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Date:   Tue, 9 Jun 2020 08:45:11 +0200
From:   Marco Felsch <m.felsch@pengutronix.de>
To:     Andrzej Hajda <a.hajda@samsung.com>
Cc:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Grant Likely <grant.likely@arm.com>,
        Saravana Kannan <saravanak@google.com>,
        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 <nd@arm.com>, kernel@pengutronix.de
Subject: Re: [PATCH v3] driver core: Break infinite loop when deferred probe
 can't be satisfied
Message-ID: <20200609064511.7nek2rhk6ebfjaia@pengutronix.de>
References: <20200324175719.62496-1-andriy.shevchenko@linux.intel.com>
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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@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.

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?
> >>
> 

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