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

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Date:	Sun, 4 Dec 2011 19:18:37 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Arnd Bergmann <arnd@...db.de>
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

Hi,

On Fri, Dec 2, 2011 at 8:33 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Friday 02 December 2011 17:12:56 Ming Lei wrote:
>> +/**
>> + * struct v4l2_fd_result - VIDIOC_G_FD_RESULT argument
>> + * @buf_index: entry, index of v4l2_buffer for face detection
>> + * @face_cnt:  return, how many faces detected from the @buf_index
>> + * @fd:                return, result of faces' detection
>> + */
>> +struct v4l2_fd_result {
>> +       __u32   buf_index;
>> +       __u32   face_cnt;
>> +       __u32   reserved[6];
>> +       struct v4l2_fd_detection *fd;
>> +};
>
>
> This data structure is not 32/64 bit safe: running a 64 bit kernel with 32 bit
> user space will see an incompatible layout.

I agree that this is not 32/64 bit safe, but I understand lib32 can handle
this correctly, otherwise many 32bit applications can't run on current
64bit kernel
since many kernel structures used by user space contained pointer,
such as struct v4l2_buffer, struct v4l2_ext_controls in v4l2 ABI.

> One way to solve this is to remove the pointer and just start the array
> directly after the __u32 members. Alternatively, you can use a __u64
> to pass the pointer in an integer representation.

So I think this need not to be solved.

>
> A nicer interface from the data structure perspective would be to
> get rid of the array altogether and always return exactly one face.

I choose to return array to user space since v4l2 ioctl has provided
this kind of support, see video_usercopy().

thanks,
--
Ming Lei
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