linux-kernel - Re: [PATCH] [media] v4l2 core: return -ENOIOCTLCMD if an ioctl doesn't exist

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Date:	Tue, 28 Jun 2011 18:50:56 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Hans Verkuil <hverkuil@...all.nl>,
	Mauro Carvalho Chehab <mchehab@...hat.com>,
	Sakari Ailus <sakari.ailus@....fi>,
	Linux Media Mailing List <linux-media@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [media] v4l2 core: return -ENOIOCTLCMD if an ioctl doesn't exist

On Tuesday 28 June 2011, Alan Cox wrote:
> > (In fact, the correct thing to do would probably be to just do
> > 
> >    #define ENOIOCTLCMD ENOTTY
> > 
> > and get rid of any translation - just giving ENOTTY a more appropriate
> > name and less chance for confusion)
> 
> Some code uses the two to separate 'the driver specific helper code
> doesn't handle this' and 'does handle this'. In that situation you take
> away the ability of a driver to override a midlayer ioctl with -ENOTTY to
> say "I don't support this even if most people do"

Right. Similarly, in compat_sys_ioctl returning -ENOIOCTLCMD from
fops->compat_ioctl means "the driver has provided no compatibility
handler for this command, need to check the global translation table",
while -ENOTTY returned from ->compat_ioctl means "this command won't
work on this device, don't bother looking at the table and don't
print an annoying message".

	Arnd
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