly. Only the
frequency divider is set during the enable phase using the value of
EDP_PWMGEN_BIT_COUNT.

This is based off the original patch series from Furquan Shaikh
<furquan@google.com>:

https://patchwork.freedesktop.org/patch/317255/?series=62326&rev=3

Changes since original patch:
* Remove unused intel_dp variable in intel_dp_aux_setup_backlight()
* Fix checkpatch issues
* Make sure that we rewrite the pwmgen bit count whenever we bring the
  panel out of D3 mode

Cc: Furquan Shaikh <furquan@google.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
---
 .../drm/i915/display/intel_display_types.h    |   3 +
 .../drm/i915/display/intel_dp_aux_backlight.c | 139 ++++++++++++------
 2 files changed, 95 insertions(+), 47 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_display_types.h b/drivers/gpu/drm/i915/display/intel_display_types.h
index 83ea04149b77..2a8d8cae638e 100644
--- a/drivers/gpu/drm/i915/display/intel_display_types.h
+++ b/drivers/gpu/drm/i915/display/intel_display_types.h
@@ -214,6 +214,9 @@ struct intel_panel {
 		u8 controller;		/* bxt+ only */
 		struct pwm_device *pwm;
 
+		/* DPCD backlight */
+		u8 pwmgen_bit_count;
+
 		struct backlight_device *device;
 
 		/* Connector and platform specific backlight functions */
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 020422da2ae2..fad470553cf9 100644
--- a/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
+++ b/drivers/gpu/drm/i915/display/intel_dp_aux_backlight.c
@@ -111,61 +111,28 @@ static bool intel_dp_aux_set_pwm_freq(struct intel_connector *connector)
 {
 	struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
 	struct intel_dp *intel_dp = enc_to_intel_dp(&connector->encoder->base);
-	int freq, fxp, fxp_min, fxp_max, fxp_actual, f = 1;
-	u8 pn, pn_min, pn_max;
+	const u8 pn = connector->panel.backlight.pwmgen_bit_count;
+	int freq, fxp, f, fxp_actual, fxp_min, fxp_max;
 
-	/* Find desired value of (F x P)
-	 * Note that, if F x P is out of supported range, the maximum value or
-	 * minimum value will applied automatically. So no need to check that.
-	 */
 	freq = dev_priv->vbt.backlight.pwm_freq_hz;
-	DRM_DEBUG_KMS("VBT defined backlight frequency %u Hz\n", freq);
 	if (!freq) {
 		DRM_DEBUG_KMS("Use panel default backlight frequency\n");
 		return false;
 	}
 
 	fxp = DIV_ROUND_CLOSEST(KHz(DP_EDP_BACKLIGHT_FREQ_BASE_KHZ), freq);
+	f = clamp(DIV_ROUND_CLOSEST(fxp, 1 << pn), 1, 255);
+	fxp_actual = f << pn;
 
-	/* Use highest possible value of Pn for more granularity of brightness
-	 * adjustment while satifying the conditions below.
-	 * - Pn is in the range of Pn_min and Pn_max
-	 * - F is in the range of 1 and 255
-	 * - FxP is within 25% of desired value.
-	 *   Note: 25% is arbitrary value and may need some tweak.
-	 */
-	if (drm_dp_dpcd_readb(&intel_dp->aux,
-			       DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &pn_min) != 1) {
-		DRM_DEBUG_KMS("Failed to read pwmgen bit count cap min\n");
-		return false;
-	}
-	if (drm_dp_dpcd_readb(&intel_dp->aux,
-			       DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &pn_max) != 1) {
-		DRM_DEBUG_KMS("Failed to read pwmgen bit count cap max\n");
-		return false;
-	}
-	pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
-	pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
-
+	/* Ensure frequency is within 25% of desired value */
 	fxp_min = DIV_ROUND_CLOSEST(fxp * 3, 4);
 	fxp_max = DIV_ROUND_CLOSEST(fxp * 5, 4);
-	if (fxp_min < (1 << pn_min) || (255 << pn_max) < fxp_max) {
-		DRM_DEBUG_KMS("VBT defined backlight frequency out of range\n");
-		return false;
-	}
-
-	for (pn = pn_max; pn >= pn_min; pn--) {
-		f = clamp(DIV_ROUND_CLOSEST(fxp, 1 << pn), 1, 255);
-		fxp_actual = f << pn;
-		if (fxp_min <= fxp_actual && fxp_actual <= fxp_max)
-			break;
-	}
 
-	if (drm_dp_dpcd_writeb(&intel_dp->aux,
-			       DP_EDP_PWMGEN_BIT_COUNT, pn) < 0) {
-		DRM_DEBUG_KMS("Failed to write aux pwmgen bit count\n");
+	if (fxp_min > fxp_actual || fxp_actual > fxp_max) {
+		DRM_DEBUG_KMS("Actual frequency out of range\n");
 		return false;
 	}
+
 	if (drm_dp_dpcd_writeb(&intel_dp->aux,
 			       DP_EDP_BACKLIGHT_FREQ_SET, (u8) f) < 0) {
 		DRM_DEBUG_KMS("Failed to write aux backlight freq\n");
@@ -178,6 +145,7 @@ static void intel_dp_aux_enable_backlight(const struct intel_crtc_state *crtc_st
 					  const struct drm_connector_state *conn_state)
 {
 	struct intel_connector *connector = to_intel_connector(conn_state->connector);
+	struct intel_panel *panel = &connector->panel;
 	struct intel_dp *intel_dp = enc_to_intel_dp(&connector->encoder->base);
 	u8 dpcd_buf, new_dpcd_buf, edp_backlight_mode;
 
@@ -197,6 +165,12 @@ static void intel_dp_aux_enable_backlight(const struct intel_crtc_state *crtc_st
 	case DP_EDP_BACKLIGHT_CONTROL_MODE_PRODUCT:
 		new_dpcd_buf &= ~DP_EDP_BACKLIGHT_CONTROL_MODE_MASK;
 		new_dpcd_buf |= DP_EDP_BACKLIGHT_CONTROL_MODE_DPCD;
+
+		if (drm_dp_dpcd_writeb(&intel_dp->aux,
+				       DP_EDP_PWMGEN_BIT_COUNT,
+				       panel->backlight.pwmgen_bit_count) < 0)
+			DRM_DEBUG_KMS("Failed to write aux pwmgen bit count\n");
+
 		break;
 
 	/* Do nothing when it is already DPCD mode */
@@ -225,20 +199,91 @@ static void intel_dp_aux_disable_backlight(const struct drm_connector_state *old
 	set_aux_backlight_enable(enc_to_intel_dp(old_conn_state->best_encoder), false);
 }
 
+static u32 intel_dp_aux_calc_max_backlight(struct intel_connector *connector)
+{
+	struct drm_i915_private *dev_priv = to_i915(connector->base.dev);
+	struct intel_dp *intel_dp = enc_to_intel_dp(&connector->encoder->base);
+	struct intel_panel *panel = &connector->panel;
+	u32 max_backlight = 0;
+	int freq, fxp, fxp_min, fxp_max, fxp_actual, f = 1;
+	u8 pn, pn_min, pn_max;
+
+	if (drm_dp_dpcd_readb(&intel_dp->aux, DP_EDP_PWMGEN_BIT_COUNT, &pn)) {
+		pn &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+		max_backlight = (1 << pn) - 1;
+	}
+
+	/* Find desired value of (F x P)
+	 * Note that, if F x P is out of supported range, the maximum value or
+	 * minimum value will applied automatically. So no need to check that.
+	 */
+	freq = dev_priv->vbt.backlight.pwm_freq_hz;
+	DRM_DEBUG_KMS("VBT defined backlight frequency %u Hz\n", freq);
+	if (!freq) {
+		DRM_DEBUG_KMS("Use panel default backlight frequency\n");
+		return max_backlight;
+	}
+
+	fxp = DIV_ROUND_CLOSEST(KHz(DP_EDP_BACKLIGHT_FREQ_BASE_KHZ), freq);
+
+	/* Use highest possible value of Pn for more granularity of brightness
+	 * adjustment while satifying the conditions below.
+	 * - Pn is in the range of Pn_min and Pn_max
+	 * - F is in the range of 1 and 255
+	 * - FxP is within 25% of desired value.
+	 *   Note: 25% is arbitrary value and may need some tweak.
+	 */
+	if (drm_dp_dpcd_readb(&intel_dp->aux,
+			      DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &pn_min) != 1) {
+		DRM_DEBUG_KMS("Failed to read pwmgen bit count cap min\n");
+		return max_backlight;
+	}
+	if (drm_dp_dpcd_readb(&intel_dp->aux,
+			      DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &pn_max) != 1) {
+		DRM_DEBUG_KMS("Failed to read pwmgen bit count cap max\n");
+		return max_backlight;
+	}
+	pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+	pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+
+	fxp_min = DIV_ROUND_CLOSEST(fxp * 3, 4);
+	fxp_max = DIV_ROUND_CLOSEST(fxp * 5, 4);
+	if (fxp_min < (1 << pn_min) || (255 << pn_max) < fxp_max) {
+		DRM_DEBUG_KMS("VBT defined backlight frequency out of range\n");
+		return max_backlight;
+	}
+
+	for (pn = pn_max; pn >= pn_min; pn--) {
+		f = clamp(DIV_ROUND_CLOSEST(fxp, 1 << pn), 1, 255);
+		fxp_actual = f << pn;
+		if (fxp_min <= fxp_actual && fxp_actual <= fxp_max)
+			break;
+	}
+
+	DRM_DEBUG_KMS("Using eDP pwmgen bit count of %d\n", pn);
+	if (drm_dp_dpcd_writeb(&intel_dp->aux,
+			       DP_EDP_PWMGEN_BIT_COUNT, pn) < 0) {
+		DRM_DEBUG_KMS("Failed to write aux pwmgen bit count\n");
+		return max_backlight;
+	}
+	panel->backlight.pwmgen_bit_count = pn;
+
+	max_backlight = (1 << pn) - 1;
+
+	return max_backlight;
+}
+
 static int intel_dp_aux_setup_backlight(struct intel_connector *connector,
 					enum pipe pipe)
 {
-	struct intel_dp *intel_dp = enc_to_intel_dp(&connector->encoder->base);
 	struct intel_panel *panel = &connector->panel;
 
-	if (intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT)
-		panel->backlight.max = 0xFFFF;
-	else
-		panel->backlight.max = 0xFF;
+	panel->backlight.max = intel_dp_aux_calc_max_backlight(connector);
+	if (!panel->backlight.max)
+		return -ENODEV;
 
 	panel->backlight.min = 0;
 	panel->backlight.level = intel_dp_aux_get_backlight(connector);
-
 	panel->backlight.enabled = panel->backlight.level != 0;
 
 	return 0;
-- 
2.21.0