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

["Pandiyan, Dhinakaran" <dhinakaran.pandiyan@intel.com>]

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?

-DK
> 
> Signed-off-by: Puthikorn Voravootivat <puthik@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%.


> +	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--) {
> +		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");
> +		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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