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[46.138.3.129]) by smtp.googlemail.com with ESMTPSA id y16sm810359lfs.145.2021.09.06.12.33.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 06 Sep 2021 12:33:26 -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> <66fbbc69-3e16-e07c-4e25-48d59d69fd3c@gmail.com> From: Dmitry Osipenko Message-ID: <7ee1a2eb-67d7-dcad-6039-05129d3822ec@gmail.com> Date: Mon, 6 Sep 2021 22:33:25 +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-pm@vger.kernel.org 06.09.2021 20:34, Ulf Hansson пишет: > On Mon, 6 Sept 2021 at 16:11, Dmitry Osipenko wrote: >> >> 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. > > Alright, so if I understand correctly, you intend to do the check for > the "error condition" of the device in the genpd->detach_dev() > callback? Yes > What exactly do you intend to do beyond this point, if you detect the > "error condition"? Locking further changes of the performance state > seems fragile too, especially if some other device/driver requires the > performance state to be raised. It sounds like you simply need to call > BUG_ON() then? I can lock it to high performance state. > Also note that a very similar problem exists, *before* the device gets > attached in the first place. More precisely, nothing prevents the > performance state from being set to a non-compatible value for an > always-on HW/device that hasn't been attached yet. So maybe you need > to set the maximum performance state at genpd initializations, then > use the ->sync_state() callback to very that all consumers have been > attached to the genpd provider, before allowing the state to be > changed/lowered? That is already done by the PD driver. https://elixir.bootlin.com/linux/latest/source/drivers/soc/tegra/pmc.c#L3790