netdev - Re: [net-next PATCH v7 1/6] Documentation: ACPI: DSD: Document MDIO PHY

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Date:   Tue, 29 Sep 2020 17:29:02 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Calvin Johnson <calvin.johnson@....nxp.com>,
        Grant Likely <grant.likely@....com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Jeremy Linton <jeremy.linton@....com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Jon <jon@...id-run.com>,
        Cristi Sovaiala <cristian.sovaiala@....com>,
        Ioana Ciornei <ioana.ciornei@....com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Madalin Bucur <madalin.bucur@....nxp.com>,
        netdev <netdev@...r.kernel.org>, "linux.cj" <linux.cj@...il.com>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        nd <nd@....com>
Subject: Re: [net-next PATCH v7 1/6] Documentation: ACPI: DSD: Document MDIO PHY

On Tue, Sep 29, 2020 at 4:49 PM Andy Shevchenko
<andy.shevchenko@...il.com> wrote:
> On Tue, Sep 29, 2020 at 5:32 PM Andrew Lunn <andrew@...n.ch> wrote:
> > On Tue, Sep 29, 2020 at 04:55:40PM +0300, Andy Shevchenko wrote:
> >  Does Tianocore, or any
> > other implementations, have the needed le32_to_cpu() calls so that
> > they can boot on a big endian CPU?
>
> Not of my knowledge.

> > Is it feasible to boot an ARM system big endian?
>
> Not an ARM guy.

Most CPUs these days support both big-endian and little-endian,
and either allow code to switch between the two modes at runtime
or are stateless in the way that you have two sets of load/store
instructions, making endianness purely a compiler construct (see
also: Intel's icc compiler has a big-endian mode using the MOVBE
instruction).

For Arm kernels, we assume that the firmware is little-endian, but
you can build a big-endian kernel that switches into big-endian
mode before doing anything else. As I said, I don't think that will
ever be used with UEFI (and ACPI, by extension), since it would
be a ton of work and few users care about big-endian kernels.

    Arnd