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Message-ID: <08018d07-79cf-cebd-aba5-214afbc5001d@quicinc.com>
Date: Fri, 1 Mar 2024 11:01:19 +0530
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Subject: Re: [PATCH V3 2/2] cpufreq: scmi: Register for limit change
 notifications
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To: Lukasz Luba <lukasz.luba@arm.com>,
        Cristian Marussi
	<cristian.marussi@arm.com>
CC: <sudeep.holla@arm.com>, <linux-arm-kernel@lists.infradead.org>,
        <pierre.gondois@arm.com>, <dietmar.eggemann@arm.com>,
        <morten.rasmussen@arm.com>, <viresh.kumar@linaro.org>,
        <rafael@kernel.org>, <linux-pm@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <quic_mdtipton@quicinc.com>,
        <linux-arm-msm@vger.kernel.org>
References: <20240227181632.659133-1-quic_sibis@quicinc.com>
 <20240227181632.659133-3-quic_sibis@quicinc.com>
 <f8bfc666-c216-44d5-a63b-99f04ff3b8ef@arm.com>
 <2608b2d8-f3b0-b4f5-f8e4-1f2242043ded@quicinc.com>
 <64c6a1bc-92f2-4f44-ab10-cbd2473746f3@arm.com>
 <18c249b2-ce8c-435b-8d65-a1770a1f294e@arm.com> <ZeBqW04f8V4dHphn@pluto>
 <7c82b316-89d9-470d-b46d-f86e81e2add3@arm.com> <ZeB0iCr9GpfUiOEg@pluto>
 <66ca73cc-8bdd-453a-951c-5e0166340edd@arm.com>
From: Sibi Sankar <quic_sibis@quicinc.com>
In-Reply-To: <66ca73cc-8bdd-453a-951c-5e0166340edd@arm.com>
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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@quicinc.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.

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