Re: [RFC PATCH v3 5/5] arm64: Use SYSTEM_OFF2 PSCI call to power off for hibernate

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On Fri, 22 Mar 2024 16:12:44 +0000,
David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
> On Fri, 2024-03-22 at 16:02 +0000, Marc Zyngier wrote:
> > On Tue, 19 Mar 2024 12:59:06 +0000,
> > David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
> > 
> > [...]
> > 
> > > +static void __init psci_init_system_off2(void)
> > > +{
> > > +       int ret;
> > > +
> > > +       ret = psci_features(PSCI_FN_NATIVE(1_3, SYSTEM_OFF2));
> > > +
> > > +       if (ret != PSCI_RET_NOT_SUPPORTED)
> > > +               psci_system_off2_supported = true;
> > 
> > It'd be worth considering the (slightly broken) case where SYSTEM_OFF2
> > is supported, but HIBERNATE_OFF is not set in the response, as the
> > spec doesn't say that this bit is mandatory (it seems legal to
> > implement SYSTEM_OFF2 without any hibernate type, making it similar to
> Such is not my understanding. If SYSTEM_OFF2 is supported, then
> HIBERNATE_OFF *is* mandatory.
> The next update to the spec is turning the PSCI_FEATURES response into
> a *bitmap* of the available features, and I believe it will mandate
> that bit zero is set.

The bitmap is already present in the current Alpha spec:

5.16.2 Implementation responsibilities


Bits[31] Reserved, must be zero.

Bits[30:0] Hibernate types supported.

All other values are reserved for future use.

and doesn't say (yet) that HIBERNATE_OFF is mandatory. Furthermore,

5.11.2 Caller responsibilities

The calling OS uses the PSCI_FEATURES API, with the SYSTEM_OFF2
function ID, to discover whether the function is present:

- If the function is implemented, PSCI_FEATURES returns the hibernate
  types supported.

- If the function is not implemented, PSCI_FEATURES returns

which doesn't say anything about which hibernate type must be
implemented. Which makes sense, as I expect it to, in the fine ARM
tradition, grow things such as "HIBERNATE_WITH_ROT13_ENCRYPTION" and
even "HIBERNATE_WITH_ERRATA_XYZ", because firmware is where people
dump their crap. And we will need some special handling for these
tasty variants.

> And if for whatever reason that SYSTEM_OFF2/HIBERNATE_OFF call
> *doesn't* work, Linux will end up doing a 'real' poweroff, first
> through EFI and then finally as a last resort with a PSCI SYSTEM_OFF.
> So it would be OK to have false positives in the detection.

I agree that nothing really breaks, but I also hold the view that
broken firmware implementations should be given the finger, specially
given that you have done this work *ahead* of the spec. I would really
like this to fail immediately on these and not even try to suspend.

With that in mind, if doesn't really matter whether HIBERNATE_OFF is
mandatory or not. We really should check for it and pretend it doesn't
exist if the correct flag isn't set.


Without deviation from the norm, progress is not possible.

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