On Mon, May 28, 2018 at 12:30:11AM +0900, Akinobu Mita wrote:
> The open() operation for the pxa_camera driver always calls s_power()
> operation to put its subdevice sensor in normal operation mode, and the
> release() operation always call s_power() operation to put the subdevice
> in power saving mode.
>
> This requires the subdevice sensor driver to keep track of its power
> state in order to avoid putting the subdevice in power saving mode while
> the device is still opened by some users.
>
> Many subdevice drivers handle it by the boilerplate code that increments
> and decrements an internal counter in s_power() like below:
>
> /*
> * If the power count is modified from 0 to != 0 or from != 0 to 0,
> * update the power state.
> */
> if (sensor->power_count == !on) {
> ret = ov5640_set_power(sensor, !!on);
> if (ret)
> goto out;
> }
>
> /* Update the power count. */
> sensor->power_count += on ? 1 : -1;
>
> However, some subdevice drivers don't handle it and may cause a problem
> with the pxa_camera driver if the video device is opened by more than
> two users at the same time.
>
> Instead of propagating the boilerplate code for each subdevice driver
> that implement s_power, this introduces an trick that many V4L2 drivers
> are using with v4l2_fh_is_singular_file().
>
> Cc: Sakari Ailus <sakari.ai...@linux.intel.com>
> Cc: Mauro Carvalho Chehab <mche...@kernel.org>
> Signed-off-by: Akinobu Mita <akinobu.m...@gmail.com>
Acked-by: Sakari Ailus <sakari.ai...@linux.intel.com>
--
Sakari Ailus
sakari.ai...@linux.intel.com
