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

[Anup Patel]

On Wed, Nov 23, 2022 at 7:17 PM Conor Dooley <conor.dooley@xxxxxxxxxxxxx> wrote:
>
> Hey Anup,
>
> (keeping all the context since you didn't reply to this mail yet)
>
> On Tue, Nov 22, 2022 at 02:57:05PM +0000, Conor Dooley wrote:
> > Hey Anup,
> >
> > I've been meaning to get back to you on this stuff for quite a while,
> > but unfortunately I've gotten distracted with other stuff every time I
> > got close. Apologies for that :(
> >
> > On Wed, Jul 27, 2022 at 07:04:57PM +0530, Anup Patel wrote:
> > > On Wed, Jul 27, 2022 at 6:05 PM Krzysztof Kozlowski
> > > <krzysztof.kozlowski@xxxxxxxxxx> wrote:
> > > >
> > > > On 27/07/2022 14:21, Anup Patel wrote:
> > > > > On Wed, Jul 27, 2022 at 5:37 PM Krzysztof Kozlowski
> > > > > <krzysztof.kozlowski@xxxxxxxxxx> wrote:
> > > > >>
> > > > >> On 27/07/2022 13:43, Anup Patel wrote:
> > > > >
> > > > > Since, there is no dedicated timer node, we use CPU compatible string
> > > > > for probing the per-CPU timer.
> > > >
> > > > Next time you add a properties:
> > > > riscv,saata-can-wake-cpu
> > > > riscv,usb-can-wake-cpu
> > > > riscv,interrupt-controller-can-wake-cpu
> > > >
> > > > and so on and keep explaining that "historically" you did not define
> > > > separate nodes, so thus must be in CPU node.
> > >
> > > This is a one-of-case with RISC-V DeviceTree where we are living with
> > > the fact that there is no timer DT node. If we add a timer DT node now
> > > then we have to deal with compatibility for existing platforms.
> >
> > I don't really understand the argument here. Perhaps this made sense a
> > few months ago, but it no longer does IMO.
> >
> > We have existing platforms that interpreted the SBI spec (or perhaps
> > predated the SBI spec in the relevant form?) differently. I've pasted it
> > several times now I feel but it's relevant so pasting it here again...
> >
> > On the subject of suspend, the RISC-V SBI spec states:
> > > Request the SBI implementation to put the calling hart in a platform
> > > specific suspend (or low power) state specified by the suspend_type
> > > parameter. The hart will automatically come out of suspended state and
> > > resume normal execution when it receives an interrupt or platform
> > > specific hardware event.
> >
> > This does not cover whether a given event actually reaches the hart or
> > not, just what the hart will do if it receives an event. For the
> > implementation on the Allwinner D1, timer events are not received during
> > suspend.
> >
> > Through-out the various bits of conversation so far, I have been
> > operating on the assumption that on PolarFire SoC, and potentially other
> > SiFive based implementations, events from the RISC-V timer do reach a
> > hart during suspend.
> > I realised while writing this response that I have never actually tested
> > it - the C3STOP flag caused problems for me during regular operation &
> > not while using some DT defined sleep states.
> > I've been learning/piecing together the bits of what is happening here as
> > time goes on, so I made an assumption that may or may not be correct, and
> > I am still oh-so-far from an understanding.
> > I just took it for granted that the existing driver worked correctly for
> > "old" SiFive stuff which MPFS is based on & figured that with ~the same
> > core complex as the fu540 that we'd behave similarly.
> > Perhaps that was not a good idea & please let me know if I've been
> > barking up the wrong tree.
> >
> > Do we know definitively what is/isn't the case for any of the existing
> > platforms?
> > I can test some stuff, but it'll take some time as it's a bad week in
> > my neck of the woods.
> >
> > > 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?
> >
> > 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?
> >
> > 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?
> > On a D1, with the C3STOP "feature" flag set, and it's custom timer
> > implementation unused, how do timers behave?
> >
> > Hopefully I've missed something blatant here Anup!
>
> So what I missed, as Anup pointed out else where, is:
>
> > me:
> > > I don't really follow. How is there a compatibility issue created by
> > > adding a new node that is not added for a new property? Both will
> > > require changes to the device tree. (You need not reply here, I am going
> > > to review the other thread, it's been on my todo list for too long. Been
> > > caught up with non-coherent stuff & our sw release cycle..)
> >
> > Adding a new timer DT node would mean, the RISC-V timer driver
> > will now be probed using the compatible to the new DT node whereas
> > the RISC-V timer driver is currently probed using CPU DT nodes.
>
> In that case, we would have to retain the ability to match against the
> "riscv". Spitballing:
> - add a new timer node
> - keep matching against "riscv"
> - look up a timer node during init w/ of_find_matching_node() that
>   contains any new timer properties
>
> I think it's unlikely that this will be the last time we have to add
> some timer properties & we should avoid doing odd things in a DT to suit
> an operating system?
>
> Would something along those lines work Anup, or am I, yet again, missing
> something?

I was already working on v3 along these lines. I will try to post a v3 this
week itself.

Regards,
Anup

>
> Thanks,
> Conor.
>