linux-kernel - Re: [PATCH] macintosh: Explicitly set llseek to no

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Date:	Mon, 16 Nov 2009 13:09:47 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Christoph Hellwig <hch@....de>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Arnd Bergmann <arndbergmann@...glemail.com>,
	Jonathan Corbet <corbet@....net>,
	LKML <linux-kernel@...r.kernel.org>, linuxppc-dev@...abs.org,
	John Kacur <jkacur@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [PATCH] macintosh: Explicitly set llseek to no_llseek in ans-lcd

On Monday 16 November 2009, Christoph Hellwig wrote:
> As mentioned before making generic_file_llseek the new default is
> probably a bad idea.  The majority of our file_operations instances
> don't actually support seeking, so no_llseek should become the new
> default if you spend some effort on converting things.  Anything that
> wants to allow seeking will have to set a llseek method.  This also
> mirrors what we do for other file operations.  None of the major ones
> has a non-trivial default, it's either silently succeeding for a
> selected few like open or release or returning an error for operatings
> that actually do something like read and write.

Ok, good point.

Do you think we should also prevent pread/pwrite for devices without
an llseek operation, like nonseekable_open does? I guess that would
be consistent.

Then there is the point that (I forgot who) brought up that changing
code to do no_llseek is actually an ABI change. Even if the file
position is never used anywhere, some random user application might
expect a chardev not to return an error when its llseek method is
called, resulting in regressions.

	Arnd <><
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