[U-Boot] [PATCH 2/3] arm: introduce _relaxed MMIO accessors

[Philipp Tomsich]

On 11.01.2019, at 01:31, Andre Przywara <andre.przywara at arm.com> wrote:
> 
> The normal MMIO accessor macros (readX/writeX) guarantee a strong ordering,
> even with normal memory accesses [1].
> For some MMIO operations (framebuffers being a prominent example) this is
> not needed, and the rather costly barrier inserted on weakly ordered
> systems like ARM costs some performance.
> To mitigate this issue, Linux introduced readX_relaxed and
> writeX_relaxed primitives, which only guarantee ordering between each
> other, so are typically faster due to the missing barrier.
> 
> We probably do not care so much about performance in U-Boot, but want to
> have those primitives for two other reasons:
> - Being able to use the _relaxed macros simplifies porting drivers from
>  Linux.
> - The missing barrier typically allows to generate smaller code, which can
>  relieve some chronically tight SPL builds.
> 
> Add those macros definitions by using the __raw versions of the
> accessors, which matches an earlier (and less complicated) version of
> the Linux implementation.
> 
> [1] https://lwn.net/Articles/698014/

No Signed-off-by?

Reviewed-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>
[On my experimental RK3399 after modifying a few drivers:]
Tested-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>