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

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Message-Id: <201106262020.20432.arnd@arndb.de>
Date:	Sun, 26 Jun 2011 20:20:20 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Mauro Carvalho Chehab <mchehab@...hat.com>
Cc:	Sakari Ailus <sakari.ailus@....fi>,
	Linux Media Mailing List <linux-media@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] [media] v4l2 core: return -ENOIOCTLCMD if an ioctl doesn't exist

On Sunday 26 June 2011 19:30:46 Mauro Carvalho Chehab wrote:
> > There was a lot of debate whether undefined ioctls on non-ttys should
> > return -EINVAL or -ENOTTY, including mass-conversions from -ENOTTY to
> > -EINVAL at some point in the pre-git era, IIRC.
> > 
> > Inside of v4l2, I believe this is handled by video_usercopy(), which
> > turns the driver's -ENOIOCTLCMD into -ENOTTY. What cases do you observe
> > where this is not done correctly and we do return ENOIOCTLCMD to
> > vfs_ioctl?
> 
> Well, currently, it is returning -EINVAL maybe due to the mass-conversions
> you've mentioned.

I mean what do you return *to* vfs_ioctl from v4l? The conversions must
have been long before we introduced compat_ioctl and ENOIOCTLCMD.

As far as I can tell, video_ioctl2 has always converted ENOIOCTLCMD into
EINVAL, so changing the vfs functions would not have any effect.

> The point is that -EINVAL has too many meanings at V4L. It currently can be
> either that an ioctl is not supported, or that one of the parameters had
> an invalid parameter. If the userspace can't distinguish between an unimplemented
> ioctl and an invalid parameter, it can't decide if it needs to fall back to
> some different methods of handling a V4L device.
> 
> Maybe the answer would be to return -ENOTTY when an ioctl is not implemented.

That is what a lot of subsystems do these days. But wouldn't that change
your ABI?

	Arnd
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