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

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Date:   Thu, 14 Nov 2019 09:55:23 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Timur Tabi <timur@...nel.org>
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 14/11/2019 06.08, Timur Tabi wrote:
> 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?
>

Depends. It's almost impossible to figure out how swab32() is defined,
so how much visibility gcc has into how it works is hard to say. But a
further complication is that the arch may not have, say (simplifying
somewhat)

#define readl(x) swab32(*(volatile u32*)x)

but instead have readl implemented as inline asm which includes the
byteswap. PPC being a case in point, where the readl is in_le32 which is
done with a lwbrx instruction, and certainly gcc couldn't in any way
change a swab32(asm("lwbrx")) into asm("lwz"). But ppc defines its own
mmio_read32be, so that's not an issue.

>> (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.

Agreed, but I'm really not prepared for trying to go down that rabbit
hole again.

Rasmus