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[2001:1c00:c0c:fe00:d2ea:f29d:118b:24dc]) by smtp.gmail.com with ESMTPSA id i6sm3566852wmb.41.2020.05.19.02.36.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 19 May 2020 02:36:35 -0700 (PDT) Subject: Re: [PATCHv2 0/7] Support inhibiting input devices To: Andrzej Pietrasiewicz , linux-input@vger.kernel.org, linux-acpi@vger.kernel.org, linux-iio@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-samsung-soc@vger.kernel.org, linux-tegra@vger.kernel.org, patches@opensource.cirrus.com, ibm-acpi-devel@lists.sourceforge.net, platform-driver-x86@vger.kernel.org Cc: "Rafael J . Wysocki" , Len Brown , Jonathan Cameron , Hartmut Knaack , Lars-Peter Clausen , Peter Meerwald-Stadler , Kukjin Kim , Krzysztof Kozlowski , Dmitry Torokhov , Shawn Guo , Sascha Hauer , Pengutronix Kernel Team , Fabio Estevam , NXP Linux Team , Vladimir Zapolskiy , Sylvain Lemieux , Laxman Dewangan , Thierry Reding , Jonathan Hunter , Barry Song , Michael Hennerich , Nick Dyer , Ferruh Yigit , Sangwon Jee , Henrique de Moraes Holschuh , kernel@collabora.com, Peter Hutterer , Benjamin Tissoires References: <20200506002746.GB89269@dtor-ws> <20200515164943.28480-1-andrzej.p@collabora.com> <842b95bb-8391-5806-fe65-be64b02de122@redhat.com> <6d9921fc-5c2f-beda-4dcd-66d6970a22fe@redhat.com> <09679de4-75d3-1f29-ec5f-8d42c84273dd@collabora.com> <2d224833-3a7e-bc7c-af15-1f803f466697@collabora.com> From: Hans de Goede Message-ID: Date: Tue, 19 May 2020 11:36:34 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: <2d224833-3a7e-bc7c-af15-1f803f466697@collabora.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-acpi-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-acpi@vger.kernel.org Hi, On 5/19/20 11:02 AM, Andrzej Pietrasiewicz wrote: > Hi Hans, Hi Dmitry, > > W dniu 18.05.2020 o 16:23, Hans de Goede pisze: >> Hi, > > > >>>>>> >>>>>> So I wonder what this series actually adds for functionality for >>>>>> userspace which can not already be achieved this way? >>>>>> >>>>>> I also noticed that you keep the device open (do not call the >>>>>> input_device's close callback) when inhibited and just throw away >>>>> >>>>> I'm not sure if I understand you correctly, it is called: >>>>> >>>>> +static inline void input_stop(struct input_dev *dev) >>>>> +{ >>>>> +    if (dev->poller) >>>>> +        input_dev_poller_stop(dev->poller); >>>>> +    if (dev->close) >>>>> +        dev->close(dev); >>>>>                  ^^^^^^^^^^^^^^^^ >>>>> +static int input_inhibit(struct input_dev *dev) >>>>> +{ >>>>> +    int ret = 0; >>>>> + >>>>> +    mutex_lock(&dev->mutex); >>>>> + >>>>> +    if (dev->inhibited) >>>>> +        goto out; >>>>> + >>>>> +    if (dev->users) { >>>>> +        if (dev->inhibit) { >>>>> +            ret = dev->inhibit(dev); >>>>> +            if (ret) >>>>> +                goto out; >>>>> +        } >>>>> +        input_stop(dev); >>>>>                  ^^^^^^^^^^^^^^^^ >>>>> >>>>> It will not be called when dev->users is zero, but if it is zero, >>>>> then nobody has opened the device yet so there is nothing to close. >>>> >>>> Ah, I missed that. >>>> >>>> So if the device implements the inhibit call back then on >>>> inhibit it will get both the inhibit and close callback called? >>>> >>> >>> That's right. And conversely, upon uninhibit open() and uninhibit() >>> callbacks will be invoked. Please note that just as with open()/close(), >>> providing inhibit()/uninhibit() is optional. >> >> Ack. >> >>>> And what happens if the last user goes away and the device >>>> is not inhibited? >>> >>> close() is called as usually. >> >> But not inhibit, hmm, see below. >> >>>> I'm trying to understand here what the difference between the 2 >>>> is / what the goal of having a separate inhibit callback ? >>>> >>> >>> Drivers have very different ideas about what it means to suspend/resume >>> and open/close. The optional inhibit/uninhibit callbacks are meant for >>> the drivers to know that it is this particular action going on. >> >> So the inhibit() callback triggers the "suspend" behavior ? >> But shouldn't drivers which are capable of suspending the device >> always do so on close() ? >> >> Since your current proposal also calls close() on inhibit() I >> really see little difference between an inhibit() and the last >> user of the device closing it and IMHO unless there is a good >> reason to actually differentiate the 2 it would be better >> to only stick with the existing close() and in cases where >> that does not put the device in a low-power mode yet, fix >> the existing close() callback to do the low-power mode >> setting instead of adding a new callback. >> >>> For inhibit() there's one more argument: close() does not return a value, >>> so its meaning is "do some last cleanup" and as such it is not allowed >>> to fail - whatever its effect is, we must deem it successful. inhibit() >>> does return a value and so it is allowed to fail. >> >> Well, we could make close() return an error and at least in the inhibit() >> case propagate that to userspace. I wonder if userspace is going to >> do anything useful with that error though... >> >> In my experience errors during cleanup/shutdown are best logged >> (using dev_err) and otherwise ignored, so that we try to clean up >> as much possible. Unless the very first step of the shutdown process >> fails the device is going to be in some twilight zone state anyways >> at this point we might as well try to cleanup as much as possible. > > What you say makes sense to me. > @Dmitry? > >> >>> All in all, it is up to the drivers to decide which callback they >>> provide. Based on my work so far I would say that there are tens >>> of simple cases where open() and close() are sufficient, out of total >>> ~400 users of input_allocate_device(): >>> >>> $ git grep "input_allocate_device(" | grep -v ^Documentation | \ >>> cut -f1 -d: | sort | uniq | wc >>>      390     390   13496 >> >> So can you explain a bit more about the cases where only having >> open/close is not sufficient?  So far I have the feeling that >> those are all we need and that we really do not need separate >> [un]inhibit callbacks. > > My primary concern was not being able to propagate inhibit() error > to userspace, and then if we have inhibit(), uninhibit() should be > there for completeness. If propagating the error to userspace can > be neglected then yes, it seems open/close should be sufficient, > even more because the real meaning of "open" is "prepare the device > for generating input events". > > To validate the idea of not introducing inhibit()/uninhibit() callbacks > to implement device inhibiting/uninhibiting let's look at > drivers/input/mouse/elan_i2c_core.c (PATCH 7/7): > > static int elan_inhibit(struct input_dev *input) > { > [...] > >     ret = mutex_lock_interruptible(&data->sysfs_mutex); >     if (ret) >         return ret; > >     disable_irq(client->irq); > >     ret = elan_disable_power(data); >     if (ret) >         enable_irq(client->irq); > [...] > } > > First, close() does not exist in this driver. Of course this can be > fixed. Then it doesn't return a value. Then, if either taking the > mutex or disabling the power fails, the close() is still deemed > successful. Is it ok? Note I also mentioned another solution for the error propagation, which would require a big "flag day" commit adding "return 0" to all existing close callbacks, but otherwise should work for your purposes: > Well, we could make close() return an error and at least in the inhibit() > case propagate that to userspace. I wonder if userspace is going to > do anything useful with that error though... And I guess we could log an error that close failed in the old close() path where we cannot propagate the error. Also why the mutex_lock_interruptible() ? If you change that to a normal mutex_lock() you loose one of the possible 2 error cases and I doubt anyone is going to do a CTRL-C of the process doing the inhibiting (or that that process starts a timer using a signal to ensure the inhibit does not take to long or some such). Regards, Hans