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Subject: Re: [PATCH] staging/vc04_services/bcm2835-camera: distinct numeration
 and names for devices
To:     Michael Kupfer <michael.kupfer@fau.de>, eric@anholt.net,
        wahrenst@gmx.net, bcm-kernel-feedback-list@broadcom.com,
        linux-rpi-kernel@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org, mchehab+samsung@kernel.org,
        linux-media@vger.kernel.org
Cc:     gregkh@linuxfoundation.org, f.fainelli@gmail.com,
        rjui@broadcom.com, sbranden@broadcom.com,
        dave.stevenson@raspberrypi.org, daniela.mormocea@gmail.com,
        devel@driverdev.osuosl.org, linux-kernel@vger.kernel.org,
        linux-kernel@i4.cs.fau.de, Kay Friedrich <kay.friedrich@fau.de>
References: <20191206085432.19962-1-michael.kupfer@fau.de>
From:   Hans Verkuil <hverkuil-cisco@xs4all.nl>
Message-ID: <3db2350b-0a6d-0693-258c-9d47f71c0627@xs4all.nl>
Date:   Fri, 10 Jan 2020 14:25:15 +0100
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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.

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