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

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Date:   Tue, 12 Nov 2019 08:14:20 +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 12/11/2019 06.17, Timur Tabi wrote:
> On Fri, Nov 8, 2019 at 7:03 AM Rasmus Villemoes
> <linux@...musvillemoes.dk> wrote:
>>
>> The QUICC engine drivers use the powerpc-specific out_be32() etc. In
>> order to allow those drivers to build for other architectures, those
>> must be replaced by iowrite32be(). However, on powerpc, out_be32() is
>> a simple inline function while iowrite32be() is out-of-line. So in
>> order not to introduce a performance regression on powerpc when making
>> the drivers work on other architectures, introduce qe_io* helpers.
> 
> Isn't it also true that iowrite32be() assumes a little-endian platform
> and always does a byte swap?
> 

No. You're probably thinking of the implementation in lib/iomap.c where
one has

#define mmio_read32be(addr) swab32(readl(addr))
unsigned int ioread32be(void __iomem *addr)
{
        IO_COND(addr, return pio_read32be(port), return
mmio_read32be(addr));
        return 0xffffffff;
}


#define mmio_write32be(val,port) writel(swab32(val),port)
void iowrite32be(u32 val, void __iomem *addr)
{
        IO_COND(addr, pio_write32be(val,port), mmio_write32be(val, addr));
}

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.

(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.