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

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Date:   Thu, 19 Mar 2020 12:23:31 +1000
From:   Peter Hutterer <peter.hutterer@...-t.net>
To:     Hans de Goede <hdegoede@...hat.com>
Cc:     Mario Limonciello <superm1@...il.com>,
        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>,
        Richard Hughes <hughsient@...il.com>
Subject: Re: [PATCH] HID: logitech-dj: issue udev change event on device
 connection

On Wed, Mar 18, 2020 at 06:20:03PM +0100, Hans de Goede wrote:
> Hi,
> 
> On 3/18/20 6:15 PM, Mario Limonciello wrote:
> > On Wed, Mar 18, 2020 at 11:19 AM Filipe Laíns <lains@...hlinux.org> 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.
> > > 
> > > 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>
> > > ---
> > >   drivers/hid/hid-logitech-dj.c | 2 ++
> > >   1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/drivers/hid/hid-logitech-dj.c b/drivers/hid/hid-logitech-dj.c
> > > index 48dff5d6b605..fcd481a0be1f 100644
> > > --- a/drivers/hid/hid-logitech-dj.c
> > > +++ b/drivers/hid/hid-logitech-dj.c
> > > @@ -1464,6 +1464,8 @@ 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(&hdev->dev.kobj, KOBJ_CHANGE);
> > >                  }
> > >                  break;
> > >          default:
> > > --
> > > 2.25.1
> > 
> > The problem that will remain here is the transition period for
> > userspace to start to rely upon
> > this.  It will have no idea whether the kernel is expected to send
> > events or not.  What do you
> > think about adding a syfs attribute to indicate that events are being
> > sent?  Or something similar?
> 
> Then we would need to support that attribute forever. IMHO the best
> option is to just make a uname call and check the kernel version, with
> the code marked to be removed in the future when kernels older then
> $version are no longer something we want to support.

Also note that we may not have access to /sys.

Cheers,
   Peter