linux-kernel - Re: [PATCH] arm64: dts: qcom: qcs404: Add PSCI cpuidle support

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Message-ID: <CAHLCerM6SVCx5vdrodh+O5nQ0uDwr7zpvOMxVYtreT9EFk22eg@mail.gmail.com>
Date:   Wed, 15 May 2019 15:26:14 +0530
From:   Amit Kucheria <amit.kucheria@...aro.org>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     Niklas Cassel <niklas.cassel@...aro.org>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Andy Gross <agross@...nel.org>,
        David Brown <david.brown@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@...aro.org>,
        Lina Iyer <lina.iyer@...aro.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        DTML <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm64: dts: qcom: qcs404: Add PSCI cpuidle support

On Mon, May 13, 2019 at 3:19 PM Sudeep Holla <sudeep.holla@....com> wrote:
>
> On Fri, May 10, 2019 at 11:58:40PM +0530, Amit Kucheria wrote:
> > On Fri, May 10, 2019 at 2:54 PM Sudeep Holla <sudeep.holla@....com> wrote:
> > >
>
> [...]
>
> > >
> > > Yes entry-method="psci" is required as per DT binding but not checked
> > > in code on arm64. We have CPU ops with idle enabled only for "psci", so
> > > there's not need to check.
> >
> > I don't see it being checked on arm32 either.
> >
>
> arm_cpuidle_get_ops in arch/arm/kernel/cpuidle.c checks the method, has
> to match "psci" for drivers/firmware/psci.c to work on arm32

That is a check for the enable-method, not entry-method.

We don't check for entry-method anywhere, AFAICT.

> [...]
>
> > >
> > > Why do you want to deprecated just because Linux kernel doesn't want to
> > > use it. That's not a valid reason IMO.
> >
> > Fair enough. Just want to make sure that it isn't some vestigial
> > property that was never used. Do you know if another OS is actually
> > using it?
> >
>
> Not that I am aware of. But Linux uses it on arm32, so it's not entirely
> unused.

entry-method is not read in Linux code (see above).

> > > > Do we expect to support PSCI platforms that might have a different
> > > > entry-method for idle states?
> > >
> > > Not on ARM64, but same DT bindings can be used for idle-states on
> > > say RISC-V and have some value other than "psci".
> >
> > Both enable-method and entry-method properties are currently only used
> > (and documented) for ARM platforms. Hence this discussion about
> > deprecation of one of them.
> >
>
> Yes, it's used on arm32 as mentioned above.

Only enable-method is checked.

> > > > Should I whip up a patch removing entry-method? Since we don't check
> > > > for it today, it won't break the old DTs either.
> > > >
> > >
> > > Nope, I don't think so. But if it's causing issues, we can look into it.
> > > I don't want to restrict the use of the bindings for ARM/ARM64 or psci only.
> >
> > Only a couple of minor issues:
> > 1. There is a trickle of DTs that need fixing up every now and then
> > because they don't use entry-method in their idle-states node. Schema
> > validation ought to fix that.
>
> I understand, scheme should fix it. This is not just restricted to this,
> it's generic DT problem. So let's hope we get schema based validation soon.
>
> > 2. A property that isn't ready by any code is a bit confusing. Perhaps
> > we can mention something to the effect in the documentation?
> >
>
> Not entirely true. We have quite a lot of bindings that are added just
> because downstream drivers use e.g. GPU and even standard ePAPR or DT
> specification has lots of bindings which OS like Linux may choose
> not to use at all. Same applies to ACPI, so I am not for removing bindings
> just because there are no users in Linux.

That is a fair point. But in those cases, the binding is probably used
by another OS. entry-method seems to an example of one that isn't used
by Linux or other OSes.

Regards,
Amit