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[PATCH v2] drm/dp: fallback to minimum when PWM bit count is zero

Christopher Obbard posted 1 patch 8 months, 3 weeks ago 
drivers/gpu/drm/display/drm_dp_helper.c | 50 ++++++++++++++++++++++-----------
1 file changed, 33 insertions(+), 17 deletions(-)
[PATCH v2] drm/dp: fallback to minimum when PWM bit count is zero
Posted by Christopher Obbard 8 months, 3 weeks ago 
According to the eDP specification (e.g., VESA eDP 1.4b, section 3.3.10.2),
if DP_EDP_PWMGEN_BIT_COUNT is less than DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN,
the sink is required to use the MIN value as the effective bit count.

Some eDP panels report DP_EDP_PWMGEN_BIT_COUNT as 0 while still providing
valid non-zero MIN and MAX capability values. This patch updates the logic
to use the CAP_MIN value in such cases, ensuring correct scaling of AUX-set
backlight brightness values.

This improves compatibility with panels like the Samsung ATNA40YK20 used
on the Lenovo T14s Gen6 (Snapdragon variant with OLED) which report a
bit count of 0 but declares an 11-bit PWM capability range.

Co-developed-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Christopher Obbard <christopher.obbard@linaro.org>
---
Changes in v2:
- Split backlight brightness patch from T14s OLED enablement series.
- Use PWMGEN_CAP_MIN rather than MAX (Dmitry).
- Rework commit message to reference eDP spec.
- Rebase on drm-misc-next.
- Link to v1: https://lore.kernel.org/all/20250325-wip-obbardc-qcom-t14s-oled-panel-v2-4-e9bc7c9d30cc@linaro.org/
---
 drivers/gpu/drm/display/drm_dp_helper.c | 50 ++++++++++++++++++++++-----------
 1 file changed, 33 insertions(+), 17 deletions(-)

diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
index dbce1c3f49691fc687fee2404b723c73d533f23d..0b843d5b634f89f144b62b30311834d118b79ba9 100644
--- a/drivers/gpu/drm/display/drm_dp_helper.c
+++ b/drivers/gpu/drm/display/drm_dp_helper.c
@@ -4083,7 +4083,7 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
 {
 	int fxp, fxp_min, fxp_max, fxp_actual, f = 1;
 	int ret;
-	u8 pn, pn_min, pn_max;
+	u8 pn, pn_min, pn_max, bl_caps;
 
 	if (!bl->aux_set)
 		return 0;
@@ -4094,8 +4094,39 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
 			    aux->name, ret);
 		return -ENODEV;
 	}
-
 	pn &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+
+	ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &pn_min);
+	if (ret != 1) {
+		drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap min: %d\n",
+			    aux->name, ret);
+		return 0;
+	}
+	pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+
+	ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &pn_max);
+	if (ret != 1) {
+		drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap max: %d\n",
+			    aux->name, ret);
+		return 0;
+	}
+	pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
+
+	ret = drm_dp_dpcd_readb(aux, DP_EDP_BACKLIGHT_ADJUSTMENT_CAP, &bl_caps);
+	if (ret != 1) {
+		bl_caps = 0;
+		drm_dbg_kms(aux->drm_dev, "%s: Failed to read backlight adjustment cap: %d\n",
+			aux->name, ret);
+	}
+
+	/*
+	 * Some eDP panels report brightness byte count support, but the byte count
+	 * reading is 0 (e.g. Samsung ATNA40YK20) so use pn_min instead.
+	 */
+	if (!pn && (bl_caps & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT)
+	    && pn_min)
+		pn = pn_min;
+
 	bl->max = (1 << pn) - 1;
 	if (!driver_pwm_freq_hz)
 		return 0;
@@ -4122,21 +4153,6 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
 	 * - FxP is within 25% of desired value.
 	 *   Note: 25% is arbitrary value and may need some tweak.
 	 */
-	ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &pn_min);
-	if (ret != 1) {
-		drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap min: %d\n",
-			    aux->name, ret);
-		return 0;
-	}
-	ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &pn_max);
-	if (ret != 1) {
-		drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap max: %d\n",
-			    aux->name, ret);
-		return 0;
-	}
-	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);

---
base-commit: ee20c69c789b6cb2179a535cf440d72b98f4a134
change-id: 20250327-wip-obbardc-qcom-t14s-oled-panel-brightness-4020865b6580

Best regards,
-- 
Christopher Obbard <christopher.obbard@linaro.org>
Re: [PATCH v2] drm/dp: fallback to minimum when PWM bit count is zero
Posted by Dmitry Baryshkov 8 months, 3 weeks ago 
On 27/03/2025 19:25, Christopher Obbard wrote:
> According to the eDP specification (e.g., VESA eDP 1.4b, section 3.3.10.2),
> if DP_EDP_PWMGEN_BIT_COUNT is less than DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN,
> the sink is required to use the MIN value as the effective bit count.
> 
> Some eDP panels report DP_EDP_PWMGEN_BIT_COUNT as 0 while still providing
> valid non-zero MIN and MAX capability values. This patch updates the logic
> to use the CAP_MIN value in such cases, ensuring correct scaling of AUX-set
> backlight brightness values.
> 
> This improves compatibility with panels like the Samsung ATNA40YK20 used
> on the Lenovo T14s Gen6 (Snapdragon variant with OLED) which report a
> bit count of 0 but declares an 11-bit PWM capability range.
> 
> Co-developed-by: Rui Miguel Silva <rui.silva@linaro.org>
> Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
> Signed-off-by: Christopher Obbard <christopher.obbard@linaro.org>
> ---
> Changes in v2:
> - Split backlight brightness patch from T14s OLED enablement series.
> - Use PWMGEN_CAP_MIN rather than MAX (Dmitry).
> - Rework commit message to reference eDP spec.
> - Rebase on drm-misc-next.
> - Link to v1: https://lore.kernel.org/all/20250325-wip-obbardc-qcom-t14s-oled-panel-v2-4-e9bc7c9d30cc@linaro.org/
> ---
>   drivers/gpu/drm/display/drm_dp_helper.c | 50 ++++++++++++++++++++++-----------
>   1 file changed, 33 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
> index dbce1c3f49691fc687fee2404b723c73d533f23d..0b843d5b634f89f144b62b30311834d118b79ba9 100644
> --- a/drivers/gpu/drm/display/drm_dp_helper.c
> +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> @@ -4083,7 +4083,7 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
>   {
>   	int fxp, fxp_min, fxp_max, fxp_actual, f = 1;
>   	int ret;
> -	u8 pn, pn_min, pn_max;
> +	u8 pn, pn_min, pn_max, bl_caps;
>   
>   	if (!bl->aux_set)
>   		return 0;
> @@ -4094,8 +4094,39 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
>   			    aux->name, ret);
>   		return -ENODEV;
>   	}
> -
>   	pn &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> +
> +	ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &pn_min);
> +	if (ret != 1) {
> +		drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap min: %d\n",
> +			    aux->name, ret);
> +		return 0;
> +	}
> +	pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> +
> +	ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &pn_max);
> +	if (ret != 1) {
> +		drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap max: %d\n",
> +			    aux->name, ret);
> +		return 0;
> +	}
> +	pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> +
> +	ret = drm_dp_dpcd_readb(aux, DP_EDP_BACKLIGHT_ADJUSTMENT_CAP, &bl_caps);
> +	if (ret != 1) {
> +		bl_caps = 0;
> +		drm_dbg_kms(aux->drm_dev, "%s: Failed to read backlight adjustment cap: %d\n",
> +			aux->name, ret);
> +	}
> +
> +	/*
> +	 * Some eDP panels report brightness byte count support, but the byte count
> +	 * reading is 0 (e.g. Samsung ATNA40YK20) so use pn_min instead.
> +	 */
> +	if (!pn && (bl_caps & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT)
> +	    && pn_min)
> +		pn = pn_min;

I wonder, what stops you from implementing this part according to the 
standard, rather than adding a hack for 0 value.

> +
>   	bl->max = (1 << pn) - 1;
>   	if (!driver_pwm_freq_hz)
>   		return 0;

-- 
With best wishes
Dmitry
Re: [PATCH v2] drm/dp: fallback to minimum when PWM bit count is zero
Posted by Christopher Obbard 8 months, 3 weeks ago 
Hi Dmitry,

On Thu, 27 Mar 2025 at 17:40, Dmitry Baryshkov
<dmitry.baryshkov@oss.qualcomm.com> wrote:
>
> On 27/03/2025 19:25, Christopher Obbard wrote:
> > According to the eDP specification (e.g., VESA eDP 1.4b, section 3.3.10.2),
> > if DP_EDP_PWMGEN_BIT_COUNT is less than DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN,
> > the sink is required to use the MIN value as the effective bit count.
> >
> > Some eDP panels report DP_EDP_PWMGEN_BIT_COUNT as 0 while still providing
> > valid non-zero MIN and MAX capability values. This patch updates the logic
> > to use the CAP_MIN value in such cases, ensuring correct scaling of AUX-set
> > backlight brightness values.
> >
> > This improves compatibility with panels like the Samsung ATNA40YK20 used
> > on the Lenovo T14s Gen6 (Snapdragon variant with OLED) which report a
> > bit count of 0 but declares an 11-bit PWM capability range.
> >
> > Co-developed-by: Rui Miguel Silva <rui.silva@linaro.org>
> > Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
> > Signed-off-by: Christopher Obbard <christopher.obbard@linaro.org>
> > ---
> > Changes in v2:
> > - Split backlight brightness patch from T14s OLED enablement series.
> > - Use PWMGEN_CAP_MIN rather than MAX (Dmitry).
> > - Rework commit message to reference eDP spec.
> > - Rebase on drm-misc-next.
> > - Link to v1: https://lore.kernel.org/all/20250325-wip-obbardc-qcom-t14s-oled-panel-v2-4-e9bc7c9d30cc@linaro.org/
> > ---
> >   drivers/gpu/drm/display/drm_dp_helper.c | 50 ++++++++++++++++++++++-----------
> >   1 file changed, 33 insertions(+), 17 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
> > index dbce1c3f49691fc687fee2404b723c73d533f23d..0b843d5b634f89f144b62b30311834d118b79ba9 100644
> > --- a/drivers/gpu/drm/display/drm_dp_helper.c
> > +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> > @@ -4083,7 +4083,7 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
> >   {
> >       int fxp, fxp_min, fxp_max, fxp_actual, f = 1;
> >       int ret;
> > -     u8 pn, pn_min, pn_max;
> > +     u8 pn, pn_min, pn_max, bl_caps;
> >
> >       if (!bl->aux_set)
> >               return 0;
> > @@ -4094,8 +4094,39 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
> >                           aux->name, ret);
> >               return -ENODEV;
> >       }
> > -
> >       pn &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> > +
> > +     ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &pn_min);
> > +     if (ret != 1) {
> > +             drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap min: %d\n",
> > +                         aux->name, ret);
> > +             return 0;
> > +     }
> > +     pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> > +
> > +     ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &pn_max);
> > +     if (ret != 1) {
> > +             drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap max: %d\n",
> > +                         aux->name, ret);
> > +             return 0;
> > +     }
> > +     pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> > +
> > +     ret = drm_dp_dpcd_readb(aux, DP_EDP_BACKLIGHT_ADJUSTMENT_CAP, &bl_caps);
> > +     if (ret != 1) {
> > +             bl_caps = 0;
> > +             drm_dbg_kms(aux->drm_dev, "%s: Failed to read backlight adjustment cap: %d\n",
> > +                     aux->name, ret);
> > +     }
> > +
> > +     /*
> > +      * Some eDP panels report brightness byte count support, but the byte count
> > +      * reading is 0 (e.g. Samsung ATNA40YK20) so use pn_min instead.
> > +      */
> > +     if (!pn && (bl_caps & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT)
> > +         && pn_min)
> > +             pn = pn_min;
>
> I wonder, what stops you from implementing this part according to the
> standard, rather than adding a hack for 0 value.

I am simply quite dense, I do not know this spec or the DRM code well
at all ;-).
I do appreciate your continued reviews though, however painful it must be.

Before I send v3, can I clarify a proper solution?
I think you are saying that I can remove the section where I read the
capabilities and instead simply have something like (in pseudocode):

/* Determine effective bit count according to eDP spec */
u8 effective_bit_count = pwm_bit_count;
if (pwm_bit_count == 0 || pwm_bit_count < cap_min) {
    effective_bit_count = cap_min;
}

I would like to introduce this new variable to make the code easier to read.

If my understanding is correct, I'll send a v3 later.


> > +
> >       bl->max = (1 << pn) - 1;
> >       if (!driver_pwm_freq_hz)
> >               return 0;
Re: [PATCH v2] drm/dp: fallback to minimum when PWM bit count is zero
Posted by Dmitry Baryshkov 8 months, 3 weeks ago 
On Thu, 27 Mar 2025 at 20:19, Christopher Obbard
<christopher.obbard@linaro.org> wrote:
>
> Hi Dmitry,
>
> On Thu, 27 Mar 2025 at 17:40, Dmitry Baryshkov
> <dmitry.baryshkov@oss.qualcomm.com> wrote:
> >
> > On 27/03/2025 19:25, Christopher Obbard wrote:
> > > According to the eDP specification (e.g., VESA eDP 1.4b, section 3.3.10.2),
> > > if DP_EDP_PWMGEN_BIT_COUNT is less than DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN,
> > > the sink is required to use the MIN value as the effective bit count.
> > >
> > > Some eDP panels report DP_EDP_PWMGEN_BIT_COUNT as 0 while still providing
> > > valid non-zero MIN and MAX capability values. This patch updates the logic
> > > to use the CAP_MIN value in such cases, ensuring correct scaling of AUX-set
> > > backlight brightness values.
> > >
> > > This improves compatibility with panels like the Samsung ATNA40YK20 used
> > > on the Lenovo T14s Gen6 (Snapdragon variant with OLED) which report a
> > > bit count of 0 but declares an 11-bit PWM capability range.
> > >
> > > Co-developed-by: Rui Miguel Silva <rui.silva@linaro.org>
> > > Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
> > > Signed-off-by: Christopher Obbard <christopher.obbard@linaro.org>
> > > ---
> > > Changes in v2:
> > > - Split backlight brightness patch from T14s OLED enablement series.
> > > - Use PWMGEN_CAP_MIN rather than MAX (Dmitry).
> > > - Rework commit message to reference eDP spec.
> > > - Rebase on drm-misc-next.
> > > - Link to v1: https://lore.kernel.org/all/20250325-wip-obbardc-qcom-t14s-oled-panel-v2-4-e9bc7c9d30cc@linaro.org/
> > > ---
> > >   drivers/gpu/drm/display/drm_dp_helper.c | 50 ++++++++++++++++++++++-----------
> > >   1 file changed, 33 insertions(+), 17 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
> > > index dbce1c3f49691fc687fee2404b723c73d533f23d..0b843d5b634f89f144b62b30311834d118b79ba9 100644
> > > --- a/drivers/gpu/drm/display/drm_dp_helper.c
> > > +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> > > @@ -4083,7 +4083,7 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
> > >   {
> > >       int fxp, fxp_min, fxp_max, fxp_actual, f = 1;
> > >       int ret;
> > > -     u8 pn, pn_min, pn_max;
> > > +     u8 pn, pn_min, pn_max, bl_caps;
> > >
> > >       if (!bl->aux_set)
> > >               return 0;
> > > @@ -4094,8 +4094,39 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
> > >                           aux->name, ret);
> > >               return -ENODEV;
> > >       }
> > > -
> > >       pn &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> > > +
> > > +     ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &pn_min);
> > > +     if (ret != 1) {
> > > +             drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap min: %d\n",
> > > +                         aux->name, ret);
> > > +             return 0;
> > > +     }
> > > +     pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> > > +
> > > +     ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &pn_max);
> > > +     if (ret != 1) {
> > > +             drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap max: %d\n",
> > > +                         aux->name, ret);
> > > +             return 0;
> > > +     }
> > > +     pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> > > +
> > > +     ret = drm_dp_dpcd_readb(aux, DP_EDP_BACKLIGHT_ADJUSTMENT_CAP, &bl_caps);
> > > +     if (ret != 1) {
> > > +             bl_caps = 0;
> > > +             drm_dbg_kms(aux->drm_dev, "%s: Failed to read backlight adjustment cap: %d\n",
> > > +                     aux->name, ret);
> > > +     }
> > > +
> > > +     /*
> > > +      * Some eDP panels report brightness byte count support, but the byte count
> > > +      * reading is 0 (e.g. Samsung ATNA40YK20) so use pn_min instead.
> > > +      */
> > > +     if (!pn && (bl_caps & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT)
> > > +         && pn_min)
> > > +             pn = pn_min;
> >
> > I wonder, what stops you from implementing this part according to the
> > standard, rather than adding a hack for 0 value.
>
> I am simply quite dense, I do not know this spec or the DRM code well
> at all ;-).

Okay, no worries. It should be pretty easy:

pn = clamp(pn_min, pn_max);

No need to check for pn being non-zero, etc.

> I do appreciate your continued reviews though, however painful it must be.

It's not, no worries.

-- 
With best wishes
Dmitry
Re: [PATCH v2] drm/dp: fallback to minimum when PWM bit count is zero
Posted by Christopher Obbard 8 months, 3 weeks ago 
On Fri, 28 Mar 2025 at 11:25, Dmitry Baryshkov
<dmitry.baryshkov@oss.qualcomm.com> wrote:
>
> On Thu, 27 Mar 2025 at 20:19, Christopher Obbard
> <christopher.obbard@linaro.org> wrote:
> >
> > Hi Dmitry,
> >
> > On Thu, 27 Mar 2025 at 17:40, Dmitry Baryshkov
> > <dmitry.baryshkov@oss.qualcomm.com> wrote:
> > >
> > > On 27/03/2025 19:25, Christopher Obbard wrote:
> > > > According to the eDP specification (e.g., VESA eDP 1.4b, section 3.3.10.2),
> > > > if DP_EDP_PWMGEN_BIT_COUNT is less than DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN,
> > > > the sink is required to use the MIN value as the effective bit count.
> > > >
> > > > Some eDP panels report DP_EDP_PWMGEN_BIT_COUNT as 0 while still providing
> > > > valid non-zero MIN and MAX capability values. This patch updates the logic
> > > > to use the CAP_MIN value in such cases, ensuring correct scaling of AUX-set
> > > > backlight brightness values.
> > > >
> > > > This improves compatibility with panels like the Samsung ATNA40YK20 used
> > > > on the Lenovo T14s Gen6 (Snapdragon variant with OLED) which report a
> > > > bit count of 0 but declares an 11-bit PWM capability range.
> > > >
> > > > Co-developed-by: Rui Miguel Silva <rui.silva@linaro.org>
> > > > Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
> > > > Signed-off-by: Christopher Obbard <christopher.obbard@linaro.org>
> > > > ---
> > > > Changes in v2:
> > > > - Split backlight brightness patch from T14s OLED enablement series.
> > > > - Use PWMGEN_CAP_MIN rather than MAX (Dmitry).
> > > > - Rework commit message to reference eDP spec.
> > > > - Rebase on drm-misc-next.
> > > > - Link to v1: https://lore.kernel.org/all/20250325-wip-obbardc-qcom-t14s-oled-panel-v2-4-e9bc7c9d30cc@linaro.org/
> > > > ---
> > > >   drivers/gpu/drm/display/drm_dp_helper.c | 50 ++++++++++++++++++++++-----------
> > > >   1 file changed, 33 insertions(+), 17 deletions(-)
> > > >
> > > > diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
> > > > index dbce1c3f49691fc687fee2404b723c73d533f23d..0b843d5b634f89f144b62b30311834d118b79ba9 100644
> > > > --- a/drivers/gpu/drm/display/drm_dp_helper.c
> > > > +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> > > > @@ -4083,7 +4083,7 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
> > > >   {
> > > >       int fxp, fxp_min, fxp_max, fxp_actual, f = 1;
> > > >       int ret;
> > > > -     u8 pn, pn_min, pn_max;
> > > > +     u8 pn, pn_min, pn_max, bl_caps;
> > > >
> > > >       if (!bl->aux_set)
> > > >               return 0;
> > > > @@ -4094,8 +4094,39 @@ drm_edp_backlight_probe_max(struct drm_dp_aux *aux, struct drm_edp_backlight_inf
> > > >                           aux->name, ret);
> > > >               return -ENODEV;
> > > >       }
> > > > -
> > > >       pn &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> > > > +
> > > > +     ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MIN, &pn_min);
> > > > +     if (ret != 1) {
> > > > +             drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap min: %d\n",
> > > > +                         aux->name, ret);
> > > > +             return 0;
> > > > +     }
> > > > +     pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> > > > +
> > > > +     ret = drm_dp_dpcd_readb(aux, DP_EDP_PWMGEN_BIT_COUNT_CAP_MAX, &pn_max);
> > > > +     if (ret != 1) {
> > > > +             drm_dbg_kms(aux->drm_dev, "%s: Failed to read pwmgen bit count cap max: %d\n",
> > > > +                         aux->name, ret);
> > > > +             return 0;
> > > > +     }
> > > > +     pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> > > > +
> > > > +     ret = drm_dp_dpcd_readb(aux, DP_EDP_BACKLIGHT_ADJUSTMENT_CAP, &bl_caps);
> > > > +     if (ret != 1) {
> > > > +             bl_caps = 0;
> > > > +             drm_dbg_kms(aux->drm_dev, "%s: Failed to read backlight adjustment cap: %d\n",
> > > > +                     aux->name, ret);
> > > > +     }
> > > > +
> > > > +     /*
> > > > +      * Some eDP panels report brightness byte count support, but the byte count
> > > > +      * reading is 0 (e.g. Samsung ATNA40YK20) so use pn_min instead.
> > > > +      */
> > > > +     if (!pn && (bl_caps & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT)
> > > > +         && pn_min)
> > > > +             pn = pn_min;
> > >
> > > I wonder, what stops you from implementing this part according to the
> > > standard, rather than adding a hack for 0 value.
> >
> > I am simply quite dense, I do not know this spec or the DRM code well
> > at all ;-).
>
> Okay, no worries. It should be pretty easy:
>
> pn = clamp(pn_min, pn_max);
>
> No need to check for pn being non-zero, etc.

I think you probably mean:
pn = clamp(pn, pn_min, pn_max);

I will look to use this macro in the next version later today & make
it more generic; thanks !


PS: I also found someone who tried to fix this in 2019; I will add the
author to CC of the next version.

https://lore.kernel.org/r/dri-devel/a766c3498d3754b598a8bf66f59a76e78ec57080.camel@intel.com/T/

Patchew 2.1-devel-b67e4c2

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