From: Sibi Sankar <firstname.lastname@example.org> To: email@example.com Cc: firstname.lastname@example.org, Kyungmin Park <email@example.com>, Chanwoo Choi <firstname.lastname@example.org>, Rob Herring <email@example.com>, Mark Rutland <firstname.lastname@example.org>, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com Subject: Re: [PATCH v3 1/2] PM / devfreq: Generic CPU frequency to device frequency mapping governor Date: Tue, 11 Sep 2018 00:15:28 +0530 Message-ID: <firstname.lastname@example.org> (raw) In-Reply-To: <email@example.com> Hi Saravana, On 2018-08-07 11:19, firstname.lastname@example.org wrote: > On 2018-08-02 14:00, email@example.com wrote: >> On 2018-08-02 02:56, MyungJoo Ham 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 >>>> takes the >>>> current frequency of each CPU frequency domain and then adjusts the >>>> frequency of the cache (or any devfreq device) based on the >>>> frequency of >>>> the CPUs. It listens to CPU frequency transition notifiers to keep >>>> itself >>>> up to date on the current CPU frequency. >>>> With the cpu-freq driver for SDM845 SoC supporting fast_switch and schedutil supporting dynamic switching wouldn't this governor be incompatible due to its reliance on transition notifiers? Is it planned to be used only with ondemand/performance governors? >>>> To decide the frequency of the device, the governor does one of the >>>> following: >>> >>> This exactly has the same purpose with "passive" governor except for >>> the >>> single part: passive governor depends on another devfreq driver, not >>> cpufreq driver. >>> >>> If both governors have many features in common, can you merge them >>> into one? >>> Passive governor also has "get_target_freq", which allows driver >>> authors >>> to define the mapping. >> >> Thanks for the review and pointing me to the passive governor. I agree >> that at a high level they are both doing the same. I can certainly >> stuff this CPU freq to dev freq mapping into passive governor, but I >> think it'll just make one huge set of code that's harder to understand >> and maintain because it trying to do different things under the hood. >> >> There are also a bunch of complexities and optimizations that come >> with the cpufreq-map governor that don't fit with the passive >> governor. >> >> 1. It's not one CPU who's frequency we have to listen to. There are >> multiple CPUs/policies we have to aggregate across. >> 2. We have to deal with big vs little having different needs/mappings. >> 3. Since it's always just CPUfreq, I can optimize the handling in the >> transition notifiers. If I have 4 different devices that are scaled >> based on CPU freq, I still use only 1 transition notifier. It becomes >> a bit harder to do with the passive governor. >> 4. It requires that the device explicitly support the passive governor >> and pick a mapping function. With cpufreq-map governor, the device >> drivers don't need to make any changes. Whoever is making a >> device/board can choose what devices to scale up base on CPU freq >> based on their board and their needs. Even an end user can say, scale >> the GPU based on my CPU based on interpolation if they choose to. >> 5. If a device has some other use for the private data, it can't work >> with passive governor, but can work with cpufreq-map governor. >> 6. I also want to improve cpufreq-map governor to handle hotplug >> correctly in later patches (needs more discussion) and that'll add >> more complexity. >> >> I think for these reasons we shouldn't combine these two governors. >> Let me know what you think. > > Friendly reminder. > > Thanks, > Saravana -- -- Sibi Sankar -- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
next prev parent reply index Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-08-02 0:57 Saravana Kannan [not found] ` <CGME20180802005756epcas4p465a12f42a0e36f0af6fd276a3a56957f@epcms1p3> 2018-08-02 9:56 ` MyungJoo Ham 2018-08-02 21:00 ` skannan 2018-08-07 5:49 ` skannan 2018-09-10 18:45 ` Sibi Sankar [this message] 2018-08-07 16:41 ` Rob Herring 2018-08-07 19:37 ` skannan 2018-08-08 8:47 ` Sudeep Holla 2018-08-08 21:18 ` skannan 2018-08-09 9:43 ` Sudeep Holla
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