setting volatile v4l2-control

While testing the LED / flash API integration patches I noticed that the v4l2-controls marked as volatile with V4L2_CTRL_FLAG_VOLATILE flag behave differently than I would expect.

Let's consider following use case:

There is a volatile V4L2_CID_FLASH_INTENSITY v4l2 control with following constraints:

min: 1
max: 100
step: 1
def: 1

1. Set the V4L2_CID_FLASH_INTENSITY control to 100.
- as a result s_ctrl op is called
2. Set flash_brightness LED sysfs attribute to 10.
3. Set the V4L2_CID_FLASH_INTENSITY control to 100.
- s_ctrl op isn't called

This way we are unable to write a new value to the device, despite that the related setting was changed from the LED subsystem level.

I would expect that if a control is marked volatile, then the v4l2-control framework should by default call g_volatile_ctrl op before set and not try to use the cached value.

Is there some vital reason for not doing this?

Quoted 21 lines · Jacek Anaszewski

While testing the LED / flash API integration patches I noticed that the v4l2-controls marked as volatile with V4L2_CTRL_FLAG_VOLATILE flag behave differently than I would expect.

Let's consider following use case:

There is a volatile V4L2_CID_FLASH_INTENSITY v4l2 control with following constraints:

min: 1
max: 100
step: 1
def: 1

1. Set the V4L2_CID_FLASH_INTENSITY control to 100.
- as a result s_ctrl op is called
2. Set flash_brightness LED sysfs attribute to 10.
3. Set the V4L2_CID_FLASH_INTENSITY control to 100.
- s_ctrl op isn't called

Quoted 8 lines

This way we are unable to write a new value to the device, despite that the related setting was changed from the LED subsystem level.

I would expect that if a control is marked volatile, then the v4l2-control framework should by default call g_volatile_ctrl op before set and not try to use the cached value.

Is there some vital reason for not doing this?

It's rather strange to have a writable volatile control. The semantics of this are ambiguous and I don't believe we have ever used such controls before.

Actually, the commit log of this patch (never merged) gives some background information about this:

http://git.linuxtv.org/cgit.cgi/hverkuil/media_tree.git/commit/?h=volatilefix

It's never been merged because I have never been certain how to handle such controls. Why do you have such controls in the first place? What is it supposed to do?