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.pandiyan@xxxxxxxxx> wrote:
What's the reason to use the lowest possible value of Pn? From what IOn 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 <puthik@xxxxxxxxxxxx>
> ---
> 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
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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