Re: [PATCH] cpuidle: riscv-sbi: Stop using non-retentive suspend

[Anup Patel]

On Wed, Nov 23, 2022 at 3:40 PM Conor Dooley <conor.dooley@xxxxxxxxxxxxx> wrote:
>
> Hey Anup,
>
> On Wed, Nov 23, 2022 at 09:56:31AM +0530, Anup Patel wrote:
> > On Tue, Nov 22, 2022 at 4:50 PM Conor Dooley <conor.dooley@xxxxxxxxxxxxx> wrote:
> > >
> > > On Tue, Nov 22, 2022 at 11:06:15AM +0530, Anup Patel wrote:
> > > > On Tue, Nov 22, 2022 at 10:46 AM Palmer Dabbelt <palmer@xxxxxxxxxxxx> wrote:
> > > > >
> > > > > On Mon, 21 Nov 2022 19:45:07 PST (-0800), anup@xxxxxxxxxxxxxx wrote:
> > > > > > On Tue, Nov 22, 2022 at 2:27 AM Palmer Dabbelt <palmer@xxxxxxxxxxxx> wrote:
> > > > > >>
> > > > > >> From: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
> > > > > >>
> > > > > >> As per [1], whether or not the core can wake up from non-retentive
> > > > > >> suspend is a platform-specific detail.  We don't have any way to encode
> > > > > >> that, so just stop using them until we've sorted that out.
> > > > > >>
> > > > > >> Link: https://github.com/riscv-non-isa/riscv-sbi-doc/issues/98#issuecomment-1288564687
> > > > > >> Fixes: 6abf32f1d9c5 ("cpuidle: Add RISC-V SBI CPU idle driver")
> > > > > >> Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
> > > > > >
> > > > > > This is just unnecessary maintenance churn and it's not the
> > > > > > right way to go. Better to fix this the right way instead of having
> > > > > > a temporary fix.
> > > > > >
> > > > > > I had already sent-out a patch series 5 months back to describe
> > > > > > this in DT:
> > > > > > https://lore.kernel.org/lkml/20220727114302.302201-1-apatel@xxxxxxxxxxxxxxxx/
> > > > > >
> > > > > > No one has commented/suggested anything (except Samuel
> > > > > > Holland and Sudeep Holla).
> > > > >
> > > > > I see some comments from Krzysztof here
> > > > > <https://lore.kernel.org/lkml/7a0477a0-9f0f-87d6-4070-30321745f4cc@xxxxxxxxxx/>
> > > > > as well.  Looks like everyone is pointing out that having our CPU nodes
> > > > > encode timers is a bad idea, my guess is that they're probably right.
> > > >
> > > > Adding a separate timer DT node, creates a new set of compatibility
> > > > issues for existing platforms. I am fine updating my series to have
> > > > separate timer DT node but do we want to go in this direction ?
> > >
> > > 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.
>
> Ahh, that is what I have missed. I'll continue my thoughts on this in
> the dt-binding thread.
>
> > > > Even if ARM has a separate timer DT node, the timers are still part
> > > > of the CPU. It depends on how we see the DT bindings aligning with
> > > > actual HW.
> > > >
> > > > >
> > > > > > Please review this series. I can quickly address comments to
> > > > > > make this available for Linux-6.2. Until this series is merged,
> > > > > > the affected platforms can simply remove non-retentive suspend
> > > > > > states from their DT.
> > > > >
> > > > > That leaves us with a dependency between kernel versions and DT
> > > > > bindings: kernels with the current driver will result in broken systems
> > > > > with the non-retentive suspend states in the DT they boot with when
> > > > > those states can't wake up the CPU.
> > >
> > > Can someone point me at a (non D1 or virt) system that has suspend
> > > states in the DT that would need fixing?
> >
> > For the QEMU virt machine, the default non-retentive suspend state was
> > tested using a temporary DTB provided separately via QEMU command
> > line. The QEMU virt machine does not have its own HART suspend
> > states so OpenSBI will functionally emulate default retentive/non-retentive
> > suspend states.
>
> So since I asked for non D1 or virt systems, that's a no & no DTs
> actually needs to be fixed :)
>
> > > > This is not a new problem we are facing. Even in the ARM world,
> > > > the DT bindings grew organically over time based on newer platform
> > > > requirements.
> > > >
> > > > Now that we have a platform which does not want the time
> > > > C3STOP feature, we need to first come-up with DT bindings
> > > > to support this platform instead of temporarily disabling
> > > > features which don't work on this platform.
> > >
> > > It's the opposite surely? It should be "now that we have a platform that
> > > *does want* the C3STOP feature", right?
> >
> > Yes, we can think this way as well.
>
> No, there's no "thinking" involved here from what I can tell. Pre-D1
> systems do not seem to need the flag and the D1 does want that flag for
> its riscv,timer. We have to operate with respect to hardware timelines
> & the corresponding software implementations, not specs in this context.
>
> If it was the case that you proposed, there would be no chance for
> regressions if someone updates their kernel but not their DT.
>
> > > > > > With all due respect, NACK to this patch from my side.
> > >
> > > As Samuel pointed out that the D1 doesn't actually use the timer in
> > > question, I think we are okay here?
> >
> > Yes, that's why D1 needs the C3STOP flag.
>
> I don't understand what you mean here, you don't appear to be replying
> to what I said.
>
> I was saying that the current D1 configuration does not actually use
> the timer-riscv driver as there's another one that has a higher rating
> & therefore we are okay to not apply this patch as my revert will not
> cause it to be put into sleep states that it cannot return from.
>
> Your reply makes no sense to me in that context.

Sorry for the confusion, I should have written a more complete sentence.

D1 does not use the RISC-V timer but it still needs to set the C3STOP
flag to inform the timer subsystem that the RISC-V timer will not work
in suspend state.

Regards,
Anup