This is not related to brightness control. This calculation is used to set
the PWM frequency.
Frequency = 27 Mhz / (F * 2^ Pn)
Lower Pn means higher value for F which mean more accuracy for this
calculation.
On Sat, May 6, 2017 at 1:35 AM, Pandiyan, Dhinakaran <
dhinakaran.pandi...@intel.com> wrote:
> On Wed, 2017-05-03 at 17:28 -0700, Puthikorn Voravootivat wrote:
> > Read desired PWM frequency from panel vbt and calculate the
> > value for divider in DPCD address 0x724 and 0x728 to match
> > that frequency as close as possible.
> >
> > Signed-off-by: Puthikorn Voravootivat <put...@chromium.org>
> > ---
> > drivers/gpu/drm/i915/intel_dp_aux_backlight.c | 71
> +++++++++++++++++++++++++++
> > 1 file changed, 71 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/i915/intel_dp_aux_backlight.c
> b/drivers/gpu/drm/i915/intel_dp_aux_backlight.c
> > index fc26fea94fd4..441ad434a82b 100644
> > --- a/drivers/gpu/drm/i915/intel_dp_aux_backlight.c
> > +++ b/drivers/gpu/drm/i915/intel_dp_aux_backlight.c
> > @@ -113,12 +113,76 @@ 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, f;
> > + 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 = DP_EDP_BACKLIGHT_FREQ_BASE / freq;
> > +
> > + /* Use lowest possible value of Pn to try to make F to be between
> 1 and
>
> What's the reason to use the lowest possible value of Pn? From what I
> understand, choosing a higher value of Pn offers more steps for
> brightness control.
>
> -DK
>
> > + * 255 while still in the range Pn_min and Pn_max
> > + */
> > + 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;
> > +
> > + f = fxp >> pn_min;
> > +
> > + for (pn = pn_min; pn < pn_max && f > 255; pn++)
> > + f >>= 1;
> > +
> > + f = clamp(f, 1, 255);
> > +
> > + 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;
> > + }
> > + 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) {
> > @@ -150,6 +214,10 @@ 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) {
> > @@ -157,6 +225,9 @@ 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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