Re: [PATCH v4] drivers: i915: Default backlight PWM frequency

[Takashi Iwai <tiwai@suse.de>]

At Fri, 11 Nov 2011 14:12:58 -0800,
Simon Que wrote:
> 
> If the firmware did not initialize the backlight PWM registers, set up a
> default PWM frequency of 200 Hz.  This is determined using the following
> formula:
> 
>   freq = refclk / (128 * pwm_max)
> 
> The PWM register allows the max PWM value to be set.  So we want to use
> the formula, where freq = 200:
> 
>   pwm_max = refclk / (128 * freq)
> 
> This patch will, in the case of missing PWM register initialization
> values, look for the reference clock frequency.  Based on that, it sets
> an appropriate max PWM value for a frequency of 200 Hz.
> 
> If no refclk frequency is found, the max PWM will be zero, which results
> in no change to the PWM registers.
> 
> Signed-off-by: Simon Que <sque@chromium.org>
> ---
>  drivers/gpu/drm/i915/intel_panel.c |   38 ++++++++++++++++++++++++++++++-----
>  1 files changed, 32 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> index f15388c..dda5de2 100644
> --- a/drivers/gpu/drm/i915/intel_panel.c
> +++ b/drivers/gpu/drm/i915/intel_panel.c
> @@ -32,6 +32,12 @@
>  
>  #define PCI_LBPC 0xf4 /* legacy/combination backlight modes */
>  
> +/* These are used to calculate a reasonable default when firmware has not
> + * configured a maximum PWM frequency, with 200Hz as the current default target.
> + */
> +#define DEFAULT_BACKLIGHT_PWM_FREQ   200
> +#define BACKLIGHT_REFCLK_DIVISOR     128
> +
>  void
>  intel_fixed_panel_mode(struct drm_display_mode *fixed_mode,
>  		       struct drm_display_mode *adjusted_mode)
> @@ -129,12 +135,32 @@ static int is_backlight_combination_mode(struct drm_device *dev)
>  	return 0;
>  }
>  
> +static void i915_set_default_max_backlight(struct drm_i915_private *dev_priv)
> +{
> +	u32 refclk_freq_mhz = 0;
> +	u32 max_pwm;
> +
> +	if (HAS_PCH_SPLIT(dev_priv->dev))
> +		refclk_freq_mhz = I915_READ(PCH_RAWCLK_FREQ) & RAWCLK_FREQ_MASK;
> +	else if (dev_priv->lvds_use_ssc)
> +		refclk_freq_mhz = dev_priv->lvds_ssc_freq;
> +
> +	max_pwm = refclk_freq_mhz * 1000000 /
> +			(BACKLIGHT_REFCLK_DIVISOR * DEFAULT_BACKLIGHT_PWM_FREQ);
> +
> +	if (HAS_PCH_SPLIT(dev_priv->dev))
> +		dev_priv->saveBLC_PWM_CTL2 = max_pwm << 16;
> +	else if (IS_PINEVIEW(dev_priv->dev))
> +		dev_priv->saveBLC_PWM_CTL = max_pwm << 17;
> +	else
> +		dev_priv->saveBLC_PWM_CTL = max_pwm << 16;

Is the pineview case really correct?
The special handling for pineview in some places in intel_panel.c is
just for omitting the bit 0, IIRC.  It doesn't mean that the value is
twice larger.

BTW, this handling of bit 0 seems necessary not only for pineview but
for the older chips (gen < 4) in general, too, as being discussed in
another thread of LKML.  915GM hits the with problem of bit-0, for
example.


Takashi