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

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Date:	Wed, 07 May 2014 09:20:17 +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

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 :)

Regards,
Jacek Anaszewski


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