Re: [PATCH v7 9/9] drm/i915: Set PWM divider to match desired frequency in vbt

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On Mon, May 15, 2017 at 4:07 PM, Pandiyan, Dhinakaran <dhinakaran.pandiyan@xxxxxxxxx> wrote:
On Fri, 2017-05-12 at 17:31 -0700, Puthikorn Voravootivat wrote:
>
>
>
> On Fri, May 12, 2017 at 5:12 PM, Pandiyan, Dhinakaran
> <dhinakaran.pandiyan@xxxxxxxxx> wrote:
>         On Thu, 2017-05-11 at 16:02 -0700, Puthikorn Voravootivat
>         wrote:
>         > Read desired PWM frequency from panel vbt and calculate the
>         > value for divider in DPCD address 0x724 and 0x728 to have
>         > as many bits as possible for PWM duty cyle for granularity
>         of
>         > brightness adjustment while the frequency is still within
>         25%
>         > of the desired frequency.
>
>         I read a few eDP panel data sheets, the PWM frequencies all
>         start from
>         ~200Hz. If the VBT chooses this lowest value to allow for more
>         brightness control, and then this patch lowers the value by
>         another 25%,
>         we'll end up below the panel allowed PWM frequency.
>
>         In fact, one of the systems I checked had PWM frequency as
>         200Hz in VBT
>         and the panel datasheet also had PWM frequency range starting
>         from
>         200Hz. Have you considered this case?
>
> The spec said "A given LCD panel typically has a limited range of
> backlight frequency capability.
> To limit the programmable frequency range, limitations are placed on
> the allowable total divider ratio with the Sink device"
>  So I think it should be auto cap to 200Hz in this case.
>
>         -DK
>         >
>         > Signed-off-by: Puthikorn Voravootivat <puthik@xxxxxxxxxxxx>
>         > ---
>         >  drivers/gpu/drm/i915/intel_dp_aux_backlight.c | 81
>         +++++++++++++++++++++++++++
>         >  1 file changed, 81 insertions(+)
>         >
>         > diff --git a/drivers/gpu/drm/i915/intel_dp_aux_backlight.c
>         b/drivers/gpu/drm/i915/intel_dp_aux_backlight.c
>         > index 0b48851013cc..6f10a2f1ab76 100644
>         > --- a/drivers/gpu/drm/i915/intel_dp_aux_backlight.c
>         > +++ b/drivers/gpu/drm/i915/intel_dp_aux_backlight.c
>         > @@ -113,12 +113,86 @@
>         intel_dp_aux_set_dynamic_backlight_percent(struct intel_dp
>         *intel_dp,
>         >       }
>         >  }
>         >
>         > +/*
>         > + * Set PWM Frequency divider to match desired frequency in
>         vbt.
>         > + * The PWM Frequency is calculated as 27Mhz / (F x P).
>         > + * - Where F = PWM Frequency Pre-Divider value programmed
>         by field 7:0 of the
>         > + *             EDP_BACKLIGHT_FREQ_SET register (DPCD
>         Address 00728h)
>         > + * - Where P = 2^Pn, where Pn is the value programmed by
>         field 4:0 of the
>         > + *             EDP_PWMGEN_BIT_COUNT register (DPCD Address
>         00724h)
>         > + */
>         > +static void 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;
>         > +
>         > +     /* 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;
>         > +     }
>         > +
>         > +     fxp = 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
>         > +      * - Effective frequency is within 25% of desired
>         frequency.
>         > +      */
>         > +     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;
>         > +     }
>         > +     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;
>         > +     }
>         > +     pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
>         > +     pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
>         > +
>         > +     fxp_min = fxp * 3 / 4;
>         > +     fxp_max = fxp * 5 / 4;
>
>
>         You are allowing fxp between +/- 25% of the actual. This isn't
>         same as
>         the "Effective frequency is within 25% of desired frequency."
>         right? The
>         frequency can vary between -20% and +33%.
>
>
> You are right.
> You want me to change commit message to reflect this or change the
> code to
> match the commit message?

I am okay with fixing the comment and commit message. Is the 25%
arbitrary or based on the distances between the possible values? Please
make a note in the comment if it's the former.


>         > +     if (fxp_min < (1 << pn_min) || (255 << pn_max) <
>         fxp_max) {



>         > +             DRM_DEBUG_KMS("VBT defined backlight frequency
>         out of range\n");
>         > +             return;
>         > +     }
>         > +
>         > +     for (pn = pn_max; pn > pn_min; pn--) {

Is there a reason this is not pn >= pn_min?
This is bug because f value will be incorrect in the case that pn == pn_min.
Thanks for catching this.
 

>         > +             f = clamp(fxp >> 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 the number of brightness control bits are changing, the max.
brightness value changes too.

Please see intel_dp_aux_setup_backlight().

I think this is fine because
- intel_dp_aux_setup_backlight() will called intel_dp_aux_enable_backlight() which 
called intel_dp_aux_set_pwm_freq() first before determine the max brightness value.
- Also, the panel I tested does not change BACKLIGHT_BRIGHTNESS_BYTE_COUNT
when changing the frequency. 


>         > +             return;
>         > +     }
>         > +     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");
>         > +             return;
>         > +     }
>         > +}
>         > +
>         >  static void intel_dp_aux_enable_backlight(struct
>         intel_connector *connector)
>         >  {
>         >       struct intel_dp *intel_dp =
>         enc_to_intel_dp(&connector->encoder->base);
>         >       uint8_t dpcd_buf = 0;
>         >       uint8_t new_dpcd_buf = 0;
>         >       uint8_t edp_backlight_mode = 0;
>         > +     bool freq_cap;
>         >
>         >       if (drm_dp_dpcd_readb(&intel_dp->aux,
>         >                       DP_EDP_BACKLIGHT_MODE_SET_REGISTER,
>         &dpcd_buf) != 1) {
>         > @@ -151,6 +225,10 @@ static void
>         intel_dp_aux_enable_backlight(struct intel_connector
>         *connector)
>         >               DRM_DEBUG_KMS("Enable dynamic brightness.\n");
>         >       }
>         >
>         > +     freq_cap = intel_dp->edp_dpcd[2] &
>         DP_EDP_BACKLIGHT_FREQ_AUX_SET_CAP;
>         > +     if (freq_cap)
>         > +             new_dpcd_buf |=
>         DP_EDP_BACKLIGHT_FREQ_AUX_SET_ENABLE;
>         > +
>         >       if (new_dpcd_buf != dpcd_buf) {
>         >               if (drm_dp_dpcd_writeb(&intel_dp->aux,
>         >                       DP_EDP_BACKLIGHT_MODE_SET_REGISTER,
>         new_dpcd_buf) < 0) {
>         > @@ -158,6 +236,9 @@ static void
>         intel_dp_aux_enable_backlight(struct intel_connector
>         *connector)
>         >               }
>         >       }
>         >
>         > +     if (freq_cap)
>         > +             intel_dp_aux_set_pwm_freq(connector);
>         > +
>         >       set_aux_backlight_enable(intel_dp, true);
>         >       intel_dp_aux_set_backlight(connector,
>         connector->panel.backlight.level);
>         >  }
>
>
>
>
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> Intel-gfx@lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/intel-gfx


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