Re: [3/3] hwmon: iio_hwmon: defer probe when no channel is found

[Guenter Roeck]

On 06/30/2016 06:59 AM, Jonathan Cameron wrote:
> On 30 June 2016 04:47:25 BST, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> > On Tue, Jun 28, 2016 at 10:18:17AM +0200, Quentin Schulz wrote:
> > > iio_channel_get_all returns -ENODEV when it cannot find either
> > phandles and
> > > properties in the Device Tree or channels whose consumer_dev_name
> > matches
> > > iio_hwmon in iio_map_list. The iio_map_list is filled in by iio
> > drivers
> > > which might be probed after iio_hwmon.
> > >
> > > It is better to defer the probe of iio_hwmon if such error is
> > returned by
> > > iio_channel_get_all in order to let a chance to iio drivers to expose
> > > channels in iio_map_list.
> > >
> > > Signed-off-by: Quentin Schulz <quentin.schulz@xxxxxxxxxxxxxxxxxx>
> > > ---
> > >   drivers/hwmon/iio_hwmon.c | 5 ++++-
> > >   1 file changed, 4 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/hwmon/iio_hwmon.c b/drivers/hwmon/iio_hwmon.c
> > > index b550ba5..c0da4d9 100644
> > > --- a/drivers/hwmon/iio_hwmon.c
> > > +++ b/drivers/hwmon/iio_hwmon.c
> > > @@ -73,8 +73,11 @@ static int iio_hwmon_probe(struct platform_device
> > *pdev)
> > >   		name = dev->of_node->name;
> > >
> > >   	channels = iio_channel_get_all(dev);
> > > -	if (IS_ERR(channels))
> > > +	if (IS_ERR(channels)) {
> > > +		if (PTR_ERR(channels) == -ENODEV)
> > > +			return -EPROBE_DEFER;
> >
> > The problem, as I see it, is with iio, which should return
> > -EPROBE_DEFER
> > in this situation.
> Agreed. New fangled stuff this deferred probing :)
> > We can not convert -ENODEV to -EPROBE_DEFER without risking that the
> > channels are _really_ not there, which would result in endless
> > "deferred"
> > messages.
> Hmm not entirely sure how we prevent that happening wherever it is done..

Outch. Better at the source, though. I didn't look at the iio code recently,
but can you detect the defer situation at least with devicetree ?

For non-devicetree situations, the only option I can think of would be
to replace the module initcall with a later initcall. That should solve
the problem if both iio_hwmon and and underlying drivers are built
into the kernel. If iio_hwmon is modular, the only real option I can
see is to make sure that all drivers it needs are loaded first.

Does this make sense ?

Guenter

> > Guenter
> >
> > >   		return PTR_ERR(channels);
> > > +	}
> > >
> > >   	st = devm_kzalloc(dev, sizeof(*st), GFP_KERNEL);
> > >   	if (st == NULL) {
> > --
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