linux-kernel - Re: [PATCH] rtc: adapt allowed RTC update error

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Date:   Wed, 02 Dec 2020 23:08:38 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Miroslav Lichvar <mlichvar@...hat.com>,
        linux-kernel@...r.kernel.org, John Stultz <john.stultz@...aro.org>,
        Prarit Bhargava <prarit@...hat.com>
Subject: Re: [PATCH] rtc: adapt allowed RTC update error

On Wed, Dec 02 2020 at 16:54, Jason Gunthorpe wrote:
> On Wed, Dec 02, 2020 at 08:21:00PM +0100, Thomas Gleixner wrote:
>> So it will not write immediately. It will run through at least one
>> retry.
>
> Right, bascially this is scheduling a WQ to do sched_sync_hw_clock()
> which will only call hrtimer_start() - seems like jsut calling
> hrtimer_start instead of queue_work above would be equivilant

Something like that.

>> I don't think the timer should be canceled if the ntp_synced() state did
>> not change. Otherwise every do_adtimex() call will cancel/restart
>> it, which does not make sense. Lemme stare at it some more.
>
> That makes sense, being conditional on the STA_UNSYNC prior to doing
> any hrtimer_start seems OK?

Yeah.
  
>> > Also x86 needs a touch, it already has RTC lib, no idea why it also
>> > provides this old path too
>> 
>> Because nobody had the stomach and/or cycles to touch it :)
>
> Hahaha yes.. I vaugely remember looking at this once..
>
> Lets see:
>
> arch/x86/kernel/kvmclock.c:     x86_platform.set_wallclock = kvm_set_wallclock;
> arch/x86/kernel/x86_init.c:             x86_platform.set_wallclock = set_rtc_noop;
> arch/x86/xen/time.c:            x86_platform.set_wallclock = xen_set_wallclock;
> arch/x86/xen/time.c:    x86_platform.set_wallclock = xen_set_wallclock;
>   All returns -ENODEV/EINVAL

You forgot to stare at the .get_wallclock() functions. That's the more
interesting part, i.e. what's behind read_persistent_clock64().

> arch/x86/kernel/x86_init.c:     .set_wallclock                  = mach_set_rtc_mmss,
>   This is already rtclib under drivers/rtc/rtc-mc146818-lib.c

That's the shared library function for setting the darn thing.
  
>   I suppose the issue here is the rtclib driver only binds via PNP and
>   very old x86 systems won't have the PNP tables? It seems doable to
>   check for a PNP device after late init and manually create a
>   platform_device for the RTC

old crap, broken BIOSes and virt. Welcome to my wonderful world :)

> arch/x86/platform/intel-mid/intel_mid_vrtc.c:   x86_platform.set_wallclock = vrtc_set_mmss;
>   This is also already in rtclib under rtc-mrst.c, and this is already
>   wired to create the rtc platform device during init
>
> So it is very close now to be able to delete all this for x86. Do you
> know of something I've missed?

Just the above :)

>> > I wonder if the cmos path could be killed off under the dead HW
>> > principle?
>> 
>> Unfortunately that code path is not that dead on x86. You need to fix
>> all the (ab)users first. :)
>
> Assuming x86 can be resolved as above, that leaves two 20 year old MIPS
> platforms and the PPC list from before. ARM is gone compared to last
> time I looked! Progress :)

Yeah. We're zooming in ....

Thanks,

        tglx