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    SubjectRe: [PATCH v3] driver core: Break infinite loop when deferred probe can't be satisfied
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    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.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.


    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?
    >>>>

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