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

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Date:   Tue, 24 Mar 2020 11:20:21 +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 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.

> 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: