linux-kernel - Re: [PATCH v8 3/4] clocksource/drivers: Add a goldfish-timer clocksource

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Date:   Sun, 16 Jan 2022 12:54:04 +0100
From:   Laurent Vivier <laurent@...ier.eu>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        John Stultz <john.stultz@...aro.org>,
        linux-rtc@...r.kernel.org, Jiaxun Yang <jiaxun.yang@...goat.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>
Subject: Re: [PATCH v8 3/4] clocksource/drivers: Add a goldfish-timer
 clocksource

Le 16/01/2022 à 11:44, Arnd Bergmann a écrit :
> On Sat, Jan 15, 2022 at 8:32 PM Laurent Vivier <laurent@...ier.eu> wrote:
> 
>> +
>> +/* goldfish endianness depends on CPU endianness */
>> +#ifdef CONFIG_CPU_BIG_ENDIAN
>> +#define goldfish_ioread32 ioread32be
>> +#define goldfish_iowrite32 iowrite32be
>> +#else
>> +#define goldfish_ioread32 ioread32
>> +#define goldfish_iowrite32 iowrite32
>> +#endif
> 
> This is not what I meant here, as you are breaking big-endian support
> for all other
> architectures in the process.
> 
> On architectures that support both big-endian and little-endian kernels, devices
> (including emulated ones) can't know which type of kernel you are running, so
> this has to be fixed by architecture. Ideally this macro should be in
> an architecture
> specific header file, but you can also just make this a check for m68k and  hope
> that qemu doesn't duplicate this bug on architectures that gain support for this
> driver in the future.

Virtio has exactly the same problem in the past (prio to v1.0), and it uses __LITTLE_ENDIAN in 
virtio_legacy_is_little_endian() to know the endianness of the device.

So the code can become:

#ifdef __LITTLE_ENDIAN
#define goldfish_ioread32 ioread32
#define goldfish_iowrite32 iowrite32
#else
#define goldfish_ioread32 ioread32be
#define goldfish_iowrite32 iowrite32be
#endif

What do you prefer: CONFIG_M68K or __LITTLE_ENDIAN?

Thanks,
Laurent