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 >>>>>>> <andriy.shevche...@linux.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. >>>> 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. 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. 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. 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? >>>>