Re: [PATCH 2/2] mtd: orion-nand: fix build error with ARMv4

From: Arnd Bergmann <arnd@arndb.de>
To: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>,
	Ezequiel Garcia <ezequiel.garcia@free-electrons.com>,
	Jingoo Han <jg1.han@samsung.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	MTD Maling List <linux-mtd@lists.infradead.org>,
	Brian Norris <computersforpeace@gmail.com>,
	David Woodhouse <dwmw2@infradead.org>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 2/2] mtd: orion-nand: fix build error with ARMv4
Date: Wed, 14 May 2014 15:09:08 +0200	[thread overview]
Message-ID: <5493671.FYnaLOl21S@wuerfel> (raw)
In-Reply-To: <CAMuHMdXD7HhG_jGJdz8hCR847tKcRC-tQnHp=xtTDYVKBc6yQQ@mail.gmail.com>

On Wednesday 14 May 2014 14:35:28 Geert Uytterhoeven wrote:
> On Wed, May 14, 2014 at 1:47 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> >> Sure, but did anyone (Arnd?) have thoughts on a better way to do this:
> >>
> >> +#ifdef CONFIG_64BIT
> >> +               buf64[i++] = readq_relaxed(io_base);
> >> +#else
> >> +               buf64[i++] = *(const volatile u64 __force *)io_base;
> >> +#endif
> >>
> >> IMHO, readq should exist on any platform that can issue a 64 bit bus
> >> transaction, and I expect many ARM's qualify.
> >
> > Well, the original problem happened specifically for the case that doesn't
> > have a 64-bit bus transaction (building for ARMv4). If we define
> > readq_relaxed, it has to be an inline assembly, in order to work for
> > drivers that require an atomic transaction, so that would have the
> > same problem. We are a bit inconsistent here though: most 32-bit
> > architectures have no readq, parisc has one that does two 32-bit accesses,
> > sh relies on the compiler, and tile apparently has a native instruction.
> >
> > It seems reasonable to replace the inline assembly in this driver with
> > a new function as a cleanup, but then how do you want to solve the case
> > of building a combined armv4/v5 kernel?
> 
> Provide two helper functions, one for v4, one for v5, and call
> them through a function pointer that's set up during driver probe?

No, that won't help. It's a compile-time problem only: This driver
is never actually used on ARMv4, but if we build a kernel that supports
both ARMv4 and later, gcc passes -march=armv4 to the assembler, which
barfs on an invalid instruction. It would be fine to make that error
message just go away because we know it will only be used on CPUs that
do have this instruction.

	Arnd