Re: [PATCH v2 1/2] dt-bindings: riscv: Add optional DT property riscv,timer-can-wake-cpu

[Conor Dooley]

Hey Samuel,

On Tue, Nov 22, 2022 at 11:43:04PM -0600, Samuel Holland wrote:
> On 11/22/22 08:57, Conor Dooley wrote:
> >> If we add a timer DT node now
> >> then we have to deal with compatibility for existing platforms.
> > 
> > In terms of what to encode in a DT, and given the spec never says that
> > the timer interrupt must arrive during suspend, we must assume, by
> > default, that no timer events arrive during suspend.
> > 
> > We have a bunch of existing platforms that may (do?) get timer events
> > during suspend, the opposite of the proposed default behaviour.
> > 
> > I'm trying to follow the line of reasoning but I fail to see how taking
> > either the property or node approach allows us to maintain behaviour for
> > exiting platforms that that do see timer events during suspend without
> > adding *something* to the DT. No matter what we add, we've got some sort
> > of backwards compatibility issue, right?
> 
> In the absence of bugs/limitations in Linux timer code (like the ones
> you are seeing on PolarFire), the backwards compatibility issue with
> setting C3STOP by default is that non-retentive idle states will be
> ignored unless:
>  1) the DT property is added (i.e. firmware upgrade), or
>  2) some other timer driver is available.
> No other behavior should be affected.

Aye, which I think is fine, in the context of platforms supported by
upstream Linux. Right now, nothing in-tree seems to use idle states:
- the SiFive stuff is more demo than anything
- we've not really got to that point with our reference PolarFire stuff
  (although I can't speak for any customers)
- the K210 is a toy (sorry Damien!)
- the StarFive lads have moved on to the jh7110
- the D1 (although it's not an in-tree config) needs C3STOP by default,
  so its behaviour is positively affected.

If there's someone with an out-of-tree idle config, there's not really
much that we can do about it?

> On the other hand, if C3STOP defaults to off, then the backwards
> compatibility issue concerns platforms that can currently boot Linux,
> but which cannot use cpuidle because they need the flag. If they were to
> upgrade their firmware, and Linux is provided a DTB that includes both
> idle states and the property, these platforms would unexpectedly fail to
> boot. (They would enter an idle state and never wake up.)
> 
> Assuming no such platforms exist, then it would actually be better to
> default C3STOP to off.

Yeah, *assuming* no such platforms exist I agree - but the D1 is one of
such platforms (albeit in a specific configuration) so I think we have
to default C3STOP to on.

> Now, this says nothing about how the property should be named -- we can
> set C3STOP based on the absence of a property, just as easily as we can
> clear C3STOP based on the presence of a property.
> 
> > I noted the above:
> > 
> >> Since, there is no dedicated timer node, we use CPU compatible string
> >> for probing the per-CPU timer.
> > 
> > If we could rely on the cpu compatible why would we need to add a
> > dt-property anyway? Forgive my naivety here, but is the timer event in
> > suspend behaviour not a "core complex" level attribute rather than a
> > something that can be consistently determined by the cpu compatible?
> 
> I do not support using either the CPU compatible (not specific enough)
> or the board compatible (too many to list, but still not specific
> enough). Consider that not all CPUs in a system may need this property.

Yeah, I was just trying to understand where Anup was coming from and
teasing out the different bits of logic. I do not think that using the
CPU compatible is a good idea - my understanding was that how a CPU with
a given compatible is integrated into a core complex determines which
timer (or interrupt etc) is capable of what.

> > Either way, we need to figure out why enabling C3STOP is causing other
> > timer issues even when we are not in some sort of sleep state & do
> > something about that - or figure out some different way to communicate
> > the behavioural differences.
> > I would expect timers to continue working "normally" with the flag set,
> > even if how they work is subtly different?
> 
> Definitely agree here. My intention was not to affect anything other
> than cpuidle behavior.
> 
> > On a D1, with the C3STOP "feature" flag set, and it's custom timer
> > implementation unused, how do timers behave?
> 
> D1 is uniprocessor, so I build with CONFIG_SMP=n. In this case,
> CONFIG_GENERIC_CLOCKEVENTS_BROADCAST=n, and thus
> __tick_broadcast_oneshot_control() returns -EBUSY, forcing
> cpuidle_enter_state() to choose a retentive idle state.

Right & that makes sense for someone building a D1 focused kernel (and
is what I do for my Nezha IIRC) but if someone builds a multiplatform
kernel you're going to end up with CONFIG_SMP=y (but I'd imagine that in
that scenario they'll have the sunxi,foo-timer's driver enabled). At
this point, it's something I should go and dig out my board for though..

I was mainly just curious if the D1 also exhibits the borked timer
behaviour that I see.

Thanks again Samuel,
Conor.