linux-kernel - Re: [PATCH v8 01/34] opp: Add dev_pm_opp

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Date:   Wed, 18 Aug 2021 18:46:10 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Ulf Hansson <ulf.hansson@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Viresh Kumar <vireshk@...nel.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Peter De Schrijver <pdeschrijver@...dia.com>,
        Mikko Perttunen <mperttunen@...dia.com>,
        Peter Chen <peter.chen@...nel.org>,
        Mark Brown <broonie@...nel.org>,
        Lee Jones <lee.jones@...aro.org>,
        Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
        Nishanth Menon <nm@...com>,
        Vignesh Raghavendra <vigneshr@...com>,
        Richard Weinberger <richard@....at>,
        Miquel Raynal <miquel.raynal@...tlin.com>,
        Lucas Stach <dev@...xeye.de>, Stefan Agner <stefan@...er.ch>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-tegra <linux-tegra@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux USB List <linux-usb@...r.kernel.org>,
        linux-staging@...ts.linux.dev, linux-spi@...r.kernel.org,
        linux-pwm@...r.kernel.org, linux-mtd@...ts.infradead.org,
        linux-mmc <linux-mmc@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        DTML <devicetree@...r.kernel.org>,
        linux-clk <linux-clk@...r.kernel.org>
Subject: Re: [PATCH v8 01/34] opp: Add dev_pm_opp_sync() helper

18.08.2021 18:43, Dmitry Osipenko пишет:
> 18.08.2021 13:08, Ulf Hansson пишет:
>> On Wed, 18 Aug 2021 at 11:50, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>>>
>>> On 18-08-21, 11:41, Ulf Hansson wrote:
>>>> On Wed, 18 Aug 2021 at 11:14, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>>>>> What we need here is just configure. So something like this then:
>>>>>
>>>>> - genpd->get_performance_state()
>>>>>   -> dev_pm_opp_get_current_opp() //New API
>>>>>   -> dev_pm_genpd_set_performance_state(dev, current_opp->pstate);
>>>>>
>>>>> This can be done just once from probe() then.
>>>>
>>>> How would dev_pm_opp_get_current_opp() work? Do you have a suggestion?
>>>
>>> The opp core already has a way of finding current OPP, that's what
>>> Dmitry is trying to use here. It finds it using clk_get_rate(), if
>>> that is zero, it picks the lowest freq possible.
>>>
>>>> I am sure I understand the problem. When a device is getting probed,
>>>> it needs to consume power, how else can the corresponding driver
>>>> successfully probe it?
>>>
>>> Dmitry can answer that better, but a device doesn't necessarily need
>>> to consume energy in probe. It can consume bus clock, like APB we
>>> have, but the more energy consuming stuff can be left disabled until
>>> the time a user comes up. Probe will just end up registering the
>>> driver and initializing it.
>>
>> That's perfectly fine, as then it's likely that it won't vote for an
>> OPP, but can postpone that as well.
>>
>> Perhaps the problem is rather that the HW may already carry a non-zero
>> vote made from a bootloader. If the consumer driver tries to clear
>> that vote (calling dev_pm_opp_set_rate(dev, 0), for example), it would
>> still not lead to any updates of the performance state in genpd,
>> because genpd internally has initialized the performance-state to
>> zero.
> 
> We don't need to discover internal SoC devices because we use
> device-tree on ARM. For most devices power isn't required at a probe
> time because probe function doesn't touch h/w at all, thus devices are
> left in suspended state after probe.
> 
> We have three components comprising PM on Tegra:
> 
> 1. Power gate
> 2. Clock state
> 3. Voltage state
> 
> GENPD on/off represents the 'power gate'.
> 
> Clock and reset are controlled by device drivers using clk and rst APIs.
> 
> Voltage state is represented by GENPD's performance level.

OPP framework couples the performance level with the clock rate.

> GENPD core assumes that at a first rpm-resume of a consumer device, its
> genpd_performance=0. Not true for Tegra because h/w of the device is
> preconfigured to a non-zero perf level initially, h/w may not support
> zero level at all.
> 
> GENPD core assumes that consumer devices can work at any performance
> level. Not true for Tegra because voltage needs to be set in accordance
> to the clock rate before clock is enabled, otherwise h/w won't work
> properly, perhaps clock may be unstable or h/w won't be latching.
> 
> Performance level should be set to 0 while device is suspended.
> Performance level needs to be bumped on rpm-resume of a device in
> accordance to h/w state before hardware is enabled.
>