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Date:   Thu, 20 Jan 2022 18:09:04 +0100
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To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Alessandro Zummo <a.zummo@towertech.it>,
        linux-rtc@vger.kernel.org, John Stultz <john.stultz@linaro.org>,
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        Jiaxun Yang <jiaxun.yang@flygoat.com>,
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        Thomas Gleixner <tglx@linutronix.de>,
        Stephen Boyd <sboyd@kernel.org>,
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References: <20220120080347.1595379-1-laurent@vivier.eu>
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 <CAK8P3a1oN8NrUjkh2X8jHQbyz42Xo6GSa=5n0gD6vQcXRjmq1Q@mail.gmail.com>
From:   Laurent Vivier <laurent@vivier.eu>
Subject: Re: [PATCH v11 2/5] tty: goldfish: introduce
 gf_ioread32()/gf_iowrite32()
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Le 20/01/2022 à 09:50, Arnd Bergmann a écrit :
> On Thu, Jan 20, 2022 at 9:03 AM Laurent Vivier <laurent@vivier.eu> wrote:
>>
>> Revert
>> commit da31de35cd2f ("tty: goldfish: use __raw_writel()/__raw_readl()")
>>
>> to use accessors defined by the architecture.
>>
>> Define by default the accessor to be little-endian as we
>> have only little-endian architectures using goldfish devices.
>>
>> Signed-off-by: Laurent Vivier <laurent@vivier.eu>
> 
> The patch looks good, but the description seems wrong to me:
> 
> Talking about "little-endian architectures" makes no sense here, the
> point is that the device was clearly defined as having little-endian
> registers, and your earlier patch broke this driver when running
> on big-endian kernels (if anyone ever tried this).
To explain why I did that:

The reference document[1] doesn't define the endianness of goldfish.

In QEMU, goldfish devices are defined with the DEVICE_NATIVE_ENDIAN flag [2], that means all the 
target architectures defined in QEMU with TARGET_WORDS_BIGENDIAN will present them as big-endian 
devices, the others as little-endian devices.

According to TARGET_WORDS_BIGENDIAN definition:

On the following QEMU target architectures (qemu-system-XXX), goldfish devices must be accessed with 
big-endian read/write:

mips, mips64, s390x, sparc, sparc64, or1k, m68k, ppc, ppc64, xtensaeb, hppa, sh4eb, microblaze

On the following QEMU target architectures, goldfish devices must be accessed with little-endian 
read/write:

arm, aarch64, alpha, avr, cris, i386, x86_64, microblazeel, mipsel, mips64el, nios2, riscv32, 
riscv64, rx, sh4, tricore, xtensa

Thanks,
Laurent

[1] https://android.googlesource.com/platform/external/qemu/+/master/docs/GOLDFISH-VIRTUAL-HARDWARE.TXT
[2] 
https://android.googlesource.com/platform/external/qemu/+/refs/heads/emu-master-dev/hw/char/goldfish_tty.c#222