From: "André Przywara" <andre.przywara@arm.com>
To: u-boot@lists.denx.de
Subject: [U-Boot] [PATCH 2/3] arm: introduce _relaxed MMIO accessors
Date: Wed, 6 Feb 2019 21:53:44 +0000 [thread overview]
Message-ID: <d0f39237-7c01-6b7a-e046-90bc83ad4fb5@arm.com> (raw)
In-Reply-To: <B1B71DAB-A3D8-498C-8C21-08BE38080E65@theobroma-systems.com>
On 06/02/2019 12:46, Philipp Tomsich wrote:
> On 11.01.2019, at 01:31, Andre Przywara <andre.przywara@arm.com> wrote:
Hi,
>>
>> 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@theobroma-systems.com>
> [On my experimental RK3399 after modifying a few drivers:]
> Tested-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Cool, many thanks for that! Out of curiosity, did you really need
*_relaxed for some reason?
Cheers,
Andre.
next prev parent reply other threads:[~2019-02-06 21:53 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-11 0:31 [U-Boot] [PATCH 0/3] arm: Introduce writel/readl_relaxed accessors Andre Przywara
2019-01-11 0:31 ` [U-Boot] [PATCH 1/3] arm: clean up asm/io.h Andre Przywara
2019-02-06 12:49 ` Philipp Tomsich
2019-01-11 0:31 ` [U-Boot] [PATCH 2/3] arm: introduce _relaxed MMIO accessors Andre Przywara
2019-02-06 12:46 ` Philipp Tomsich
2019-02-06 21:53 ` André Przywara [this message]
2019-02-07 0:32 ` Philipp Tomsich
2019-01-11 0:31 ` [U-Boot] [PATCH 3/3] sunxi: H6: use writel_relaxed for DRAM timing register accesses Andre Przywara
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