linux-kernel - Re: [PATCH] ARM: add get_user() support for 8 byte types

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Message-Id: <201211151439.41393.arnd@arndb.de>
Date:	Thu, 15 Nov 2012 14:39:41 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Rob Clark <rob.clark@...aro.org>
Cc:	"Russell King - ARM Linux" <linux@....linux.org.uk>,
	linux-arm-kernel@...ts.infradead.org, patches@...aro.org,
	linux-kernel@...r.kernel.org, linux-omap@...r.kernel.org,
	dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH] ARM: add get_user() support for 8 byte types

On Thursday 15 November 2012, Rob Clark wrote:
> > I still haven't heard a conclusive argument why we need to use get_user()
> > rather than copy_from_user() in the DRM code. Is this about a fast path
> > where you want to shave off a few cycles for each call, or does this
> > simplify the code structure, or something else?
> 
> well, it is mostly because it seemed like a good idea to first try to
> solve the root issue, rather than having to fix things up in each
> driver when someone from x86-world introduces a 64b get_user()..

As pointed out by hpa earlier, x86-32 doesn't have a 64b get_user
either. I don't think we have a lot of drivers that are used only
on 64-bit x86 and on 32-bit ARM but not on 32-bit x86.

> There are some other arch's that don't have 64b get_user(), but I
> don't think any that have any DRM drivers.  If 64b get_user() is
> really not intended to be supported by all archs, it is better to
> remove it from x86 and the other arch's that do currently support it,
> rather than making it possible to write drivers that are broken on
> some archs.

The majority of architectures we support have PCI and should be able
to build the regular (radeon, nouveau, MGA, VIA, ...) DRM drivers
AFAICT.

	Arnd
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