Re: S_CTRL must be called twice to set volatile controls

[Jacek Anaszewski <j.anaszewski@samsung.com>]

Hi Sakari,

On 04/29/2015 02:01 PM, Sakari Ailus wrote:
> Hi Jacek,
>
> On Wed, Apr 29, 2015 at 10:58:20AM +0200, Jacek Anaszewski wrote:
>> Hi Hans,
>>
>> On 04/29/2015 09:53 AM, Hans Verkuil wrote:
>>> Hi Jacek,
>>>
>>> On 04/29/15 09:33, Jacek Anaszewski wrote:
>>>> Hi,
>>>>
>>>> After testing my v4l2-flash helpers patch [1] with the recent patches
>>>> for v4l2-ctrl.c ([2] and [3]) s_ctrl op isn't called despite setting
>>>> the value that should be aligned to the other step than default one.
>>>>
>>>> This happens for V4L2_CID_FLASH_TORCH_INTENSITY control with
>>>> V4L2_CTRL_FLAG_VOLATILE flag.
>>>>
>>>> The situation improves after setting V4L2_CTRL_FLAG_EXECUTE_ON_WRITE
>>>> flag for the control. Is this flag required now for volatile controls
>>>> to be writable?
>>>
>>> Yes, you need that if you want to be able to write to a volatile control.
>>>
>>> It was added for exactly that purpose.
>>
>> Thanks for the explanation.
>>
>>> Why is V4L2_CID_FLASH_TORCH_INTENSITY volatile? Volatile typically only
>>> makes sense if the hardware itself is modifying the value without the
>>> software knowing about it.
>>
>> This can be the case for the flash LED devices that can reduce torch
>> current when battery voltage level falls below predefined threshold.
>
> Can the LED flash actually tell about this, other than reading this through
> the faults?

MAX77693-flash device supports this.

> I don't know of a LED flash that could do this, so I wonder if
> we could make these controls non-volatile, as the reason I originally though
> why they were volatile (the V4L2 control framework being more or less just a
> front-end) was not a valid one.

The other reason for keeping the controls volatile is the fact that
LED subsystem has brightness_get op, which can be implemented by
drivers to report the current level actually set.

> I guess this could be done later on on top of the patch you currently have.
>


-- 
Best Regards,
Jacek Anaszewski