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

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Date:	Tue, 6 May 2014 12:10:59 +0300
From:	Sakari Ailus <sakari.ailus@....fi>
To:	Jacek Anaszewski <j.anaszewski@...sung.com>
Cc:	linux-media@...r.kernel.org, linux-leds@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	s.nawrocki@...sung.com, a.hajda@...sung.com,
	kyungmin.park@...sung.com
Subject: Re: [PATCH/RFC v3 5/5] media: Add registration helpers for V4L2
 flash sub-devices

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 .

-- 
Regards,

Sakari Ailus
e-mail: sakari.ailus@....fi	XMPP: sailus@...iisi.org.uk
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