linux-kernel - Re: [PATCH v6 3/4] clocksource/drivers: Add a goldfish-timer clocksource

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Date:   Fri, 14 Jan 2022 12:03:41 +0100
From:   Laurent Vivier <laurent@...ier.eu>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        John Stultz <john.stultz@...aro.org>,
        linux-rtc@...r.kernel.org,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>
Subject: Re: [PATCH v6 3/4] clocksource/drivers: Add a goldfish-timer
 clocksource

Le 14/01/2022 à 11:46, Arnd Bergmann a écrit :
> On Thu, Jan 13, 2022 at 9:19 PM Laurent Vivier <laurent@...ier.eu> wrote:
>>
>> Add a clocksource based on the goldfish-rtc device.
>>
>> Signed-off-by: Laurent Vivier <laurent@...ier.eu>
>> ---
>>   drivers/clocksource/Kconfig          |   7 ++
>>   drivers/clocksource/Makefile         |   1 +
>>   drivers/clocksource/timer-goldfish.c | 130 +++++++++++++++++++++++++++
>>   include/clocksource/timer-goldfish.h |  12 +++
>>   4 files changed, 150 insertions(+)
>>   create mode 100644 drivers/clocksource/timer-goldfish.c
>>   create mode 100644 include/clocksource/timer-goldfish.h
>>
>> diff --git a/drivers/clocksource/Kconfig b/drivers/clocksource/Kconfig
>> index f65e31bab9ae..6ca9bb78407d 100644
>> --- a/drivers/clocksource/Kconfig
>> +++ b/drivers/clocksource/Kconfig
>> @@ -711,4 +711,11 @@ config MICROCHIP_PIT64B
>>            modes and high resolution. It is used as a clocksource
>>            and a clockevent.
>>
>> +config GOLDFISH_TIMER
>> +       bool "Clocksource using goldfish-rtc"
>> +       select RTC_CLASS
>> +       select RTC_DRV_GOLDFISH
> 
> This should probably be
> 
>            depends on M68K || COMPILE_TEST
>            depends on RTC_DRV_GOLDFISH
> 
> A driver should never 'select' another user-selectable subsystem

ok

> 
>> +static int goldfish_timer_set_oneshot(struct clock_event_device *evt)
>> +{
>> +       struct goldfish_timer *timerdrv = ced_to_gf(evt);
>> +       void __iomem *base = timerdrv->base;
>> +
>> +       __raw_writel(0, base + TIMER_ALARM_HIGH);
>> +       __raw_writel(0, base + TIMER_ALARM_LOW);
>> +       __raw_writel(1, base + TIMER_IRQ_ENABLED);
> 
> As mentioned elsewhere, the __raw_* accessors are not portable, please
> use readl()/writel() here, or possibly ioread32_be()/iowrite32_be() for
> the big-endian variant.

We can't use readl()/writel() here because it's supposed to read from a little endian device, and 
goldfish endianness depends on the endianness of the machine.

For goldfish, readl()/writel() works fine on little-endian machine but not on big-endian machine.

On m68k, you have:

#define readl(addr)      in_le32(addr)
#define writel(val,addr) out_le32((addr),(val))

and with goldfish it's wrong as the device is not little-endian, it is big-endian like the machine.

same comment with ioread32_be()/iowrite32_be(): it will work on big-endian machine not on little-endian.

We need an accessor that doesn't byteswap the value, that accesses it natively, and in all other 
parts of the kernel __raw_writel() and __raw_readl() are used.

Thanks,
Laurent