Re: [PATCH/RFC v3 5/5] media: Add registration helpers for V4L2 flash sub-devices

[Jacek Anaszewski]

Hi Sakari,

On 05/07/2014 09:58 AM, Sakari Ailus wrote:
> Hi Jacek,
>
> On Wed, May 07, 2014 at 09:20:17AM +0200, Jacek Anaszewski wrote:
> > On 05/06/2014 11:10 AM, Sakari Ailus wrote:
> > > Hi Jacek,
> > >
> > > On Tue, May 06, 2014 at 08:44:41AM +0200, Jacek Anaszewski wrote:
> > > > Hi Sakari,
> > > >
> > > > On 05/02/2014 01:06 PM, Sakari Ailus wrote:
> > > >
> > > > > > > > [...]
> > > > > > > > > > +static inline enum led_brightness v4l2_flash_intensity_to_led_brightness(
> > > > > > > > > > +					struct led_ctrl *config,
> > > > > > > > > > +					u32 intensity)
> > > > > > > > >
> > > > > > > > > Fits on a single line.
> > > > > > > > >
> > > > > > > > > > +{
> > > > > > > > > > +	return intensity / config->step;
> > > > > > > > >
> > > > > > > > > Shouldn't you first decrement the minimum before the division?
> > > > > > > >
> > > > > > > > Brightness level 0 means that led is off. Let's consider following case:
> > > > > > > >
> > > > > > > > intensity - 15625
> > > > > > > > config->step - 15625
> > > > > > > > intensity / config->step = 1 (the lowest possible current level)
> > > > > > >
> > > > > > > In V4L2 controls the minimum is not off, and zero might not be a possible
> > > > > > > value since minimum isn't divisible by step.
> > > > > > >
> > > > > > > I wonder how to best take that into account.
> > > > > >
> > > > > > I've assumed that in MODE_TORCH a led is always on. Switching
> > > > > > the mode to MODE_FLASH or MODE_OFF turns the led off.
> > > > > > This way we avoid the problem with converting 0 uA value to
> > > > > > led_brightness, as available torch brightness levels start from
> > > > > > the minimum current level value and turning the led off is
> > > > > > accomplished on transition to MODE_OFF or MODE_FLASH, by
> > > > > > calling brightness_set op with led_brightness = 0.
> > > > >
> > > > > I'm not sure if we understood the issue the same way. My concern was that if
> > > > > the intensity isn't a multiple of step (but intensity - min is), the above
> > > > > formula won't return a valid result (unless I miss something).
> > > >
> > > > Please note that v4l2_flash_intensity_to_led_brightness is called only
> > > >from s_ctrl callback, and thus it expects to get the intensity aligned
> > > > to the step value, so it will always be a multiple of step.
> > > > Is it possible that s_ctrl callback would be passed a non-aligned
> > > > control value?
> > >
> > > In a nutshell: value - min is aligned but value is not. Please see
> > > validate_new() in drivers/media/v4l2-core/v4l2-ctrls.c .
> >
> > Still, to my mind, value is aligned.
> >
> > Below I execute the calculation steps one by one
> > according to the V4L2_CTRL_TYPE_INTEGER case in the
> > validate_new function:
> >
> > c->value = 35000
> >
> > val = c->value + step / 2;       // 35000 + 15625 / 2 = 42812
> > val = clamp(val, min, max);      // val = 42812
> > offset = val - min;              // 42812 - 15625 = 27187
> > offset = step * (offset / step); // 15625 * (27187 / 15625) = 15625
> > c->value = min + offset;         // 15625 + 15625 = 31250
> >
> > Value is aligned to the nearest step.
> >
> > Please spot any discrepancies in my way of thinking if there
> > are any :)
>
> min is aligned to step above. This is not necessarily the case. And if min
> is not aligned, neither is value.

Thanks for spotting this. Below are improved versions of the conversion
functions. Please let me know if you have any comments.

static inline
enum led_brightnessv4l2_flash_intensity_to_led_brightness(
                                        struct led_ctrl *config,
                                        u32 intensity)
{
        return ((intensity - config->min) / config->step) + 1;
}

static inline
u32 v4l2_flash_led_brightness_to_intensity(
                                        struct led_ctrl *config,
                                        enum led_brightness brightness)
{
        return ((brightness - 1) * config->step) + config->min;
}

Regards,
Jacek Anaszewski
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