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Date: Tue, 07 Jun 2011 09:17:50 -0300
From: Mauro Carvalho Chehab <mchehab@redhat.com>
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To: Sakari Ailus <sakari.ailus@maxwell.research.nokia.com>
CC: Sylwester Nawrocki <snjw23@gmail.com>,
	Hans Verkuil <hverkuil@xs4all.nl>,
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	Kim HeungJun <riverful@gmail.com>, andrew.b.adams@gmail.com,
	Sung Hee Park <shpark7@stanford.edu>
Subject: Re: [RFC v4] V4L2 API for flash devices
References: <4DC2F131.6090407@maxwell.research.nokia.com> <201105071446.56843.hverkuil@xs4all.nl> <4DC5849A.9050806@gmail.com> <4DC7151E.8070601@maxwell.research.nokia.com> <4DC9A2D0.2060709@gmail.com> <4DD2DBDC.6060303@maxwell.research.nokia.com> <4DD4464C.30702@gmail.com> <4DD4D0D2.7030609@maxwell.research.nokia.com>
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Em 19-05-2011 05:12, Sakari Ailus escreveu:
> Sylwester Nawrocki wrote:

>>>> These were mostly fixed point arithmetic numbers in [32-bit numerator/
>>>> 32-bit denominator] form carrying exposure time, shutter speed, aperture,
>>>> brightness, flash, etc. information. The tags could be read from ISP after
>>>> it buffered a frame in its memory and processed it.
>>>> In case of a JPEG image format the tags can be embedded into the main
>>>> image file. But the image processors not always supported that so we used
>>>> to have an ioctl for the purpose of retrieving the metadata in user space.
>>>> In some cases it is desired to read data directly from the driver rather
>>>> than parsing a relatively large buffer.
>>>> It would be good to have a uniform interface for passing such data to
>>>> applications. I think in that particular use case a control id/value pair
>>>> sequences would do.
 
> - Which formats are your rational numbers in? A kernel interface can't
> really have floating point numbers, so there would need to be a sane way
> to pass these to user space.

The V4L2 API has support for rational numbers. The frame rate is specified as a
rational number. There's a struct for that:

struct v4l2_fract {
	__u32   numerator;
	__u32   denominator;
};

Cheers,
Mauro