Re: [PATCH v2 17/18] cpufreq: add support for CPU DVFS based on SCMI message protocol

From: Sudeep Holla <sudeep.holla@arm.com>
To: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Sudeep Holla <sudeep.holla@arm.com>,
	ALKML <linux-arm-kernel@lists.infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	DTML <devicetree@vger.kernel.org>,
	Roy Franz <roy.franz@cavium.com>,
	Harb Abdulhamid <harba@codeaurora.org>,
	Nishanth Menon <nm@ti.com>, Arnd Bergmann <arnd@arndb.de>,
	Loc Ho <lho@apm.com>, Alexey Klimov <alexey.klimov@arm.com>,
	Ryan Harkin <Ryan.Harkin@arm.com>,
	Jassi Brar <jassisinghbrar@gmail.com>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	linux-pm@vger.kernel.org
Subject: Re: [PATCH v2 17/18] cpufreq: add support for CPU DVFS based on SCMI message protocol
Date: Wed, 9 Aug 2017 11:15:49 +0100	[thread overview]
Message-ID: <d98703ce-7e66-4172-8d36-03c5200e3208@arm.com> (raw)
In-Reply-To: <20170809100651.GA29383@vireshk-i7>

On 09/08/17 11:06, Viresh Kumar wrote:
> On 09-08-17, 10:59, Sudeep Holla wrote:
>> On 09/08/17 05:18, Viresh Kumar wrote:
> 
>>> This stores the same handle pointer which is stored in the global variable
>>> below. Right? Why keep a local variable here at all ?
>>
>> Yes, you are right. Initially, started with just private pointers and
>> then added global. I was thinking of calling devm_scmi_handle_get per
>> policy to reflect the refcount correctly and drop global variable. Let
>> me know what you think.
> 
> A refcount of 1 should be fine as well, i.e. For the cpufreq driver. Why would
> SCMI care if we manage multiple policies here ? Unless it makes something within
> SCMI core better.
> 

Not really, just we can get rid of global pointer which may be need in
system with multiple scmi instances, but that's long way to go.

>>> This is something special which is used only when we are returning indexes and
>>> I am not sure if this will have benefit here. I will rather return 0 here.
>>> That's what other drivers are doing.
>>
>> Indeed had 0 initially but changed as per Juri's suggestion.
> 
> Maybe he suggested doing that in the fast switch routine ? As that's the normal
> protocol there. Though I have sent a patch today to propose using 0 there as
> well (you cc'd).
> 

Yes, saw that. I have changed both to 0 for now. I will watch that
thread and update if necessary before next posting.

>> But is 0
>> treated as failure and still running at current OPP ?
> 
> You have used that in the ->get() routine. So the OPP isn't changing, but we are
> just trying to fetch it. cpufreq core doesn't do a lot with the value returned
> from here, but at one place we break early if 0 is returned. And so all drivers
> are returning that.
>

Agreed, I assumed _INVALID is new thing and changed at both target_indes
and fast_switch.

>> and not 0KHz I assume.
> 
> Yeah, 0 KHz is dead CPU really :)
> 

:)

>>> I suppose any CPU can change the frequency of any other CPU here, right? You
>>> must set policy->dvfs_possible_from_any_cpu = true, from ->init() then.
>>>
>>
>> OK, I missed to see something like that exists, will do.
> 
> Fairly recent stuff, present in pm/linux-next only.
> 

Oh OK.

>>>> +	/*
>>>> +	 * But we need OPP table to function so if it is not there let's
>>>> +	 * give platform code chance to provide it for us.
>>>> +	 */
>>>
>>> How are we getting the OPPs? DT or non DT ?
>>>
>>
>> Non DT :), from the firmware.
> 
> I would improve the above comment in that case to clearly say that OPPs are
> added by the platform, lets wait for it.
> 

Done

-- 
Regards,
Sudeep