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

[Sakari Ailus]

Hi Jacek,

On Fri, May 09, 2014 at 09:18:55AM +0200, Jacek Anaszewski wrote:
> 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;

V4L2 control integer values are signed, thus s32 instead of u32. Otherwise
looks good to me.

-- 
Regards,

Sakari Ailus
e-mail: sakari.ailus@xxxxxx	XMPP: sailus@xxxxxxxxxxxxxx
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