linux-kernel - Re: [PATCH V3 2/2] cpufreq: scmi: Register for limit change notifications

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Message-ID: <ZeB0iCr9GpfUiOEg@pluto>
Date: Thu, 29 Feb 2024 12:11:52 +0000
From: Cristian Marussi <cristian.marussi@....com>
To: Lukasz Luba <lukasz.luba@....com>
Cc: Sibi Sankar <quic_sibis@...cinc.com>, sudeep.holla@....com,
	linux-arm-kernel@...ts.infradead.org, pierre.gondois@....com,
	dietmar.eggemann@....com, morten.rasmussen@....com,
	viresh.kumar@...aro.org, rafael@...nel.org,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	quic_mdtipton@...cinc.com, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH V3 2/2] cpufreq: scmi: Register for limit change
 notifications

On Thu, Feb 29, 2024 at 11:45:41AM +0000, Lukasz Luba wrote:
> 
> 
> On 2/29/24 11:28, Cristian Marussi wrote:
> > On Thu, Feb 29, 2024 at 10:22:39AM +0000, Lukasz Luba wrote:
> > > 
> > > 
> > > On 2/29/24 09:59, Lukasz Luba wrote:
> > > > 
> > > > 
> > > > On 2/28/24 17:00, Sibi Sankar wrote:
> > > > > 
> > > > > 
> > > > > On 2/28/24 18:54, Lukasz Luba wrote:
> > > > > > 
> > > > > > 
> > > > > > On 2/27/24 18:16, Sibi Sankar wrote:
> > > > > > > Register for limit change notifications if supported and use
> > > > > > > the throttled
> > > > > > > frequency from the notification to apply HW pressure.
> > > > > 
> > > > > Lukasz,
> > > > > 
> > > > > Thanks for taking time to review the series!
> > > > > 
> > > > > > > 
> > > > > > > Signed-off-by: Sibi Sankar <quic_sibis@...cinc.com>
> > > > > > > ---
> > > > > > > 
> > > > > > > v3:
> > > > > > > * Sanitize range_max received from the notifier. [Pierre]
> > > > > > > * Update commit message.
> > > > > > > 
> > > > > > > ï¿½ drivers/cpufreq/scmi-cpufreq.c | 29 ++++++++++++++++++++++++++++-
> > > > > > > ï¿½ 1 file changed, 28 insertions(+), 1 deletion(-)
> > > > > > > 
> > > > > > > diff --git a/drivers/cpufreq/scmi-cpufreq.c
> > > > > > > b/drivers/cpufreq/scmi-cpufreq.c
> > > > > > > index 76a0ddbd9d24..78b87b72962d 100644
> > > > > > > --- a/drivers/cpufreq/scmi-cpufreq.c
> > > > > > > +++ b/drivers/cpufreq/scmi-cpufreq.c
> > > > > > > @@ -25,9 +25,13 @@ struct scmi_data {
> > > > > > > ï¿½ï¿½ï¿½ï¿½ï¿½ int domain_id;
> > > > > > > ï¿½ï¿½ï¿½ï¿½ï¿½ int nr_opp;
> > > > > > > ï¿½ï¿½ï¿½ï¿½ï¿½ struct device *cpu_dev;
> > > > > > > +ï¿½ï¿½ï¿½ struct cpufreq_policy *policy;
> > > > > > > ï¿½ï¿½ï¿½ï¿½ï¿½ cpumask_var_t opp_shared_cpus;
> > > > > > > +ï¿½ï¿½ï¿½ struct notifier_block limit_notify_nb;
> > > > > > > ï¿½ };
> > > > > > > +const struct scmi_handle *handle;
> > > > 
> > > > I've missed this bit here.
> > > 
> > > So for this change we actually have to ask Cristian or Sudeep
> > > because I'm not sure if we have only one 'handle' instance
> > > for all cpufreq devices.
> > > 
> > > If we have different 'handle' we cannot move it to the
> > > global single pointer.
> > > 
> > > Sudeep, Cristian what do you think?
> > 
> > I was just replying noticing this :D .... since SCMI drivers can be
> > probed multiple times IF you defined multiple scmi top nodes in your DT
> > containing the same protocol nodes, they receive a distinct sdev/handle/ph
> > for each probe...so any attempt to globalize these wont work...BUT...
> > 
> > ...this is a bit of a weird setup BUT it is not against the spec and it can
> > be used to parallelize more the SCMI accesses to disjont set of resources
> > within the same protocol (a long story here...) AND this type of setup is
> > something that it is already used by some other colleagues of Sibi working
> > on a different line of products (AFAIK)...
> > 
> > So, for these reasons, usually, all the other SCMI drivers have per-instance
> > non-global references to handle/sdev/ph....
> > 
> > ...having said that, thought, looking at the structure of CPUFReq
> > drivers, I am not sure that they can stand such a similar setup
> > where multiple instances of this same driver are probed
> > 
> > .... indeed the existent *ph refs above is already global....so it wont already
> > work anyway in case of multiple instances now...
> > 
> > ...and if I look at how CPUFreq expects the signature of scmi_cpufreq_get_rate()
> > to be annd how it is implemented now using the global *ph reference, it is
> > clearly already not working cleanly on a multi-instance setup...
> > 
> > ...now...I can imagine how to (maybe) fix the above removing the globals and
> > fixing this, BUT the question, more generally, is CPUFreq supposed to work at all in
> > this multi-probed mode of operation ?
> > Does it even make sense to be able to support this in CPUFREQ ?
> > 
> > (as an example in cpufreq,c there is static global cpufreq_driver
> >   pointing to the arch-specific configured driver BUT that also holds
> >   some .driver_data AND that cleraly wont be instance specific if you
> >   probe multiple times and register with CPUFreq multiple times...)
> > 
> >   More questions than answers here :D
> > 
> 
> Thanks Cristian for instant response. Yes, indeed now we have more
> questions :) (which is good). But that's good description of the
> situation.
> 
> So lets consider a few option what we could do now:
> 1. Let Sibi add another global state the 'handle' but add
>    a BUG_ON() or WARN_ON() in the probe path if the next
>    'handle' instance is different than already set in global.
>    This would simply mean that we don't support (yet)
>    such configuration in a platform. As you said, we
>    already have the *ph global, so maybe such platforms
>    with multiple instances for this particular cpufreq and
>    performance protocol don't exist yet.

Yes this is the quickst way (and a WARN_ON() is better I'd say) but there
are similar issues of "unicity" currently already with another vendor SCMI
drivers and custom protocol currently under review, so I was thinking to
add a new common mechanism in SCMI to handle this ... not thought about
this really in depth and I want to chat with Sudeep about this...

> 2. Ask Sibi to wait with this change, till we refactor the
>    exiting driver such that it could support easily those
>    multiple instances. Then pick up this patch set.
>    Although, we would also like to have those notifications from our
>    Juno SCP reference FW, so the feature is useful.
> 3. Ask Sibi to refactor his patch to somehow get the 'handle'
>    in different way, using exiting code and not introduce this global.
> 

> IHMO we could do this in steps: 1. and then 2. When
> we create some mock platform to test this refactoring we can
> start cleaning it.
> 

Both of these options really beg an answer to my original previous q
question...if we somehow enable this multi-probe support in the
scmi-cpufreq.c driver by avoiding glbals refs, does this work at all in
the context of CPUFreq ?

..or it is just that CPUFreq cannot handle such a configuration (and
maybe dont want to) and so the only solution here is just 1. at first and
then a common refined mechanism (as mentioned above) to ensure this "unicity"
of the probes for some drivers ?

I'm not familiar enough to grasp if this "multi-probed" mode of operation is
allowed/supported by CPUFreq and, more important, if it makes any sense
at all to be a supported mode...

Thanks,
Cristian