Re: [PATCH] rtc: class: support hctosys from modular RTC drivers

[Alexandre Belloni <alexandre.belloni@bootlin.com>]

On 27/12/2019 12:23:23-0800, Steve Muckle wrote:
> On Fri, Nov 15, 2019 at 5:36 AM Alexandre Belloni <
> alexandre.belloni@bootlin.com> wrote:
> 
> > On 06/11/2019 15:37:49-0800, Steve Muckle wrote:
> > > On 11/6/19 3:19 PM, Alexandre Belloni wrote:
> > > > On 06/11/2019 11:46:25-0800, Steve Muckle wrote:
> > > > > Due to distribution constraints it may not be possible to statically
> > > > > compile the required RTC driver into the kernel.
> > > > >
> > > > > Expand RTC_HCTOSYS support to cover all RTC devices (statically
> > compiled
> > > > > or not) by checking at the end of RTC device registration whether the
> > > > > time should be synced.
> > > > >
> > > >
> > > > This does not really help distributions because most of them will still
> > > > have "rtc0" hardcoded and rtc0 is often the rtc that shouldn't be used.
> > >
> > > Just for my own edification, why is that? Is rtc0 normally useless on PC
> > for
> > > some reason?
> > >
> >
> > On PC, rtc0 is probably fine which is not the case for other
> > architectures where rtc0 is the SoC RTC and is often not battery backed.
> >
> > > On the platforms I'm working with I believe it can be assured that rtc0
> > will
> > > be the correct rtc. That doesn't help typical distributions though.
> > >
> > > What about a kernel parameter to optionally override the rtc hctosys
> > device
> > > at runtime?
> > >
> >
> > What about keeping that in userspace instead which is way easier than
> > messing with kernel parameters?
> >
> 
> This should ideally happen before file systems are mounted so I don't see
> many alternatives for communicating which RTC should be used. Android uses
> the kernel command line for userspace parameters as well and that's an
> option but that defeats part of the value of doing it in userspace IMO.
> There's also device tree but I'm not sure this belongs there.
> 
> Hctosys is also saving and restoring the system time on suspend/resume. It
> seems more efficient to me to do this (which happens very frequently on an
> Android device) in the kernel as opposed to in userspace.
> 
> If I set the initial system time from the rtc in userspace but continue to
> rely on the hctosys suspend/resume code, as it stands there will be a
> window after the rtc driver is loaded but before the system time is set
> where if suspend is entered, the correct time in the rtc will be lost.
> 
> > > Can't you move away from HCTOSYS and do the correct thing in userspace
> > > > instead of the crap hctosys is doing?
> > >
> > > Yes, I just figured it's a small change, and if hctosys can be made to
> > work
> > > might as well use that.
> >
> > The fact is that hctosys is more related to time keeping than it is to
> > the RTC subsytem. It also does a very poor job setting the system time
> > because adding 0.5s is not the smartest thing to do. The rtc granularity
> > is indeed 1 second but is can be very precisely set.
> >
> 
> No argument with that, but millions of devices successfully rely on it
> today. AFAICT this simple patch doesn't make anything worse. Together with
> a change to support a kernel parameter for runtime rtc selection, it should
> allow RTC drivers to be modularized on many systems. Can it be adopted as a
> stopgap measure?

I've applied this patch for 5.7 after removing the unnecessary
reformatting of the kconfig help.