Re: [PATCH 1/3] driver core: enable drivers to use deferred probe from init

From: Luis R. Rodriguez
Date: Mon Jul 28 2014 - 11:38:20 EST

On Mon, Jul 28, 2014 at 03:12:11PM +0000, Yuval Mintz wrote:
> > +static int __driver_probe_device(struct device_driver *drv, struct
> > +device *dev) {
> > + if (drv->delay_probe && !dev->init_delayed_probe) {
> > + dev_info(dev, "Driver %s requests probe deferral on init\n",
> > + drv->name);
> > + dev->init_delayed_probe = true;
> > + driver_deferred_probe_add(dev);
> > + return -EPROBE_DEFER;
> > + }
> > +
> > + return really_probe(dev, drv);
> > +}
> Perhaps this is a silly question, but what guarantees that the deferred probe
> list will actually be triggered, e.g., in case the delayed device is the last device
> in the system?

The dev->init_delayed_probe is used to ensure that we'd add the device to the
deferred probe list once making this a per device thing if the driver has the
field delay_probe set to true. This technically also allows this to be a per
device thing so with some more work we could enable drivers to only enable this
for specific devices but at this point this did not seem required.

> [From drivers/base/dd.c - "A successful driver probe will trigger moving all
> devices from the pending to the active list so that the workqueue will
> eventually retry them]

I had not noticed this, thanks for pointing this out, in this case
if __driver_probe_device() is still used to retrigger a probe it will
be added back to the deferred list but since dev->init_delayed_probe
is still false. I checked the original commit that added this feature
d1c3414c but in the code I see that bus_probe_device(dev) is used and
I only see the device itself being removed from the deferred probe list,
nothing else.

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