Re: [RFC PATCH] videodev2.h: introduce VIDIOC_DQEXTEVENT

[Hans Verkuil <hverkuil@xs4all.nl>]

On 1/28/19 10:21 AM, Sakari Ailus wrote:
> Hi Hans,
> 
> Thanks for the patch.
> 
> On Sat, Jan 26, 2019 at 12:06:19PM +0100, Hans Verkuil wrote:
>> This patch adds an extended version of VIDIOC_DQEVENT that:
>>
>> 1) is Y2038 safe by using a __u64 for the timestamp
>> 2) needs no compat32 conversion code
>> 3) is able to handle control events from 64-bit control types
>>    by changing the type of the minimum, maximum, step and default_value
>>    field to __u64
>>
>> All drivers and frameworks will be using this, and v4l2-ioctl.c would be the
>> only place where the old event ioctl and structs are used.
>>
>> Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
>> ---
>> Please let me know if there are additional requests for such a new ioctl.
>>
>> Note that I am using number 104 for the ioctl, but perhaps it would be better to
>> use an unused ioctl number like 1 or 3. There are quite a few holes in the
>> ioctl numbers. We currently have only 82 ioctls, yet are up to ioctl number 103.
>> ---
>> diff --git a/include/uapi/linux/videodev2.h b/include/uapi/linux/videodev2.h
>> index 9a920f071ff9..969e775b8c25 100644
>> --- a/include/uapi/linux/videodev2.h
>> +++ b/include/uapi/linux/videodev2.h
>> @@ -2303,6 +2303,37 @@ struct v4l2_event {
>>  	__u32				reserved[8];
>>  };
>>
>> +struct v4l2_event_ext_ctrl {
>> +	__u32 changes;
>> +	__u32 type;
>> +	union {
>> +		__s32 value;
>> +		__s64 value64;
>> +	};
>> +	__s64 minimum;
>> +	__s64 maximum;
>> +	__s64 step;
>> +	__s64 default_value;
>> +	__u32 flags;
>> +};
>> +
>> +struct v4l2_ext_event {
>> +	__u32				type;
>> +	__u32				id;
>> +	union {
>> +		struct v4l2_event_vsync		vsync;
>> +		struct v4l2_event_ext_ctrl	ctrl;
>> +		struct v4l2_event_frame_sync	frame_sync;
>> +		struct v4l2_event_src_change	src_change;
>> +		struct v4l2_event_motion_det	motion_det;
>> +		__u8				data[64];
>> +	} u;
> 
> If I'd change something in the event IOCTL, I'd probably put the reserved
> fields here. That'd allow later taking some for the use of the event data
> if needed.

Good point, I'll do that.

> I might also increase the size of the event data. 64 bytes is not that
> much. But you indeed end up copying it around all the time... So it's a
> trade-off.

I decided to leave this alone. I think by putting the reserved array after
the union (nice idea) we allow for such future extension should it be
necessary.

> 
>> +	__u64				timestamp;
>> +	__u32				pending;
>> +	__u32				sequence;
>> +	__u32				reserved[8];
>> +};
>> +
>>  #define V4L2_EVENT_SUB_FL_SEND_INITIAL		(1 << 0)
>>  #define V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK	(1 << 1)
>>
>> @@ -2475,6 +2506,7 @@ struct v4l2_create_buffers {
>>  #define VIDIOC_DBG_G_CHIP_INFO  _IOWR('V', 102, struct v4l2_dbg_chip_info)
>>
>>  #define VIDIOC_QUERY_EXT_CTRL	_IOWR('V', 103, struct v4l2_query_ext_ctrl)
>> +#define	VIDIOC_DQEXTEVENT	 _IOR('V', 104, struct v4l2_ext_event)
> 
> How do you plan to name the new buffer handling IOCTLs? I.e. with or
> without underscores around "EXT"?

It's a good question. In my old patch I named them VIDIOC_EXT_QBUF etc. See:
https://git.linuxtv.org/hverkuil/media_tree.git/commit/?h=v4l2-buffer&id=a95549df06d9900f3559afdbb9da06bd4b22d1f3

So I think I should probably rename this to VIDIOC_EXT_DQEVENT.

Alternatively, perhaps we should ditch the _ext_ usage and instead use a
version suffix: VIDIOC_DQEVENT_V2.

The problem with EXT is that if you want to make a newer version of such a
control, you can't just name it EXT_EXT, that would be silly. But naming it
_V3 would be fine.

Frankly, the extended control ioctls have that problem, also due to awful
64 bit alignment issues. It would be really nice to have _V3 versions of
those ioctls that do not require compat32 code.

Feedback on this would be very welcome!

Regards,

	Hans

> 
>>
>>  /* Reminder: when adding new ioctls please add support for them to
>>     drivers/media/v4l2-core/v4l2-compat-ioctl32.c as well! */
>