[PATCH] io: prevent compiler reordering on the default writeX() implementation

[Russell King - ARM Linux]

On Fri, Mar 30, 2018 at 10:29:58AM -0400, Sinan Kaya wrote:
> The default implementation of mapping writeX() to __raw_writeX() is wrong.
> writeX() has stronger ordering semantics. Compiler is allowed to reorder
> __raw_writeX().
> 
> In the abscence of a write barrier or when using a strongly ordered
> architecture, writeX() should at least have a compiler barrier in
> it to prevent commpiler from clobbering the execution order.

You want the barrier _before_ the call to __raw_writel() - you need to
ensure that writes to memory are emitted by the compiler _before_ the
write to the hardware - the write to the hardware may start DMA, and it
may be reading data that the program thinks it previously wrote.

Similarly, for readl(), you need the barrier after __raw_readl() to
ensure that other reads in the program aren't scheduled before a
potential DMA status register read.

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