linux-kernel - Re: [PATCH v12 2/5] tty: goldfish: introduce gf_ioread32()/gf

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Date:   Wed, 26 Jan 2022 18:32:08 +0100
From:   Laurent Vivier <laurent@...ier.eu>
To:     Greg KH <greg@...ah.com>
Cc:     linux-kernel@...r.kernel.org, linux-rtc@...r.kernel.org,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-m68k@...ts.linux-m68k.org, Arnd Bergmann <arnd@...db.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        John Stultz <john.stultz@...aro.org>, stable@...r.kernel.org
Subject: Re: [PATCH v12 2/5] tty: goldfish: introduce
 gf_ioread32()/gf_iowrite32()

Le 26/01/2022 à 14:41, Greg KH a écrit :
> On Fri, Jan 21, 2022 at 09:07:35PM +0100, Laurent Vivier wrote:
>> Revert
>> commit da31de35cd2f ("tty: goldfish: use __raw_writel()/__raw_readl()")
> 
> Why?
> 
>> and define gf_ioread32()/gf_iowrite32() to be able to use accessors
>> defined by the architecture.
> 
> What does this do?
> 
>>
>> Cc: stable@...r.kernel.org # v5.11+
>> Fixes: da31de35cd2f ("tty: goldfish: use __raw_writel()/__raw_readl()")
>> Signed-off-by: Laurent Vivier <laurent@...ier.eu>
>> ---
>>   drivers/tty/goldfish.c   | 20 ++++++++++----------
>>   include/linux/goldfish.h | 15 +++++++++++----
>>   2 files changed, 21 insertions(+), 14 deletions(-)
...
>> -- 
>> 2.34.1
>>
> 
> This feels like a step backwards.  Why keep this level of indirection
> for no good reason?
> 

It was proposed by Arnd on my previous iteration of the series when I wanted to update goldfish-rtc 
in the same way:

https://lore.kernel.org/all/CAK8P3a1H6-sd_+FqnOq0Zhj=L51EWuW5VCcYeTENcp3+PkTC4Q@mail.gmail.com/

Keeping __raw_XXX() functions works on most of the cases except if the current CPU endianness can 
differ from the architecture one (like a ppc64le kernel (little-endian) running a ppc64 machine 
(big-endian)).

The best solution would be to update QEMU to set the device as a little-endian one, but google 
didn't merge its implemention in upstream QEMU and this would break all other OSes using goldfish 
devices on big-endian architectures.

Thanks,
Laurent