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Date: Thu, 26 Mar 2020 15:01:22 +0000 From: Grant Likely <grant.likely@....com> To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, Saravana Kannan <saravanak@...gle.com> Cc: a.hajda@...sung.com, artem.bityutskiy@...ux.intel.com, balbi@...nel.org, broonie@...nel.org, fntoth@...il.com, gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org, peter.ujfalusi@...com, rafael@...nel.org, kernel-team@...roid.com, nd <nd@....com> Subject: Re: [PATCH v3] driver core: Break infinite loop when deferred probe can't be satisfied 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 <andriy.shevchenko@...ux.intel.com> 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. 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. 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. g.
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