Message-ID: <a576209f-ffda-4891-82e9-21f153b57a26@arm.com>
Date: Fri, 22 Mar 2024 10:45:30 +0000
From: Lukasz Luba <lukasz.luba@....com>
To: Sibi Sankar <quic_sibis@...cinc.com>
Cc: sudeep.holla@....com, Cristian Marussi <cristian.marussi@....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

Hi Sibi,

On 3/1/24 05:31, Sibi Sankar wrote:
> 
> 
> On 2/29/24 19:45, Lukasz Luba wrote:
>>
>>
>> On 2/29/24 12:11, Cristian Marussi wrote:
>>> 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.
>>>>>>>>>>
>>>>>>>>>> � drivers/cpufreq/scmi-cpufreq.c | 29 
>>>>>>>>>> ++++++++++++++++++++++++++++-
>>>>>>>>>> � 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 {
>>>>>>>>>> ����� int domain_id;
>>>>>>>>>> ����� int nr_opp;
>>>>>>>>>> ����� struct device *cpu_dev;
>>>>>>>>>> +��� struct cpufreq_policy *policy;
>>>>>>>>>> ����� cpumask_var_t opp_shared_cpus;
>>>>>>>>>> +��� struct notifier_block limit_notify_nb;
>>>>>>>>>> � };
>>>>>>>>>> +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.
> 
> I should be able to volunteer a platform to test against when
> we have things ready.
> 
>>>>
>>>
>>> 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 ?
>>
>> I don't know yet.
>>
>>>
>>> ...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 ?
>>
>> This sounds reasonable.
>>
>>>
>>> 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...
>>>
>>
>> OK, let me check some stuff in the code and think for a while on that.
>> Thanks Cristian!
>>
>> Sibi, please give me a few days. In the meantime you can continue
>> on the 'boost' patch set probably.
> 
> sure, thanks. I've plenty things to send out so no hurry ;)
> 
> -Sibi
> 

I've went through the cpufreq. It's quite complicated how those
policies, cpus, drivers are setup. Although, IHMO we should be
safe with you current proposal in this patch.

As we discussed with Cristian, we can take that approach further.

Therefore, you can add:

Reviewed-by: Lukasz Luba <lukasz.luba@....com>

Regards,
Lukasz

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