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

Puthikorn Voravootivat puthik at chromium.org
Tue May 16 20:56:49 UTC 2017


On Tue, May 16, 2017 at 1:29 PM, Pandiyan, Dhinakaran <
dhinakaran.pandiyan at intel.com> wrote:

> On Tue, 2017-05-16 at 11:07 -0700, Puthikorn Voravootivat wrote:
> >
> >
> > On Mon, May 15, 2017 at 11:21 PM, Pandiyan, Dhinakaran
> > <dhinakaran.pandiyan at intel.com> wrote:
> >         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 at intel.com> 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 at intel.com> 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 at chromium.org>
> >         >         >         > ---
> >         >         >         >
> >         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?
> > The bug is just for loop that exit too soon.
> >
> Not sure I got that, what's the invalid "f" case that you see with
> pn==pn_min?
>
>
> if the for loop has pn > pn_min, when pn == pn_min we will exit the loop
first
without running the  f = clamp(fxp >> pn, 1, 255); in the next line.

This would fix with pn >= pn_min in for loop.

We guarantee that the break statement will be called and pn won't be pn_min
- 1
because we check (fxp_min < (1 << pn_min) || (255 << pn_max) < fxp_max)
before
entering this for loop.


> >         Would DIV_ROUND_CLOSEST allow you to use pn_min?
> > It does not related to the point above, but DIV_ROUND_CLOSEST still
> > better than just using right shift. I will change to that in next
> > version.
> >
> >
> >         >
> >         >         >         > +             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.
> >
> >
> > It is suppose to be 0xFF or 0xFFFF only.
> >
> >
> > From eDP spec:
> > 0x702 bit 2 BACKLIGHT_BRIGHTNESS_BYTE_COUNT
> > 0 = Indicates that the Sink device supports a 1-byte setting for
> > backlight brightness
> > 1 = Indicates that the Sink device supports a 2-byte setting for the
> > backlight brightness
> >
> >
> > 0x722 EDP_BACKLIGHT_BRIGHTNESS_MSB
> > The actual number of assigned bits for the backlight brightness PWM
> > generator is set by
> > field 4:0 of the EDP_PWMGEN_BIT_COUNT register (DPCD Address 00724h).
> > Assigned bits are allocated to the MSB of the enabled register
> > combination
> >
> ^This makes it somewhat clear but gets confusing again.
> >
> > This means that if PWM_GEN_BIT_COUNT is less than 16 then the panel
> > will ignore
> > the least significant bit in EDP_BACKLIGHT_BRIGHTNESS register.
> >
> >
> > For example,
> > if  BACKLIGHT_BRIGHTNESS_BYTE_COUNT == 1 and PWM_GEN_BIT_COUNT = 16
> > and EDP_BACKLIGHT_BRIGHTNESS == 0xabcd
> > In this case, all bits in EDP_BACKLIGHT_BRIGHTNESS will be used.
> >
> > 0xabcd means 0xabcd / 0xffff or (43981 / 65535) or 67.111%
> >
> >
> > if  BACKLIGHT_BRIGHTNESS_BYTE_COUNT == 1 and PWM_GEN_BIT_COUNT = 12
> > and EDP_BACKLIGHT_BRIGHTNESS == 0xabcd
> >
> > In this case, the last 4 bits will be discarded.
> > 0xabcd means 0xabc / 0xfff or 2748 / 4095 or 67.106%
> >
> >
> > I think it should be fine to just have 8 or 16 bits of the max
> > brightness and let panel drop
> > the unneed bit to simplify the driver code.
> >
>
> Note:
> The number of assigned or controllable bits will have the ability to
> provide full brightness control. Examples:
>>  For 10-bit operation, 000h = 0% brightness, and 3FFh = 100% brightness.
>
> Going by the logic you stated, for a 10-bit control, the MSB and LSB
> registers have to be written FFFFh to set 100% brightness with the last
> 6 bits dropped off by the hardware.
>
> I don't like the way the spec leaves this to interpretation. Have you
> experimented with both 0x0ABC and 0xABCD for the 12-bit, BYTE_COUNT==1
> example you gave?
>
> Yes, already test with the real panel.

Here is better test case than 0xABCD
0xFFFF with 6, 12, 16 shows very bright screen with same brightness in all
case. (All 100%)
0x0FFF with 6, 12, 16 bits shows dim screen. 6 bit case is slightly dimmer
than other case.
Note that the brightness are 4.76% / 6.23% / 6.25% in 6 / 12 / 16 bits case.

You can apply this patch for easy testing
https://patchwork.kernel.org/patch/6241481/


>
> >
> >
> >         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.
> >
> > Lets get this patch set done first. I can send in another patch after
> > this is go in to optimize this.
>
> Sure. This patch should be good to go with answers to my questions. The
> only thing remaining would be getting an ACK from Jani for adding a new
> parameter for dynamic backlight control.
>
>
> -DK
>
> > Probably need to add new member in struct drm_i915_private
> >
> >         >
> >         >
> >         >         >         > +             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 at lists.freedesktop.org
> >         >         >
> >         https://lists.freedesktop.org/mailman/listinfo/intel-gfx
> >         >
> >         >
> >         >
> >         > _______________________________________________
> >         > Intel-gfx mailing list
> >         > Intel-gfx at lists.freedesktop.org
> >         > https://lists.freedesktop.org/mailman/listinfo/intel-gfx
> >
> >
> >
> >
> > _______________________________________________
> > Intel-gfx mailing list
> > Intel-gfx at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/intel-gfx
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.freedesktop.org/archives/dri-devel/attachments/20170516/b053da22/attachment-0001.html>


More information about the dri-devel mailing list