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

[Grant Likely]

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@xxxxxxxxxxxxxxx> wrote:
[...]
> > > > ...but extcon driver is still missing...
> > > >
> > > > [ÂÂ 22.283174] platform dwc3.0.auto: Added to deferred list
> > > > [ÂÂ 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.