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SubjectRe: [PATCH v3] driver core: Break infinite loop when deferred probe can't be satisfied
On 26/03/2020 15:01, 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
>>> <> wrote:
>>>> ...but extcon driver is still missing...
>>>> [   22.283174] platform Added to deferred list
>>>> [   22.288513] platform
>>>> 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 ( 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.

If it is difficult to determine whether extcon is available before
creating the child devices, then there is a way to solve it that still
leverages the driver core. You could refactor dwc3_core_init_mode() into
a set of separate probe() routines, one for each mode. dwc3_probe()
could register the matching child device just before it exits. Then the
driver core will take care of calling it again at a later point in time
without tearing down the ulpi device probe.


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