From: Arnd Bergmann <arnd@arndb.de>
To: van <van.wanless@eqware.net>
Subject: Re: File I/O from within a driver
Date: Wed, 25 May 2005 14:18:44 +0200
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On Middeweken 25 Mai 2005 07:15, van wrote:
> ?The structure of media files is complex and I'd rather the calling application
> ?didn't need to have any knowledge of that structure. ?But how can the driver
> ?do the necessary read() operations?  
> ?
> I could, for example, have the application pass an open file descriptor in to
> my driver via an ioctl() call; if I understand matters correctly, my driver
> could then call sys_read(). ?I've never done anything like that before, never
> expected to need to, and it doesn't feel right.   

_if_ you want to read the file, use fget() and vfs_read() on the file
descriptor you get passed. It is however considered rather bad style to
do file I/O from drivers. As Brian Gerst said, better use mmap in user
space and pass the pointer via ioctl() or write().

> Can anyone suggest the *proper* way to accomplish this?

Your assumption that the driver should parse the media file structure
is probably wrong. You should rather do as much as possible in a user
space library. Pass a file name to a library call and have that 
work with all the complex parts of the file format, then define an
ioctl interface for the driver on a relatively low level.

Or even better, don't use ioctl() at all but implement only read()/write()
in the driver. E.g. for MPEG acceleration, you might want to have an
interface where you write a series of macro blocks to the character
device and read back pixel data.

	Arnd <><
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