[PATCH] rtc: Allow rtc drivers to specify the tv_nsec value for ntp

[Alexandre Belloni]

On 23/11/2017 at 08:04:39 -0700, Jason Gunthorpe wrote:
> On Thu, Nov 23, 2017 at 10:54:56AM +0100, Alexandre Belloni wrote:
> 
> > So I'm discovering that this patch made it upstream silently. While it
> > somewhat solves the issue for some RTCs, it is not a proper solution for
> > most.
> 
> It was cc'd all the right lists?
> 

It wasn't sent to the RTC ML but that's fine.

> > With this patch, set_offset_nsec will be hardcoded in the driver and
> > this basically work for the mc146818 but many RTCs are connected on a
> > slow bus (let's say i2c) and that bus may have various latencies
> > depending on the platform so the value actually depends on the platform
> > rather than on the RTC itself.
> 
> The intention of the patch is for the actual RTC driver to set the
> value to what it needs.
> 
> If some RTCs are dealing with variable latency busses then they should
> do an bus IO to measure their bus latency then tweak their value.
> 
> In any event, having the correct offset for the RTC chip (eg 0.5 vs 0)
> is a big improvement, even if there are some small I2C latencies.
> 
> > I really think that we should simply consider dropping hctosys,
> > systohc and update_persistent_clock. Most distributions have been
> 
> Like Russell has said already, the kernel is the only place that could
> actually have all the information needed. User space would have to use
> some kind of trail and error approach to figure out what the offset
> should be.
> 

And I don't see how this is different from doing it in the kernel. You
currently don't give any feedback loop so the RTC driver doesn't know
whether it's own offset is correct or not.

> You can't really measure the offset without doing a time set, many
> embedded RTCs do not hook up interrupts, etc.
> 

If you don't hook up the interrupt, then all you did is completely
pointless because you will never be able to read the correct time from
the RTC, you'll have up to a 1s offset.


-- 
Alexandre Belloni, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com