On Tue, 2020-03-24 at 11:20 +0100, Bastien Nocera wrote:
> On Wed, 2020-03-18 at 19:27 +0000, Filipe Laíns wrote:
> > As discussed in the mailing list:
> > 
> > > Right now the hid-logitech-dj driver will export one node for each
> > > connected device, even when the device is not connected. That
> > > causes
> > > some trouble because in userspace we don't have have any way to
> > > know if
> > > the device is connected or not, so when we try to communicate, if
> > > the
> > > device is disconnected it will fail.
> 
> Why is it a problem that user-space communication fails? Note that
> sending a signal without any way to fetch the state means that it's
> always going to be racy.

It failing is not the problem. The problem is knowing when the device
is available again. Right now the only way to do that is to listen for
events or periodically ping it.

We want to only export the HID++ hidraw node when the device is
available but that will take a while. We will have to test and sync up
userspace. I also want to write tests for the driver before, to make
sure there are no regressions. We had a thread discussing this, IIRC
you were in CC.

> > The solution reached to solve this issue is to trigger an udev change
> > event when the device connects, this way userspace can just wait on
> > those connections instead of trying to ping the device.
> > 
> > Signed-off-by: Filipe Laíns <lains@archlinux.org>
> > 
> > ---
> > 
> > v2:
> >   - Issue udev change event on the connected hid device, not on the
> >   receiver
> > 
> > ---
> >  drivers/hid/hid-logitech-dj.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/hid/hid-logitech-dj.c b/drivers/hid/hid-
> > logitech-dj.c
> > index 48dff5d6b605..282e57dd467d 100644
> > --- a/drivers/hid/hid-logitech-dj.c
> > +++ b/drivers/hid/hid-logitech-dj.c
> > @@ -1412,6 +1412,7 @@ static int logi_dj_dj_event(struct hid_device
> > *hdev,
> >  {
> >  	struct dj_receiver_dev *djrcv_dev = hid_get_drvdata(hdev);
> >  	struct dj_report *dj_report = (struct dj_report *) data;
> > +	struct dj_device *dj_dev;
> >  	unsigned long flags;
> >  
> >  	/*
> > @@ -1447,7 +1448,9 @@ static int logi_dj_dj_event(struct hid_device
> > *hdev,
> >  
> >  	spin_lock_irqsave(&djrcv_dev->lock, flags);
> >  
> > -	if (!djrcv_dev->paired_dj_devices[dj_report->device_index]) {
> > +	dj_dev = djrcv_dev->paired_dj_devices[dj_report->device_index];
> > +
> > +	if (!dj_dev) {
> >  		/* received an event for an unknown device, bail out */
> >  		logi_dj_recv_queue_notification(djrcv_dev, dj_report);
> >  		goto out;
> > @@ -1464,6 +1467,8 @@ static int logi_dj_dj_event(struct hid_device
> > *hdev,
> >  		if (dj_report-
> > > report_params[CONNECTION_STATUS_PARAM_STATUS] ==
> >  		    STATUS_LINKLOSS) {
> >  			logi_dj_recv_forward_null_report(djrcv_dev,
> > dj_report);
> > +		} else {
> > +			kobject_uevent(&dj_dev->hdev->dev.kobj,
> > KOBJ_CHANGE);
> >  		}
> >  		break;
> >  	default:
-- 
Filipe Laíns