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

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Date:	Tue, 06 May 2014 08:44:41 +0200
From:	Jacek Anaszewski <j.anaszewski@...sung.com>
To:	Sakari Ailus <sakari.ailus@....fi>
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 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?

Regards,
Jacek Anaszewski

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