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Subject: Re: [PATCH] HID: logitech-dj: issue udev change event on device
 connection
To:     Mario Limonciello <superm1@gmail.com>,
        =?UTF-8?Q?Filipe_La=c3=adns?= <lains@archlinux.org>
Cc:     Jiri Kosina <jikos@kernel.org>,
        Benjamin Tissoires <benjamin.tissoires@redhat.com>,
        linux-input@vger.kernel.org, linux-kernel@vger.kernel.org,
        Peter Hutterer <peter.hutterer@redhat.com>,
        Richard Hughes <hughsient@gmail.com>
References: <20200318161906.3340959-1-lains@archlinux.org>
 <CA+EcB1MoTXMaueJfRHf51A5PU4oiKSJXrHazfTEvifZK54OrLQ@mail.gmail.com>
From:   Hans de Goede <hdegoede@redhat.com>
Message-ID: <e8ea0c2e-445f-21e2-a248-3368f26bf391@redhat.com>
Date:   Wed, 18 Mar 2020 18:20:03 +0100
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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.

Regards,

Hans