linux-kernel - Re: [PATCH] dw_mmc: Add Synopsys DesignWare mmc host driver.

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Date:	Sun, 12 Dec 2010 14:47:50 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Will Newton <will.newton@...il.com>
Cc:	Chris Ball <cjb@...top.org>, Matt Fleming <matt@...sole-pimps.org>,
	linux-mmc@...r.kernel.org,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] dw_mmc: Add Synopsys DesignWare mmc host driver.

On Sun, Dec 12, 2010 at 02:31:40PM +0000, Will Newton wrote:
> On Sun, Dec 12, 2010 at 2:11 PM, Russell King - ARM Linux
> <linux@....linux.org.uk> wrote:
> > Maybe invent CONFIG_HAVE_MMIO_64BIT which architectures can select as
> > appropriate?
> 
> Wouldn't it be simpler to have a fallback readq/writeq implementation
> like the below?
> 
> static inline u64 __raw_readq(const volatile void __iomem *addr)
> {
>         return *(const volatile u64 __force *) addr;
> }
> 
> It won't break any existing hardware (if your SoC bus does not support
> 64bit accesses you are unlikely to have peripherals that require it)
> and would avoid a number of #ifdefs and/or Kconfig dependencies.

Firstly, why, then, are we discussing fixing dw_mmc.c so it builds on ARM?
Just make its configuration option conditional on !ARM and that specific
problem is solved.

Secondly, providing such a fallback implementation is asking people to
use it, and they'll expect it to work.  Their driver will build and
apparantly work, but because the compiler will use ldrd/strd for it, a
perfectly timed interrupt could cause data corruption.

It's unsafe to provide 64-bit MMIO operations on hardware which is unable
to perform 64-bit MMIO atomically - it will lead to really subtle driver
bugs.
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