Re: [PATCH] staging/vc04_services/bcm2835-camera: distinct numeration and names for devices

[Hans Verkuil <hverkuil-cisco@xs4all.nl>]

On 1/10/20 3:35 PM, Dave Stevenson wrote:
> Hi Hans
> 
> On Fri, 10 Jan 2020 at 13:25, Hans Verkuil <hverkuil-cisco@xs4all.nl> wrote:
>>
>> Hi Michael, Kay,
>>
>> On 12/6/19 9:54 AM, Michael Kupfer wrote:
>>> Create a static atomic counter for numerating cameras.
>>> Use the Media Subsystem Kernel Internal API to create distinct
>>> device-names, so that the camera-number (given by the counter)
>>> matches the camera-name.
>>>
>>> Co-developed-by: Kay Friedrich <kay.friedrich@fau.de>
>>> Signed-off-by: Kay Friedrich <kay.friedrich@fau.de>
>>> Signed-off-by: Michael Kupfer <michael.kupfer@fau.de>
>>> ---
>>>  .../vc04_services/bcm2835-camera/bcm2835-camera.c        | 9 ++++++---
>>>  1 file changed, 6 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/staging/vc04_services/bcm2835-camera/bcm2835-camera.c b/drivers/staging/vc04_services/bcm2835-camera/bcm2835-camera.c
>>> index beb6a0063bb8..be5f90a8b49d 100644
>>> --- a/drivers/staging/vc04_services/bcm2835-camera/bcm2835-camera.c
>>> +++ b/drivers/staging/vc04_services/bcm2835-camera/bcm2835-camera.c
>>> @@ -60,6 +60,9 @@ MODULE_PARM_DESC(max_video_width, "Threshold for video mode");
>>>  module_param(max_video_height, int, 0644);
>>>  MODULE_PARM_DESC(max_video_height, "Threshold for video mode");
>>>
>>> +/* camera instance counter */
>>> +static atomic_t camera_instance = ATOMIC_INIT(0);
>>> +
>>>  /* global device data array */
>>>  static struct bm2835_mmal_dev *gdev[MAX_BCM2835_CAMERAS];
>>>
>>> @@ -1870,7 +1873,6 @@ static int bcm2835_mmal_probe(struct platform_device *pdev)
>>>
>>>               /* v4l2 core mutex used to protect all fops and v4l2 ioctls. */
>>>               mutex_init(&dev->mutex);
>>> -             dev->camera_num = camera;
>>>               dev->max_width = resolutions[camera][0];
>>>               dev->max_height = resolutions[camera][1];
>>>
>>> @@ -1886,8 +1888,9 @@ static int bcm2835_mmal_probe(struct platform_device *pdev)
>>>               dev->capture.fmt = &formats[3]; /* JPEG */
>>>
>>>               /* v4l device registration */
>>> -             snprintf(dev->v4l2_dev.name, sizeof(dev->v4l2_dev.name),
>>> -                      "%s", BM2835_MMAL_MODULE_NAME);
>>> +             dev->camera_num = v4l2_device_set_name(&dev->v4l2_dev,
>>> +                                                    BM2835_MMAL_MODULE_NAME,
>>> +                                                    &camera_instance);
>>>               ret = v4l2_device_register(NULL, &dev->v4l2_dev);
>>>               if (ret) {
>>>                       dev_err(&pdev->dev, "%s: could not register V4L2 device: %d\n",
>>>
>>
>> Actually, in this specific case I would not use v4l2_device_set_name().
>>
>> Instead just use:
>>
>>                 snprintf(dev->v4l2_dev.name, sizeof(dev->v4l2_dev.name),
>>                          "%s-%u", BM2835_MMAL_MODULE_NAME, camera);
>>
>> It would be even better if there would be just one top-level v4l2_device used
>> for all the camera instances. After all, there really is just one platform
>> device for all of the cameras, and I would expect to see just a single
>> v4l2_device as well.
>>
>> It doesn't hurt to have multiple v4l2_device structs, but it introduces a
>> slight memory overhead since one would have been sufficient.
> 
> Doesn't that make all controls for all cameras common? The struct
> v4l2_ctrl_handler is part of struct v4l2_device.
> 
> Or do we:
> - ditch the use of ctrl_handler in struct v4l2_device
> - create and initialise a ctrl_handler per camera on an internal
> structure so we retain the control state
> - assign ctrl_handler in struct v4l2_fh to it every time a file handle
> on the device is opened?

You can set ctrl_handlers at three levels: v4l2_device (common to all
video devices), video_device (specific to a single video device), or
for v4l2_fh (specific to that filehandle only).

In this case you would assign it to the video_device:

	dev->vdev.ctrl_handler = &dev->ctrl_handler;

> 
> And if we only have one struct v4l2_device then is there the
> possibility of the unique names that Michael and Kay are trying to
> introduce?
> 
> I'm a little confused as to whether there really is a gain in having a
> single v4l2_device. In this case the two cameras are independent
> devices, even if they are loaded by a single platform driver.

There probably is little to no gain. Hence my proposal to just stick the
camera index after the module name.

Regards,

	Hans

> 
>   Dave
> 
>> v4l2_device_set_name() is meant for pci-like devices. And it really
>> is a bit overkill to have it as a helper function.
>>
>> Regards,
>>
>>         Hans