Messages in this thread | | | From | Arnd Bergmann <> | Subject | Re: [PATCH] ARM: Gemini: Add support for PCI BUS | Date | Mon, 29 Nov 2010 17:45:22 +0100 |
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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.
Arnd
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