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Tue, 9 Jun 2020 13:02:13 +0000 (GMT) Received: from eusmgms1.samsung.com (unknown [182.198.249.179]) by eusmtrp2.samsung.com (KnoxPortal) with ESMTP id 20200609130213eusmtrp243a4f42078299f9f813dd3c5b948fe95~W4cAQJYJC1791317913eusmtrp2Z; Tue, 9 Jun 2020 13:02:13 +0000 (GMT) X-AuditID: cbfec7f5-a29ff7000001ed1a-a2-5edf88556086 Received: from eusmtip2.samsung.com ( [203.254.199.222]) by eusmgms1.samsung.com (EUCPMTA) with SMTP id 8B.03.08375.5588FDE5; Tue, 9 Jun 2020 14:02:13 +0100 (BST) Received: from [106.210.85.205] (unknown [106.210.85.205]) by eusmtip2.samsung.com (KnoxPortal) with ESMTPA id 20200609130212eusmtip20a30ff72a9370bdbb70b7bc1abfa53ba~W4b-MVaLQ0625706257eusmtip2F; Tue, 9 Jun 2020 13:02:12 +0000 (GMT) Subject: Re: [PATCH v3] driver core: Break infinite loop when deferred probe can't be satisfied To: Marco Felsch Cc: Andy Shevchenko , Grant Likely , Saravana Kannan , artem.bityutskiy@linux.intel.com, balbi@kernel.org, broonie@kernel.org, fntoth@gmail.com, gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org, peter.ujfalusi@ti.com, rafael@kernel.org, kernel-team@android.com, nd , kernel@pengutronix.de From: Andrzej Hajda Message-ID: <12859a82-6402-71fc-7d51-20ec40999f88@samsung.com> Date: Tue, 9 Jun 2020 15:02:11 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; 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charset="utf-8" X-RootMTR: 20200608091722eucas1p2fa8a4ac15c70e5a6e03c4babdf9f96b7 X-EPHeader: CA CMS-TYPE: 201P X-CMS-RootMailID: 20200608091722eucas1p2fa8a4ac15c70e5a6e03c4babdf9f96b7 References: <20200324175719.62496-1-andriy.shevchenko@linux.intel.com> <20200325032901.29551-1-saravanak@google.com> <20200325125120.GX1922688@smile.fi.intel.com> <295d25de-f01e-26de-02d6-1ac0c149d828@arm.com> <20200326163110.GD1922688@smile.fi.intel.com> <20200608091712.GA28093@pengutronix.de> <437de51b-37e9-d8d1-19c7-137a9265bf45@samsung.com> <20200609064511.7nek2rhk6ebfjaia@pengutronix.de> <20200609121029.nfhgilpu5meoygoa@pengutronix.de> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09.06.2020 14:10, Marco Felsch wrote: > On 20-06-09 11:27, Andrzej Hajda wrote: >> On 09.06.2020 08:45, Marco Felsch wrote: >>> On 20-06-08 13:11, Andrzej Hajda wrote: >>>> On 08.06.2020 11:17, Marco Felsch wrote: >>>>> On 20-03-26 18:31, Andy Shevchenko wrote: >>>>>> On Thu, Mar 26, 2020 at 03:01:22PM +0000, Grant Likely wrote: >>>>>>> On 25/03/2020 12:51, Andy Shevchenko wrote: >>>>>>>> On Tue, Mar 24, 2020 at 08:29:01PM -0700, Saravana Kannan wrote: >>>>>>>>> On Tue, Mar 24, 2020 at 5:38 AM Andy Shevchenko wrote: >>>>>>>>>> Consider the following scenario. >>>>>>>>>> >>>>>>>>>> The main driver of USB OTG controller (dwc3-pci), which has the following >>>>>>>>>> functional dependencies on certain platform: >>>>>>>>>> - ULPI (tusb1210) >>>>>>>>>> - extcon (tested with extcon-intel-mrfld) >>>>>>>>>> >>>>>>>>>> Note, that first driver, tusb1210, is available at the moment of >>>>>>>>>> dwc3-pci probing, while extcon-intel-mrfld is built as a module and >>>>>>>>>> won't appear till user space does something about it. >>>>>>>>>> >>>>>>>>>> This is depicted by kernel configuration excerpt: >>>>>>>>>> >>>>>>>>>> CONFIG_PHY_TUSB1210=y >>>>>>>>>> CONFIG_USB_DWC3=y >>>>>>>>>> CONFIG_USB_DWC3_ULPI=y >>>>>>>>>> CONFIG_USB_DWC3_DUAL_ROLE=y >>>>>>>>>> CONFIG_USB_DWC3_PCI=y >>>>>>>>>> CONFIG_EXTCON_INTEL_MRFLD=m >>>>>>>>>> >>>>>>>>>> In the Buildroot environment the modules are probed by alphabetical ordering >>>>>>>>>> of their modaliases. The latter comes to the case when USB OTG driver will be >>>>>>>>>> probed first followed by extcon one. >>>>>>>>>> >>>>>>>>>> So, if the platform anticipates extcon device to be appeared, in the above case >>>>>>>>>> we will get deferred probe of USB OTG, because of ordering. >>>>>>>>>> >>>>>>>>>> Since current implementation, done by the commit 58b116bce136 ("drivercore: >>>>>>>>>> deferral race condition fix") counts the amount of triggered deferred probe, >>>>>>>>>> we never advance the situation -- the change makes it to be an infinite loop. >>>>>>>>> Hi Andy, >>>>>>>>> >>>>>>>>> I'm trying to understand this sequence of steps. Sorry if the questions >>>>>>>>> are stupid -- I'm not very familiar with USB/PCI stuff. >>>>>>>> Thank you for looking into this. My answer below. >>>>>>>> >>>>>>>> As a first thing I would like to tell that there is another example of bad >>>>>>>> behaviour of deferred probe with no relation to USB. The proposed change also >>>>>>>> fixes that one (however, less possible to find in real life). >>>>>>>> >>>>>>>>>> ---8<---8<--- >>>>>>>>>> >>>>>>>>>> [ 22.187127] driver_deferred_probe_trigger <<< 1 >>>>>>>>>> >>>>>>>>>> ...here is the late initcall triggers deferred probe... >>>>>>>>>> >>>>>>>>>> [ 22.191725] platform dwc3.0.auto: deferred_probe_work_func in deferred list >>>>>>>>>> >>>>>>>>>> ...dwc3.0.auto is the only device in the deferred list... >>>>>>>>> Ok, dwc3.0.auto is the only unprobed device at this point? >>>>>>>> Correct. >>>>>>>> >>>>>>>>>> [ 22.198727] platform dwc3.0.auto: deferred_probe_work_func 1 <<< counter 1 >>>>>>>>>> >>>>>>>>>> ...the counter before mutex is unlocked is kept the same... >>>>>>>>>> >>>>>>>>>> [ 22.205663] platform dwc3.0.auto: Retrying from deferred list >>>>>>>>>> >>>>>>>>>> ...mutes has been unlocked, we try to re-probe the driver... >>>>>>>>>> >>>>>>>>>> [ 22.211487] bus: 'platform': driver_probe_device: matched device dwc3.0.auto with driver dwc3 >>>>>>>>>> [ 22.220060] bus: 'platform': really_probe: probing driver dwc3 with device dwc3.0.auto >>>>>>>>>> [ 22.238735] bus: 'ulpi': driver_probe_device: matched device dwc3.0.auto.ulpi with driver tusb1210 >>>>>>>>>> [ 22.247743] bus: 'ulpi': really_probe: probing driver tusb1210 with device dwc3.0.auto.ulpi >>>>>>>>>> [ 22.256292] driver: 'tusb1210': driver_bound: bound to device 'dwc3.0.auto.ulpi' >>>>>>>>>> [ 22.263723] driver_deferred_probe_trigger <<< 2 >>>>>>>>>> >>>>>>>>>> ...the dwc3.0.auto probes ULPI, we got successful bound and bumped counter... >>>>>>>>>> >>>>>>>>>> [ 22.268304] bus: 'ulpi': really_probe: bound device dwc3.0.auto.ulpi to driver tusb1210 >>>>>>>>> So where did this dwc3.0.auto.ulpi come from? >>>>>>>>> Looks like the device is created by dwc3_probe() through this call flow: >>>>>>>>> dwc3_probe() -> dwc3_core_init() -> dwc3_core_ulpi_init() -> >>>>>>>>> dwc3_ulpi_init() -> ulpi_register_interface() -> ulpi_register() >>>>>>>> Correct. >>>>>>>> >>>>>>>>>> [ 22.276697] platform dwc3.0.auto: Driver dwc3 requests probe deferral >>>>>>>>> Can you please point me to which code patch actually caused the probe >>>>>>>>> deferral? >>>>>>>> Sure, it's in drd.c. >>>>>>>> >>>>>>>> if (device_property_read_string(dev, "linux,extcon-name", &name) == 0) { >>>>>>>> edev = extcon_get_extcon_dev(name); >>>>>>>> if (!edev) >>>>>>>> return ERR_PTR(-EPROBE_DEFER); >>>>>>>> return edev; >>>>>>>> } >>>>>>>> >>>>>>>>>> ...but extcon driver is still missing... >>>>>>>>>> >>>>>>>>>> [ 22.283174] platform dwc3.0.auto: Added to deferred list >>>>>>>>>> [ 22.288513] platform dwc3.0.auto: driver_deferred_probe_add_trigger local counter: 1 new counter 2 >>>>>>>>> I'm not fully aware of all the USB implications, but if extcon is >>>>>>>>> needed, why can't that check be done before we add and probe the ulpi >>>>>>>>> device? That'll avoid this whole "fake" probing and avoid the counter >>>>>>>>> increase. And avoid the need for this patch that's touching the code >>>>>>>>> code that's already a bit delicate. >>>>>>>>> Also, with my limited experience with all the possible drivers in the >>>>>>>>> kernel, it's weird that the ulpi device is added and probed before we >>>>>>>>> make sure the parent device (dwc3.0.auto) can actually probe >>>>>>>>> successfully. >>>>>>>> As I said above the deferred probe trigger has flaw on its own. >>>>>>>> Even if we fix for USB case, there is (and probably will be) others. >>>>>>> Right here is the driver design bug. A driver's probe() hook should *not* >>>>>>> return -EPROBE_DEFER after already creating child devices which may have >>>>>>> already been probed. >>>>>> Any documentation statement for this requirement? >>>>>> >>>>>> By the way, I may imagine other mechanisms that probe the driver on other CPU >>>>>> at the same time (let's consider parallel modprobes). The current code has a >>>>>> flaw with that. >>>>> Hi, >>>>> >>>>> sorry for picking this up again but I stumbled above the same issue >>>>> within the driver imx/drm driver which is using the component framework. >>>>> I end up in a infinity boot loop if I enabled the HDMI (which is the >>>>> DesignWare bridge device) and the LVDS support and the LVDS bind return >>>>> with EPROBE_DEFER. There are no words within the component framework docs >>>>> which says that this is forbidden. Of course we can work-around the >>>>> driver-core framework but IMHO this shouldn't be the way to go. I do not >>>>> say that we should revert the commit introducing the regression but we >>>>> should address this not only by extending the docs since the most >>>>> drm-drivers are using the component framework and can end up in the same >>>>> situation. >>>> I am not sure why do you think this is similar issue. >>> Because I see trying to bind the device over and over.. >>> >>>> Please describe the issue in more detail. Which drivers defers probe and >>>> why, and why do you have infinite loop. >>> As said I'm currently on the imx-drm driver. The iMX6 devices are >>> using the synopsis HDMI IP core and so they are using this bridge device >>> driver (drivers/gpu/drm/bridge/synopsys/). The imx-drm driver can be >>> build module wise. As example I enabled the LDB and the HDMI support. >>> The HDMI driver is composed as platform driver with different >>> (sub-)drivers and devices. Those devices are populated by the HDMI core >>> driver _probe() function and triggers a driver_deferred_probe_trigger() >>> after the driver successfully probed. The LDB driver bind() returns >>> -EPROBE_DEFER because the panel we are looking for depends on a defered >>> regulator device. Now the defered probe code tries to probe the defered >>> devices again because the local-trigger count was changed by the HDMI >>> driver and we are in the never ending loop. >>> >>>> In general deferring probe from bind is not forbidden, but it should be >>>> used carefully (as everything in kernel :) ). Fixing deferring probe >>>> issues in many cases it is a matter of figuring out 'dependency loops' >>>> and breaking them by splitting device initialization into more than one >>>> phase. >>> We are on the way of splitting the imx-drm driver but there are many >>> other DRM drivers using the component framework. As far as I can see the >>> sunxi8 driver is component based and uses the same HDMI driver. I'm with >>> Andy that we should fix that on the common/core place. >> >> I have looked at the drivers and I see the main issue I see is that imx >> drivers performs resource acquisition in bind phase. > As I said we are working on this. > >> I think rule of >> thumb should be "do not expose yourself, until you are ready", which in >> this case means "do not call component_add, until resources are >> acquired" - ie resource acquisition should be performed in probe. > Hm.. there are is no documentation which forbid this use-case. I thought > that the component framework bind() equals the driver probe() function.. In this particular case (components vs deferred probe interaction) I guess the source code is the only documentation. > >> I use >> this approach mainly to avoid multiple deferred re-probes, but it should >> solve also this issue, so even if there will be solution to "deferred >> probe issues" in core it would be good to fix imx drivers. > Pls, see my above comments. It is not only the imx driver. Also we > shouldn't expect that driver-developers will follow a rule which is > not written somewhere. As I wrote above this is only my advice and my experience, if you have better idea regarding drivers/documentation/core please post appropriate patches. Regards Andrzej > > Regards, > Marco > >> Regards >> >> Andrzej >> >> >>> Regards, >>> Marco >>> >>>> Regards >>>> >>>> Andrzej >>>> >>>> >>>>>>> It can be solved by refactoring the driver probe routine. If a resource is >>>>>>> required to be present, then check that it is available early; before >>>>>>> registering child devices. >>>>>> We fix one and leave others. >>>>> E.g. the imx-drm and the sunxi driver... >>>>> >>>>> Regards, >>>>> Marco >>>>> >>>>>>> The proposed solution to modify driver core is fragile and susceptible to >>>>>>> side effects from other probe paths. I don't think it is the right approach. >>>>>> Have you tested it on your case? Does it fix the issue? >>>>>>