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

André Przywara andre.przywara at arm.com
Wed Feb 6 21:53:44 UTC 2019

On 06/02/2019 12:46, Philipp Tomsich wrote:
> 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?

Doh, indeed. Got so excited about my commit message, that I forgot the
obvious (and I only think about -s *after* hitting Enter).

> 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>

Cool, many thanks for that! Out of curiosity, did you really need
*_relaxed for some reason?


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