Re: [PATCH 4/4] backlight: pwm_bl: Set scale type for brightness curves specified in the DT

[Matthias Kaehlcke <mka@chromium.org>]

Hi Daniel,

On Fri, Jun 21, 2019 at 02:10:19PM +0100, Daniel Thompson wrote:
> On 13/06/2019 20:43, Matthias Kaehlcke wrote:
> > Check if a brightness curve specified in the device tree is linear or
> > not and set the corresponding property accordingly. This makes the
> > scale type available to userspace via the 'scale' sysfs attribute.
> > 
> > To determine if a curve is linear it is compared to a interpolated linear
> > curve between min and max brightness. The curve is considered linear if
> > no value deviates more than +/-5% of ${brightness_range} from their
> > interpolated value.
> > 
> > Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> > ---
> >   drivers/video/backlight/pwm_bl.c | 25 +++++++++++++++++++++++++
> >   1 file changed, 25 insertions(+)
> > 
> > diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
> > index f067fe7aa35d..912407b6d67f 100644
> > --- a/drivers/video/backlight/pwm_bl.c
> > +++ b/drivers/video/backlight/pwm_bl.c
> > @@ -404,6 +404,26 @@ int pwm_backlight_brightness_default(struct device *dev,
> >   }
> >   #endif
> > +static bool pwm_backlight_is_linear(struct platform_pwm_backlight_data *data)
> > +{
> > +	unsigned int nlevels = data->max_brightness + 1;
> > +	unsigned int min_val = data->levels[0];
> > +	unsigned int max_val = data->levels[nlevels - 1];
> > +	unsigned int slope = (100 * (max_val - min_val)) / nlevels;
> 
> Why 100 (rather than a power of 2)?

I guess it came from the decimal part of my brain, I can change it to
128 ;-)

> It would also be good to have a comment here saying what the maximum
> quantization error is. Doesn't have to be over complex just mentioning
> something like the following (assuming you agree that its true ;-) ):
> 
>   Multiplying by XXX means that even in pathalogical cases such as
>   (max_val - min_val) == nlevels then the error at max_val is less than
>   1%.

Sounds good, thanks for the suggestion!

> With a suitable comment in the fixed point code:
> Acked-by: Daniel Thompson <daniel.thompson@linaro.org>

Thanks