Date:   Tue, 9 Jun 2020 00:30:04 -0700
From:   Saravana Kannan <saravanak@...gle.com>
To:     Marco Felsch <m.felsch@...gutronix.de>
Cc:     Andrzej Hajda <a.hajda@...sung.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Grant Likely <grant.likely@....com>,
        Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>,
        Felipe Balbi <balbi@...nel.org>,
        Mark Brown <broonie@...nel.org>, Ferry Toth <fntoth@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Peter Ujfalusi <peter.ujfalusi@...com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Android Kernel Team <kernel-team@...roid.com>,
        nd <nd@....com>, kernel@...gutronix.de
Subject: Re: [PATCH v3] driver core: Break infinite loop when deferred probe
 can't be satisfied

On Mon, Jun 8, 2020 at 11:45 PM Marco Felsch <m.felsch@...gutronix.de> 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.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.
> > >> 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'm not opposed to fixing this at the common/core level if that's
possible, but Andy's patch still has a bug where it might never probe
a device. I still haven't seen an answer to this.
https://lore.kernel.org/lkml/CAGETcx_3+YH_LmNUCAAk1OaXk6noHEXxcE+ckkoBqKJJhtpDjQ@mail.gmail.com/

-Saravana

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