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.