Re: [PATCH v10 3/3] drm/i915: Set PWM divider to match desired frequency in vbt

From: "Pandiyan, Dhinakaran" <dhinakaran.pandiyan@intel.com>
To: "puthik@chromium.org" <puthik@chromium.org>
Cc: "dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	"intel-gfx@lists.freedesktop.org"
	<intel-gfx@lists.freedesktop.org>
Subject: Re: [PATCH v10 3/3] drm/i915: Set PWM divider to match desired frequency in vbt
Date: Wed, 31 May 2017 03:40:51 +0000	[thread overview]
Message-ID: <1496203279.17832.9.camel@dk-H97M-D3H> (raw)
In-Reply-To: <20170527014209.186331-4-puthik@chromium.org>

The patch looks good overall, it would have been easier to merge if
you'd sent this as the first patch in this version. Some comments
inline.

On Fri, 2017-05-26 at 18:42 -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 divisor is still
> within 25% of the desired value.
> 
> Signed-off-by: Puthikorn Voravootivat <puthik@chromium.org>
> ---
>  drivers/gpu/drm/i915/intel_dp_aux_backlight.c | 79 +++++++++++++++++++++++++++
>  1 file changed, 79 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/intel_dp_aux_backlight.c b/drivers/gpu/drm/i915/intel_dp_aux_backlight.c
> index f55af41ce3bd..a8d485a29f29 100644
> --- a/drivers/gpu/drm/i915/intel_dp_aux_backlight.c
> +++ b/drivers/gpu/drm/i915/intel_dp_aux_backlight.c
> @@ -116,6 +116,81 @@ 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 int 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;

nit: unnecessary initialization for 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 -1;
> +	}
> +
> +	fxp = DIV_ROUND_CLOSEST(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
> +	 * - FxP is within 25% of desired value.
> +	 *   Note: 25% is arbitrary value and may need some tweak.
> +	 */
> +	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 -EIO;

The error numbers are not propagated outside, so I don't see a real need
for them. bool should suffice.

> +	}
> +	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 -EIO;
> +	}
> +	pn_min &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> +	pn_max &= DP_EDP_PWMGEN_BIT_COUNT_MASK;
> +
> +	fxp_min = DIV_ROUND_CLOSEST(fxp * 3, 4);
> +	fxp_max = DIV_ROUND_CLOSEST(fxp * 5, 4);
> +	if (fxp_min < (1 << pn_min) || (255 << pn_max) < fxp_max) {
> +		DRM_DEBUG_KMS("VBT defined backlight frequency out of range\n");
> +		return -ERANGE;
> +	}
> +
> +	for (pn = pn_max; pn >= pn_min; pn--) {
> +		f = clamp(DIV_ROUND_CLOSEST(fxp, 1 << 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 -EIO;
> +	}
> +	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 -EIO;
> +	}
> +	return 0;
> +}
> +
>  static void intel_dp_aux_enable_backlight(struct intel_connector *connector)
>  {
>  	struct intel_dp *intel_dp = enc_to_intel_dp(&connector->encoder->base);
> @@ -152,6 +227,10 @@ static void intel_dp_aux_enable_backlight(struct intel_connector *connector)
>  		DRM_DEBUG_KMS("Enable dynamic brightness.\n");
>  	}
>  
> +	if (intel_dp->edp_dpcd[2] & DP_EDP_BACKLIGHT_FREQ_AUX_SET_CAP)
> +		if (!intel_dp_aux_set_pwm_freq(connector))

nit: This doesn't read right to me, looking at the function name it
seems like you are proceeding when it failed.

> +			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) {

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