[libcamera-devel] [PATCH v5 5/6] v4l2-subdev: add VIDIOC_SUBDEV_QUERYCAP ioctl

[Hans Verkuil]

On 29/04/2020 10:18, Sakari Ailus wrote:
> Hi Jacopo,
> 
> On Wed, Apr 29, 2020 at 10:09:49AM +0200, Jacopo Mondi wrote:
>> Hi Sakari,
>>
>> On Wed, Apr 29, 2020 at 12:28:58AM +0300, Sakari Ailus wrote:
>>> Hi Jacopo,
>>>
>>> On Tue, Apr 28, 2020 at 11:06:08PM +0200, Jacopo Mondi wrote:
>>>> From: Hans Verkuil <hans.verkuil at cisco.com>
>>>>
>>>> While normal video/radio/vbi/swradio nodes have a proper QUERYCAP ioctl
>>>> that apps can call to determine that it is indeed a V4L2 device, there
>>>> is currently no equivalent for v4l-subdev nodes. Adding this ioctl will
>>>> solve that, and it will allow utilities like v4l2-compliance to be used
>>>> with these devices as well.
>>>>
>>>> SUBDEV_QUERYCAP currently returns the version and subdev_caps of the
>>>> subdevice. Define as the initial set of subdev_caps the read-only or
>>>> read/write flags, to signal to userspace which set of IOCTLs are
>>>> available on the subdevice.
>>>>
>>>> Signed-off-by: Hans Verkuil <hans.verkuil at cisco.com>
>>>> Signed-off-by: Jacopo Mondi <jacopo at jmondi.org>
>>>> ---
>>>>  drivers/media/v4l2-core/v4l2-subdev.c | 12 ++++++++++++
>>>>  include/uapi/linux/v4l2-subdev.h      | 15 +++++++++++++++
>>>>  2 files changed, 27 insertions(+)
>>>>
>>>> diff --git a/drivers/media/v4l2-core/v4l2-subdev.c b/drivers/media/v4l2-core/v4l2-subdev.c
>>>> index f3fe515b8ccb..b8c0071aa4d0 100644
>>>> --- a/drivers/media/v4l2-core/v4l2-subdev.c
>>>> +++ b/drivers/media/v4l2-core/v4l2-subdev.c
>>>> @@ -15,6 +15,7 @@
>>>>  #include <linux/types.h>
>>>>  #include <linux/videodev2.h>
>>>>  #include <linux/export.h>
>>>> +#include <linux/version.h>
>>>>
>>>>  #include <media/v4l2-ctrls.h>
>>>>  #include <media/v4l2-device.h>
>>>> @@ -331,6 +332,17 @@ static long subdev_do_ioctl(struct file *file, unsigned int cmd, void *arg)
>>>>  	int rval;
>>>>
>>>>  	switch (cmd) {
>>>> +	case VIDIOC_SUBDEV_QUERYCAP: {
>>>> +		struct v4l2_subdev_capability *cap = arg;
>>>> +
>>>> +		memset(cap, 0, sizeof(*cap));
>>>> +		cap->version = LINUX_VERSION_CODE;
>>>> +		cap->subdev_caps |= ro_subdev ? V4L2_SUBDEV_CAP_RO_SUBDEV
>>>> +					      : V4L2_SUBDEV_CAP_RW_SUBDEV;
>>>> +
>>>> +		return 0;
>>>> +	}
>>>> +
>>>>  	case VIDIOC_QUERYCTRL:
>>>>  		/*
>>>>  		 * TODO: this really should be folded into v4l2_queryctrl (this
>>>> diff --git a/include/uapi/linux/v4l2-subdev.h b/include/uapi/linux/v4l2-subdev.h
>>>> index 03970ce30741..89dc8f2ba6b3 100644
>>>> --- a/include/uapi/linux/v4l2-subdev.h
>>>> +++ b/include/uapi/linux/v4l2-subdev.h
>>>> @@ -155,9 +155,24 @@ struct v4l2_subdev_selection {
>>>>  	__u32 reserved[8];
>>>>  };
>>>>
>>>> +/**
>>>> + * struct v4l2_subdev_capability - subdev capabilities
>>>> + * @device_caps: the subdev capabilities, see V4L2_SUBDEV_CAP_*.
>>>> + */
>>>> +struct v4l2_subdev_capability {
>>>> +	__u32 version;
>>>> +	__u32 subdev_caps;
>>>
>>> How do you intend to address additional fields being added to the struct in
>>> the future? Something else than what's been done in V4L2 traditionally?
>>>
>>
>> I'm not sure I get what you mean here, so I assume I don't know what
>> "has been done in V4L2 traditionally" and why what I have here goes
>> against it...
> 
> I can't help noticing you have no reserved fields in your IOCTL argument
> struct. That has generally been the way V4L2 IOCTLs have been extended when
> there's been a need to.
> 
> Media controller adopted a different approach to that recently, based on
> the argument size. We've discussed doing that on V4L2 but I don't think
> a decision has been made.
> 

While I agree that using the argument size to do 'versioning' of the API
is a better solution, the fact is that historically we always used a 'reserved'
field. And I think we need to do that here as well. It's consistent with
the existing subdev ioctls, so I would be in favor of adding a 'u32 reserved[6];'
field.

If there are such strong feelings against it that it wouldn't be merged, then
we can always just leave it as-is. It's not worth blocking this patch just
because of that.

BTW, one thing that should be changed is the name 'subdev_caps': just name it
'capabilities'. It's a field in a struct v4l2_subdev_capability, so it is
already obvious that this is subdev specific.

Regards,

	Hans