linux-kernel - Re: [PATCH] m68k: Remove read_persistent

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Message-ID: <CAMuHMdXPnKGJYLPm-DrmtMiJ30z0f=+5EJMp3NuuibuP_3A-3w@mail.gmail.com>
Date:   Mon, 23 Apr 2018 11:47:35 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Baolin Wang <baolin.wang@...aro.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Mark Brown <broonie@...nel.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Finn Thain <fthain@...egraphics.com.au>
Subject: Re: [PATCH] m68k: Remove read_persistent_clock()

Hi Arnd,

On Mon, Apr 23, 2018 at 11:28 AM, Arnd Bergmann <arnd@...db.de> wrote:
> On Mon, Apr 23, 2018 at 11:07 AM, Geert Uytterhoeven
> <geert@...ux-m68k.org> wrote:
>> On Fri, Apr 20, 2018 at 5:22 PM, Arnd Bergmann <arnd@...db.de> wrote:
>>> On Thu, Apr 19, 2018 at 8:22 AM, Baolin Wang <baolin.wang@...aro.org> wrote:
>>>> The read_persistent_clock() uses a timespec, which is not year 2038 safe
>>>> on 32bit systems. Moreover on m68k architecture, we have implemented generic
>>>> RTC drivers that can be used to compensate the system suspend time. So
>>>> we can remove the obsolete read_persistent_clock().
>>>>
>>>> Signed-off-by: Baolin Wang <baolin.wang@...aro.org>
>>>
>>> I'm not sure about this one: it's still possible to turn off
>>> CONFIG_RTC_DRV_GENERIC
>>> on M68KCLASSIC, and in that case we still need a read_persistent_clock64()
>>> implementation. Or we use your patch but make CONFIG_RTC_DRV_GENERIC
>>> mandatory.
>>
>> Typically (in the defconfigs) CONFIG_RTC_DRV_GENERIC is either "m",
>> or not set.
>>
>> And in both cases, a platform-specific RTC class driver may or may not be
>> builtin or loaded. We have them for some Amigas (RTC_DRV_MSM6242 or
>> RTC_DRV_RP5C01).
>>
>> I've never been an expert of timekeeping code...
>
> Some extra background on this:
>
> read_persistent_clock64/read_persistent_clock is primarily needed when you
> have a real time source that is better than the one exposed in the RTC class
> driver.  For platforms doing suspend/resume, the timekeeping code will
> first try calling read_persistent_clock64() but fall back to the RTC
> if that fails.
> On only a few architectures like m68k, read_persistent_clock64() falls
> back to reading the RTC hardware, which means it will still work even if
> the RTC_DRV_GENERIC driver is not loaded. Removing that code will
> still work correctly as long as the generic RTC driver does get loaded
> before suspend. On platforms without suspend/resume, none of this matters.

M68k-with-MMU[*] does not support suspend/resume.

[*] Probably the PM dependency should be updated for Coldfire with MMU?

> The other user of read_persistent_clock() is to set the initial time at boot.
> This again can be done by the RTC subsystem if the correct RTC driver
> is built-in and CONFIG_RTC_HCTOSYS is set. Alexandre is planning
> to remove that option and instead force early user space to load the
> RTC driver and then sync it manually using the sysfs knob for it.
>
> If you have ancient user space that doesn't do this, you might still
> rely on read_persistent_clock() to set the boot time.

Yeah, we have some ancient userspace (old ramdisk from just after the
a.out to ELF switch ;-), which worked fine last time I tried ;-)

But this should not be considered a dependency, as most people will
run e.g. Debian/ports, which I assume is a modern userspace.

>> Should we get rid of ARCH_USES_GETTIMEOFFSET?
>> it seems m68k and two ARM platforms are the last users.
>> What needs to be done?
>
> This is entirely unrelated, but generally speaking it would be
> great to convert m68k away from ARCH_USES_GETTIMEOFFSET,
> yes.
>
> The first step would be to get rid of all the dummy gettimeoffset() functions
> that return a constant (like dn_gettimeoffset()).
>
> The larger part of that effort would be to turn each clock implementation
> in m68k into a real clocksource driver. This is probably straightforward,
> but I don't know the details (adding LinusW to Cc, he's done this on
> some ARM hardware in the past).

OK, more work to do...

>>> See below for a version I did a while ago (but never submitted as I got
>>> distracted).
>>
>> Thanks, I can apply it, if it is deemed correct ;-)
>
> Please apply Finn's bugfix for the month-off bug first, that one is more
> urgent. I've rebased my patch on top of his and resent that one.

Sure. Will do. Thanks!

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