Re: [PATCH v2] drivers: i915: Default backlight PWM frequency

[Olof Johansson <olof@lixom.net>]

Hi,

A collection of nits below.


Thanks,

-Olof

On Thu, Nov 10, 2011 at 07:48:55PM -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/i915_reg.h    |    1 +
>  drivers/gpu/drm/i915/intel_panel.c |   37 +++++++++++++++++++++++++++++------
>  2 files changed, 31 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> index 5d5def7..a832028 100644
> --- a/drivers/gpu/drm/i915/i915_reg.h
> +++ b/drivers/gpu/drm/i915/i915_reg.h
> @@ -3275,6 +3275,7 @@
>  #define  PWM_POLARITY_ACTIVE_HIGH2	(0 << 28)
>  
>  #define BLC_PWM_PCH_CTL2	0xc8254
> +#define BLC_PWM_PCH_FREQ_SHIFT	16
>  
>  #define PCH_PP_STATUS		0xc7200
>  #define PCH_PP_CONTROL		0xc7204
> diff --git a/drivers/gpu/drm/i915/intel_panel.c b/drivers/gpu/drm/i915/intel_panel.c
> index f15388c..f865e52 100644
> --- a/drivers/gpu/drm/i915/intel_panel.c
> +++ b/drivers/gpu/drm/i915/intel_panel.c
> @@ -32,6 +32,10 @@
>  
>  #define PCI_LBPC 0xf4 /* legacy/combination backlight modes */
>  
> +/* For computing default PWM settings */
> +#define DEFAULT_BACKLIGHT_PWM_FREQ   200
> +#define BACKLIGHT_REFCLK_DIVISOR     128

I would a bit more comment above. Maybe something like:

/* These are used to calculate a reasonable default when firmware has not
 * configured a maximum PWM frequency, with 200Hz as current target.
 */

> +
>  void
>  intel_fixed_panel_mode(struct drm_display_mode *fixed_mode,
>  		       struct drm_display_mode *adjusted_mode)
> @@ -129,12 +133,31 @@ static int is_backlight_combination_mode(struct drm_device *dev)
>  	return 0;
>  }
>  
> +static u32 i915_get_default_max_backlight(struct drm_i915_private *dev_priv)

Having a 'get' function that has side effects is misleading.

'i915_set_default_max_backlight' is a better name (and not returning the new
max).

> +{
> +	u32 refclk_freq_mhz = 0;
> +	u32 max_pwm = 0;

No need to set max_pwm to 0.

Empty line between variables and code.

> +	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 << BLC_PWM_PCH_FREQ_SHIFT;
> +	else
> +		dev_priv->saveBLC_PWM_CTL = max_pwm <<
> +			BACKLIGHT_MODULATION_FREQ_SHIFT;
> +	return max_pwm;
> +}
> +
>  static u32 i915_read_blc_pwm_ctl(struct drm_i915_private *dev_priv)
>  {
>  	u32 val;
>  
> -	/* Restore the CTL value if it lost, e.g. GPU reset */
> -
> +	/* Restore the CTL value if it was lost, e.g. GPU reset */
>  	if (HAS_PCH_SPLIT(dev_priv->dev)) {
>  		val = I915_READ(BLC_PWM_PCH_CTL2);
>  		if (dev_priv->saveBLC_PWM_CTL2 == 0) {
> @@ -168,11 +191,11 @@ u32 intel_panel_get_max_backlight(struct drm_device *dev)
>  
>  	max = i915_read_blc_pwm_ctl(dev_priv);
>  	if (max == 0) {
> -		/* XXX add code here to query mode clock or hardware clock
> -		 * and program max PWM appropriately.
> -		 */
> -		printk_once(KERN_WARNING "fixme: max PWM is zero.\n");
> -		return 1;
> +		/* If backlight PWM registers have not been set, set them to */
> +		/* default backlight PWM settings. */

/* Kernel multi-line comment style
 * is like this.
 */

> +		max = i915_get_default_max_backlight(dev_priv);
> +		i915_read_blc_pwm_ctl(dev_priv);

max = i915_read_..

> +		return max;

If you return here you won't be doing the same math as below, that looks like
a bug.

>  	}
>  
>  	if (HAS_PCH_SPLIT(dev)) {


-Olof