linux-kernel - Re: [PATCH v4 04/47] soc: fsl: qe: introduce qe

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Date:   Wed, 13 Nov 2019 23:08:28 -0600
From:   Timur Tabi <timur@...nel.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Qiang Zhao <qiang.zhao@....com>, Li Yang <leoyang.li@....com>,
        Christophe Leroy <christophe.leroy@....fr>,
        Scott Wood <oss@...error.net>, linuxppc-dev@...ts.ozlabs.org,
        lkml <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v4 04/47] soc: fsl: qe: introduce qe_io{read,write}*
 wrappers

On 11/12/19 1:14 AM, Rasmus Villemoes wrote:
> but that's because readl and writel by definition work on little-endian
> registers. I.e., on a BE platform, the readl and writel implementation
> must themselves contain a swab, so the above would end up doing two
> swabs on a BE platform.

Do you know whether the compiler optimizes-out the double swab?

> (On PPC, there's a separate definition of mmio_read32be, namely
> writel_be, which in turn does a out_be32, so on PPC that doesn't
> actually end up doing two swabs).
> 
> So ioread32be etc. have well-defined semantics: access a big-endian
> register and return the result in native endianness.

It seems weird that there aren't any cross-arch lightweight 
endian-specific I/O accessors.