linux-kernel - Re: [PATCH] ARM: Gemini: Add support for PCI BUS

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Message-ID: <4CF3F687.6040801@gmail.com>
Date:	Mon, 29 Nov 2010 20:52:55 +0200
From:	Paulius Zaleckas <paulius.zaleckas@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	linux-arm-kernel@...ts.infradead.org,
	Hans Ulli Kroll <ulli.kroll@...glemail.com>,
	Russell King <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: Gemini: Add support for PCI BUS

On 11/29/2010 06:45 PM, Arnd Bergmann wrote:
> On Monday 29 November 2010, Paulius Zaleckas wrote:
>>> The I/O ordering is probably not what you think it is.
>>> There is no ordering guarantee between __raw_writel and
>>> spin_lock/spin_unlock, so you really should be using
>>> readl/writel.
>>
>> No he really should NOT use readl/writel. The ONLY difference
>> between readl/writel and __raw_readl/__raw_writel is endianess
>> conversion. __raw_*l is not doing it. Which to use depend only
>> on HW.
>
> There are many differences between readl and __raw_readl, including
>
> * __raw_readl does not have barriers and does not serialize with
>    spinlocks, so it breaks on out-of-order CPUs.
> * __raw_readl does not have a specific endianess, while readl is
>    fixed little-endian, just as the hardware is in this case.
>    The endian-conversion is a NOP on little-endian ARM, but required
>    if you actually run on a big-endian ARM (you don't).
> * __raw_readl may not be atomic, gcc is free to split the access
>    into byte wise reads (it normally does not, unless you mark
>    the pointer __attribute__((packed))).
>
> In essence, it is almost never a good idea to use __raw_readl, and
> the double underscores should tell you so.

You are wrong:

Since CONFIG_ARM_DMA_MEM_BUFFERABLE is NOT defined for FA526 core,
no barriers are in use when using readl. It just translates into
le32_to_cpu(__raw_readl(x)). Now this CPU has physical pin for endianess
configuration and if you will chose big-endian you will fail to read
internal registers, because they ALSO change endianess and le32_to_cpu()
will screw it. However it is different when accessing registers through
PCI bus, then you need to use readl().
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