Re: [PATCH v4] dt-bindings: dvfs: Add support for generic performance domains

[Ulf Hansson <ulf.hansson@linaro.org>]

On Wed, 20 Oct 2021 at 12:25, Sudeep Holla <sudeep.holla@arm.com> wrote:
>
> On Fri, Oct 15, 2021 at 11:17:18AM +0200, Ulf Hansson wrote:
> > On Thu, 14 Oct 2021 at 16:56, Sudeep Holla <sudeep.holla@arm.com> wrote:
> > >
> > > On Thu, Oct 14, 2021 at 12:56:46PM +0200, Ulf Hansson wrote:
> > > > On Mon, 17 May 2021 at 18:14, Sudeep Holla <sudeep.holla@arm.com> wrote:
> > > > >
> > > > > The CLKSCREW attack [0] exposed security vulnerabilities in energy management
> > > > > implementations where untrusted software had direct access to clock and
> > > > > voltage hardware controls. In this attack, the malicious software was able to
> > > > > place the platform into unsafe overclocked or undervolted configurations. Such
> > > > > configurations then enabled the injection of predictable faults to reveal
> > > > > secrets.
> > > > >
> > > > > Many Arm-based systems used to or still use voltage regulator and clock
> > > > > frameworks in the kernel. These frameworks allow callers to independently
> > > > > manipulate frequency and voltage settings. Such implementations can render
> > > > > systems susceptible to this form of attack.
> > > > >
> > > > > Attacks such as CLKSCREW are now being mitigated by not having direct and
> > > > > independent control of clock and voltage in the kernel and moving that
> > > > > control to a trusted entity, such as the SCP firmware or secure world
> > > > > firmware/software which are to perform sanity checking on the requested
> > > > > performance levels, thereby preventing any attempted malicious programming.
> > > > >
> > > > > With the advent of such an abstraction, there is a need to replace the
> > > > > generic clock and regulator bindings used by such devices with a generic
> > > > > performance domains bindings.
> > > > >
> > > > > [0] https://www.usenix.org/conference/usenixsecurity17/technical-sessions/presentation/tang
> > > > >
> > > > > Link: https://lore.kernel.org/r/20201116181356.804590-1-sudeep.holla@arm.com
> > > > > Cc: Rob Herring <robh+dt@kernel.org>
> > > > > Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
> > > > > Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
> > > >
> > > > Hi Sudeep/Viresh/Rob,
> > > >
> > > > I noticed this binding recently got accepted, so I guess I have missed
> > > > the opportunity to provide you with a few comments.
> > > >
> > >
> > > Sorry for not cc-ing you, wasn't aware of the below mentioned intersection,
> > > so assumed you are not one of the interested parties.
> > >
> > > > In any case, I would like to ask a few questions. In particular, am I
> > > > trying to understand why the power-domains bindings [1] can't be used
> > > > for this?
> > > >
> > >
> > > One reason I can think of is on some platforms, the power domains are
> > > completely controlled by the firmware and not exposed to the OSPM.
> > > This is mostly applicable for CPU devices(Platform co-ordinated PSCI)
> >
> > See below.
> >
> > >
> > > > The power-domains are capable of dealing with "performance" through
> > > > the "operating-points-v2" DT property, which maps to the generic OPP
> > > > bindings [2]. I wonder why that isn't sufficient here? Can you please
> > > > elaborate?
> > > >
> > >
> > > Even if the power domains are exposed to the OSPM, the OPPs can be
> > > firmware enumerated rather than DT. Not sure if it is possible to
> > > represent such systems in the above mentioned bindings. IIUC, the genpd
> > > uses clock and regulator apis to drive the performance, but these
> > > platforms have f/w interface to drive the OPPs(abstracted).
> >
> > Genpd doesn't operate on clock rates or voltage levels. Instead
> > "performance" is just an integer value for genpd. What a performance
> > index means, is genpd provider specific.
> >
>
> Understood.
>
> > In other words, it becomes the responsibility for the genpd provider
> > to map a performance state index to an OPP, for example. So far,
> > providers have used the generic OPP DT bindings to do this, but for
> > sure, we don't have to limit ourselves to this. So, if OPP tables can
> > be enumerated by FW, rather than specified in DT, that should
> > certainly be possible to support.
> >
> > BTW, these are genpd provider callbacks, that needs to be implemented
> > to let it control performance. Perhaps that helps to understand
> > things.
> >
> > int (*set_performance_state)(struct generic_pm_domain *genpd, unsigned
> > int state);
> > unsigned int (*opp_to_performance_state)(struct generic_pm_domain
> > *genpd, struct dev_pm_opp *opp);
> >
>
> Looks like this can be used for devices but what about CPUs ?

Yes, that should work. dev_pm_genpd_set_performance_state() takes a
struct device* as an in-parameter.

The struct device to use would typically be the one that you receive
from dev_pm_domain_attach_by_name(). We already do this for some other
cpufreq drivers, so this works fine.

>
> > >
> > > I am happy to know if there are ways to support such systems with the
> > > options you have mentioned above.
> >
> > As far as I understand, the "performance domains" DT bindings that
> > $subject patch introduces, allows us to group devices into domains, to
> > let them be "performance controlled" together. Right?
> >
>
> Or independently. It doesn't matter.
>
> > Unless I am missing something, it looks like power domains DT bindings
> > already offer this for us. Yes, certainly, the DT doc [1] needs an
> > updated description to better explain this, but other than that we
> > should be fine, don't you think?
> >
>
> As I mentioned about, the main question is what if firmware doesn't
> want to expose power domain details to OSPM like PC co-ordinated PSCI
> idle states while it wants to either group CPUs or leave them as
> individual in order to get per-CPU DVFS requests and aggregate them
> in the firmware. It does something similar for idle states already.

Yes, that can be modeled too.

Just let each CPU node point to its own separate power-domain and also
*don't* model the parent power-domain, instead leave this to be
managed by the FW.

Kind regards
Uffe