linux-kernel - Re: [PATCH v2] HID: logitech-dj: issue udev change event on device connection

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Message-ID: <3c20624ea9d88f13d6f1a768735f08a4a8a59ea6.camel@archlinux.org>
Date:   Tue, 24 Mar 2020 14:10:57 +0000
From:   Filipe Laíns <lains@...hlinux.org>
To:     Bastien Nocera <hadess@...ess.net>, Jiri Kosina <jikos@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Hutterer <peter.hutterer@...hat.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Mario Limonciello <superm1@...il.com>,
        Richard Hughes <hughsient@...il.com>
Subject: Re: [PATCH v2] HID: logitech-dj: issue udev change event on device
 connection

On Tue, 2020-03-24 at 15:03 +0100, Bastien Nocera wrote:
> On Tue, 2020-03-24 at 13:46 +0000, Filipe Laíns wrote:
> > 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.
> 
> If I need to remember some old thread to know what we're talking about,
> then that means that the commit message is probably not good enough...

Should I put in the commit message what is planned next?

> Please add some links to the relevant discussion on bug forums if
> there's interesting data there too.

You can find the discussion in [1][2].

[1] https://www.spinics.net/lists/linux-input/thrd2.html#65615
[2] https://www.spinics.net/lists/linux-input/msg65615.html

> > > > 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@...hlinux.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

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