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

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On Mon, 2017-05-15 at 17:43 -0700, Puthikorn Voravootivat wrote:
> 
> 
> 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.
>  

Isn't that a side-effect using the right shift operation for division?
Would DIV_ROUND_CLOSEST allow you to use pn_min?

>         
>         >         > +             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. 

The only values I see being set for max brightness are 0xFFFF and 0xFF  

if (intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_BRIGHTNESS_BYTE_COUNT)
	panel->backlight.max = 0xFFFF;
else
	panel->backlight.max = 0xFF;

I can't see where you are setting this based on Pn. Can you please point
out? From what I understand, max should be 2^Pn.

Also, won't this function be called every time _enable_backlight() is
called? Isn't doing this from setup sufficient? I guess, fixing it is an
optimization that'd be nice to have but not necessary.
>         
>         
>         >         > +             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);
>         >         >  }
>         >
>         >
>         >
>         >
>         
>         > _______________________________________________
>         > Intel-gfx mailing list
>         > Intel-gfx@xxxxxxxxxxxxxxxxxxxxx
>         > https://lists.freedesktop.org/mailman/listinfo/intel-gfx
>         
> 
> 
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