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

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Message-ID: <CAK8P3a1doCAdW4-=C0rKAvm+LUwsZyJUTOqz_JafWrFWzADiyw@mail.gmail.com>
Date:   Mon, 23 Apr 2018 13:47:41 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
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()

On Mon, Apr 23, 2018 at 11:47 AM, Geert Uytterhoeven
<geert@...ux-m68k.org> wrote:
> 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?

Ok, so for the suspend case on m68k, can can totally ignore
read_persistent_clock64(): classic doesn't have suspend and
coldfire doesn't implement mach_hwclock() callbacks, right?

For reference, this is the set of machines implementing mach_hwclk:

arch/m68k/68000/m68328.c:  mach_hwclk = m68328_hwclk;
arch/m68k/68000/m68EZ328.c:  mach_hwclk = m68328_hwclk;
arch/m68k/68000/m68VZ328.c:     mach_hwclk = m68328_hwclk;
arch/m68k/apollo/config.c:      mach_hwclk           = dn_dummy_hwclk; /* */
arch/m68k/atari/config.c:               mach_hwclk = atari_tt_hwclk;
arch/m68k/atari/config.c:               mach_hwclk = atari_mste_hwclk;
arch/m68k/bvme6000/config.c:    mach_hwclk           = bvme6000_hwclk;
arch/m68k/hp300/config.c:       mach_hwclk           = hp300_hwclk;
arch/m68k/mac/config.c: mach_hwclk = mac_hwclk;
arch/m68k/mvme147/config.c:     mach_hwclk              = mvme147_hwclk;
arch/m68k/mvme16x/config.c:    mach_hwclk           = mvme16x_hwclk;
arch/m68k/q40/config.c: mach_hwclk = q40_hwclk;
arch/m68k/sun3/config.c:        mach_hwclk           =  sun3_hwclk;
arch/m68k/sun3x/config.c:       mach_hwclk           = sun3x_hwclk;

>> 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.

Ok. In that case, a possible cleanup for the old time handling
could be to move each of the hwclk implementations over to use
their own RTC driver instead of a mach_hwclk function with
generic_rtc.

It seems that the mach_set_ss and nach_set_mmss
callbacks can simply get removed already, they are
never called. Similarly, the mach_get_rtc_pll() and
mach_get_rtc_pll() callbacks are only used on q40, and
could be removed after changing the q40 over to using
its own RTC driver, or registering the ioctl operation itself.

      Arnd