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

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Date:   Tue, 24 Mar 2020 15:24:02 +0100
From:   Bastien Nocera <hadess@...ess.net>
To:     Filipe Laíns <lains@...hlinux.org>,
        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 14:10 +0000, Filipe Laíns wrote:
> 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?

You should put in everything that explains why this change is needed,
and how user-space is supposed to use that information. Nobody should
have to refer back to the mailing-list thread to find this out.

> > 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

I was in CC: because "might be maintaining one of the affected
userspace stacks", but I don't know what that stack might be for me. Is
it for upower (which I only work on seldom and definitely don't
maintain), or something else?

> > > > > 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: