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Subject: Re: [PATCH v3] driver core: Break infinite loop when deferred probe
 can't be satisfied
From:   Grant Likely <grant.likely@arm.com>
To:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Saravana Kannan <saravanak@google.com>
Cc:     a.hajda@samsung.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>
References: <20200324175719.62496-1-andriy.shevchenko@linux.intel.com>
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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 
>>> <andriy.shevchenko@linux.intel.com> wrote:
[...]
>>>> ...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.
> 
> 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.

Cheers,
g.