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[46.138.3.129]) by smtp.googlemail.com with ESMTPSA id t20sm756427lfk.157.2021.09.06.07.11.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 06 Sep 2021 07:11:27 -0700 (PDT) Subject: Re: [PATCH 1/3] PM: domains: Drop the performance state vote for a device at detach To: Ulf Hansson Cc: "Rafael J . Wysocki" , Viresh Kumar , Linux PM , Dmitry Baryshkov , Jonathan Hunter , Thierry Reding , Rajendra Nayak , Stephan Gerhold , Bjorn Andersson , Linux ARM , Linux Kernel Mailing List References: <20210902101634.827187-1-ulf.hansson@linaro.org> <20210902101634.827187-2-ulf.hansson@linaro.org> <08335cd4-7dc8-3b8b-d63f-374585ffa373@gmail.com> <6603212d-f36c-afff-6222-8125de5b7b79@gmail.com> From: Dmitry Osipenko Message-ID: <66fbbc69-3e16-e07c-4e25-48d59d69fd3c@gmail.com> Date: Mon, 6 Sep 2021 17:11:26 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 06.09.2021 13:24, Ulf Hansson пишет: > On Sun, 5 Sept 2021 at 10:26, Dmitry Osipenko wrote: >> >> 03.09.2021 17:03, Ulf Hansson пишет: >>> On Fri, 3 Sept 2021 at 11:58, Dmitry Osipenko wrote: >>>> >>>> 03.09.2021 11:22, Ulf Hansson пишет: >>>>> On Fri, 3 Sept 2021 at 08:01, Dmitry Osipenko wrote: >>>>>> >>>>>> 02.09.2021 13:16, Ulf Hansson пишет: >>>>>>> When a device is detached from its genpd, genpd loses track of the device, >>>>>>> including its performance state vote that may have been requested for it. >>>>>>> >>>>>>> Rather than relying on the consumer driver to drop the performance state >>>>>>> vote for its device, let's do it internally in genpd when the device is >>>>>>> getting detached. In this way, we makes sure that the aggregation of the >>>>>>> votes in genpd becomes correct. >>>>>> >>>>>> This is a dangerous behaviour in a case where performance state >>>>>> represents voltage. If hardware is kept active on detachment, say it's >>>>>> always-on, then it may be a disaster to drop the voltage for the active >>>>>> hardware. >>>>>> >>>>>> It's safe to drop performance state only if you assume that there is a >>>>>> firmware behind kernel which has its own layer of performance management >>>>>> and it will prevent the disaster by saying 'nope, I'm not doing this'. >>>>>> >>>>>> The performance state should be persistent for a device and it should be >>>>>> controlled in a conjunction with runtime PM. If platform wants to drop >>>>>> performance state to zero on detachment, then this behaviour should be >>>>>> specific to that platform. >>>>> >>>>> I understand your concern, but at this point, genpd can't help to fix this. >>>>> >>>>> Genpd has no information about the device, unless it's attached to it. >>>>> For now and for these always on HWs, we simply need to make sure the >>>>> device stays attached, in one way or the other. >>>> >>>> This indeed requires to redesign GENPD to make it more coupled with a >>>> device, but this is not a real problem for any of the current API users >>>> AFAIK. Ideally the state should be persistent to make API more universal. >>> >>> Right. In fact this has been discussed in the past. In principle, the >>> idea was to attach to genpd at device registration, rather than at >>> driver probe. >>> >>> Although, this is not very easy to implement - and it seems like the >>> churns to do, have not been really worth it. At least so far. >>> >>>> >>>> Since for today we assume that device should be suspended at the time of >>>> the detachment (if the default OPP state isn't used), it may be better >>>> to add a noisy warning message if pstate!=0, keeping the state untouched >>>> if it's not zero. >>> >>> That would just be very silly in my opinion. >>> >>> When the device is detached (suspended or not), it may cause it's PM >>> domain to be powered off - and there is really nothing we can do about >>> that from the genpd point of view. >>> >>> As stated, the only current short term solution is to avoid detaching >>> the device. Anything else, would just be papering of the issue. >> >> What about to re-evaluate the performance state of the domain after >> detachment instead of setting the state to zero? > > I am not suggesting to set the performance state of the genpd to zero, > but to drop a potential vote for a performance state for the *device* > that is about to be detached. By removing the vote of the *device*, you will drop the performance state of the genpd. If device is active and it's wrong to drop its state, then you may cause the damage. > Calling genpd_set_performance_state(dev, 0), during detach will have > the same effect as triggering a re-evaluation of the performance state > for the genpd, but after the detach. Yes >> This way PD driver may >> take an action on detachment if performance isn't zero, before hardware >> is crashed, for example it may emit a warning. > > Not sure I got that. Exactly when do you want to emit a warning and > for what reason? > > Do you want to add a check somewhere to see if > 'gpd_data->performance_state' is non zero - and then print a warning? I want to check the 'gpd_data->performance_state' from the detachment callback and emit the warning + lock further performance changes in the PD driver since it's a error condition.