linux-kernel - Re: [PATCH] PM / devfreq: Generic cpufreq governor

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Date:   Wed, 1 Aug 2018 17:03:29 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Saravana Kannan <skannan@...eaurora.org>,
        MyungJoo Ham <myungjoo.ham@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Chanwoo Choi <cw00.choi@...sung.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Cc:     Sudeep Holla <sudeep.holla@....com>, linux-pm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PM / devfreq: Generic cpufreq governor



On 28/07/18 04:56, Saravana Kannan wrote:
> Many CPU architectures have caches that can scale independent of the CPUs.
> Frequency scaling of the caches is necessary to make sure the cache is not
> a performance bottleneck that leads to poor performance and power. The same
> idea applies for RAM/DDR.
> 
> To achieve this, this patch adds a generic devfreq governor that can listen
> to the frequency transitions of each CPU frequency domain and then adjusts
> the frequency of the cache (or any devfreq device) based on the frequency
> of the CPUs.
> 
> To decide the frequency of the device, the governor does one of the
> following:
> 
> * Uses a CPU frequency to device frequency mapping table
>   - Either one mapping table used for all CPU freq policies (typically used
>     for system with homogeneous cores/clusters that have the same OPPs.
>   - One mapping table per CPU freq policy (typically used for ASMP systems
>     with heterogeneous CPUs with different OPPs)
> 
> OR
> 
> * Scales the device frequency in proportion to the CPU frequency. So, if
>   the CPUs are running at their max frequency, the device runs at its max
>   frequency.  If the CPUs are running at their min frequency, the device
>   runs at its min frequency. And interpolated for frequencies in between.
> 

Is this solution for the old generation of SDM ?
I have seen newer ones have some kind of firmware interface/hardware to
deal with CPUFreq. Do you need this solution for them too ?

If yes, why ? IMO firmware can arbitrate various requests for frequency
scaling and do the *right thing* for the platform. Having OSPM sending
separate requests for such bus/interconnect might end up with conflicts.
No ?

-- 
Regards,
Sudeep