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

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Date:   Fri, 14 Jan 2022 12:12:03 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Laurent Vivier <laurent@...ier.eu>
Cc:     Arnd Bergmann <arnd@...db.de>,
        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>,
        Stephen Boyd <sboyd@...nel.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>
Subject: Re: [PATCH v6 3/4] clocksource/drivers: Add a goldfish-timer clocksource

Hi Laurent,

On Fri, Jan 14, 2022 at 12:03 PM Laurent Vivier <laurent@...ier.eu> wrote:
> 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:
> >> +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.

Hence Arnd's suggestion to define custom accessors in the Goldfish
RTC driver, that map to {read,write}l() on little-endian, and to
io{read,write}32_be() on big-endian.

BTW, I'd go for io{read,write}32() on little endian instead, for
symmetry.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds