Re: [PATCH] rtc: adapt allowed RTC update error

Alexandre Belloni
2020-12-03T16:08:39Z

On 03/12/2020 11:52:49-0400, Jason Gunthorpe wrote:
> On Thu, Dec 03, 2020 at 03:10:47AM +0100, Alexandre Belloni wrote:
>
> > IIRC, used in conjunction with rtc_hctosys which also adds
> > inconditionnaly 500ms this can ends up with the system time
> > being one second away from the wall clock time which NTP will take quite
> > some time to remove.
>
> I can't remember the details, but this was not the intention.
>
> As long as systohc and hctosys exist then the design goal of rtclib
> should be to provide sub-second accuracy on the round trip of time
> through the RTC.
>
> Otherwise what is the point?
>

hctosys never had a sub second accuracy because there is no way to
accurately read the rtc time without being ready to wait up to a second.
I have patches doing exactly that but I'm pretty sure nobody wants to
pay the price and have a kernel that boots significantly slower.
Especially since that would basically affect everyone since systemd
requires that hctosys is enabled.

--
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering