[Linux-kernel-mentees] Advice on fixing the bug of MSFT0001:00 04F3:Touchpad being handled by hid_multitouch by mistake

Coiby Xu coiby.xu at gmail.com
Fri Sep 4 14:59:16 UTC 2020

On Fri, Sep 04, 2020 at 10:16:51AM +0200, Benjamin Tissoires wrote:
>On Tue, Aug 11, 2020 at 11:15 AM Coiby Xu <coiby.xu at gmail.com> wrote:
>> Hi,
>> I'm working on a touchpad device issue as reported on
>> https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1887190.
>> This touchpad device MSFT0001:00 04F3:Touchpad should be handled by
>> hid_rmi. But currently hid-core.c chooses hid_multitouch by mistake,
>>      1. When scanning this device's report descriptor, HID_DG_CONTACTID
>>         usage is found. Thus group HID_GROUP_MULTITOUCH is assigned to
>>         the device.
>>      2. The flag HID_SCAN_FLAG_MT_WIN_8 is also found. Thus group
>>         HID_GROUP_MULTITOUCH_WIN_8 is assigned to the device.
>>      3. hid-multitouch.c claims handling devices with the group of
>>          static const struct hid_device_id mt_devices[] = {
>>                 /* Generic MT device */
>>                 /* Generic Win 8 certified MT device */
>>                 {  .driver_data = MT_CLS_WIN_8,
>>                         HID_DEVICE(HID_BUS_ANY, HID_GROUP_MULTITOUCH_WIN_8,
>>                                 HID_ANY_ID, HID_ANY_ID) },
>>                 { }
>>          };
>> There are several potential solutions,
>>      - Let the device vendor fix this problem since this device's report
>>        descriptor shouldn't have the HID_DG_CONTACTID usage.
>>      - Make it a special case by setting the device's group to
>>        HID_GROUP_RMI in hid_scan_report when vendor id and product ID
>>        are matched.
>>      - hid-quirks.c seems to be designed to handle special cases, is it
>>        suitable for this case?
>AFAIU, the touchpad doesn't work at all with hid-multitouch. So I
>guess the best is to add the VID/PID to hid-quirks.c in
>hid_have_special_driver[], and add it to the hid-rmi driver too.
>This way, you will ensure hid-rmi will pick up the device all the time.

Thank you for the advice! I have exactly adopted this approach by looking
at commit e9287099ba6539bccb20cd791269186f3ae28b85
("HID: rmi: Add support for the touchpad in the Razer Blade 14 laptop")
as an example.

My previous email is a bit misleading because 0x04F3 is the vendor code
of ELAN while hid-rmi is for the Synaptics touchpad. And actually this
laptop model of Lenovo Legion-5 15ARH05 is shipped with both kinds of

- for the Synaptics touchpad, hid-rmi could almost handle it perfectly
   except the clicking is not sensitive enough. I need to let my finger
   linger on the touchpad for a while. I notice when I click on the touchpad,
   an HID report would be received by hid-recorder. But evtest couldn't receive
   any EV_ event. If hid-multitouch is handling the device, the cursor
   won't move but 2-4 finger touching events could still be received by

- for the ELAN touchpad, only HID reporters can be read and parsed by
   hid-core then two input devices (mouse and touchpad) could created by
   hid-multitouch as seen from /proc/bus/input/devices. But hid-recorder
   could never get any HID report.

AFAIU, isn't hid-multitouch supposed to be the implementation of Windows
Precision Touchpad? And since Precision touchpad is mandatory for
Windows 10 notebooks and this laptop model of Lenovo Legion-5 15ARH05
seem to have been certificated by Windows 10, does it mean by theory
hid-multiouch could handle these two touchpad devices?

Anyway, it seems I need to install Windows Driver Kit to capture&analyze
HID reports to see what's happening. Or do you have any suggestion?

Best regards,

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