linux-kernel - Re: [RFC PATCH v1 5/7] media: v4l2: introduce two IOCTLs for face detection

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Date:	Tue, 6 Dec 2011 14:41:25 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Mauro Carvalho Chehab <mchehab@...radead.org>,
	Tony Lindgren <tony@...mide.com>, Greg KH <greg@...ah.com>,
	linux-kernel@...r.kernel.org,
	Sylwester Nawrocki <snjw23@...il.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>, linux-omap@...r.kernel.org
Subject: Re: [RFC PATCH v1 5/7] media: v4l2: introduce two IOCTLs for face detection

On Tuesday 06 December 2011, Ming Lei wrote:
> > Using an array added to the end of the v4l2_fd_result structure
> > rather than a pointer would really make this easier IMHO.
> 
> I have tried to do this, but video_usercopy needs a few changes
> to handle array args if no indirect pointer is passed to kernel.

Ah, I see. Or you would have to encode the array size into the
ioctl command, which is also ugly in a different way.

> I am not sure if media guys are happy to accept the changes, :-)

Maybe Mauro can comment on which solution he prefers then, given
the choice between:

1. adding another handler in drivers/media/video/v4l2-compat-ioctl32.c

2. passing a pointer that is casted to __u64 in user space an back
   in the kernel

3. extending video_usercopy in some way to make this work, preferably
   in a generic way.

4. using a variable command number like
   #define VIDIOC_G_FD_RESULT(num)	_IOC(_IOC_READ|_IOC_WRITE,'V', 95, \
		sizeof(struct v4l2_fd_result) + (num) * sizeof(struct v4l2_fd_detection)

5. requiring the interface to be simplified to return only a single
   struct v4l2_fd_detection at a time

I agree that none of these are nice. My preferred option would be last one,
but I don't know how performance critical the interface is or if it would
cause any races that you want to avoid.

	Arnd
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