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

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Date:   Wed, 18 Mar 2020 12:15:13 -0500
From:   Mario Limonciello <superm1@...il.com>
To:     Filipe Laíns <lains@...hlinux.org>
Cc:     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>,
        Richard Hughes <hughsient@...il.com>
Subject: Re: [PATCH] HID: logitech-dj: issue udev change event on device connection

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?