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SubjectRe: [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@archlinux.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@archlinux.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

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