linux-kernel - Re: Touchpad multitouch leaves ghost fingers

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Message-ID: <iav7hzeaarxifwxk7zlfnt6vipqkp4h4ldt634exlvcswz62gj@a7ongaeduylz>
Date: Tue, 23 Sep 2025 18:30:33 +0200
From: Benjamin Tissoires <bentiss@...nel.org>
To: Yinon Burgansky <yinonburgansky@...il.com>
Cc: jikos@...nel.org, linux-input@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: Touchpad multitouch leaves ghost fingers

On Sep 22 2025, Yinon Burgansky wrote:
> On Mon, Sep 22, 2025 at 7:51 PM Benjamin Tissoires <bentiss@...nel.org> wrote:
> > Well, I was meaning that I would provide a HID-BPF MR ready to install
> > for you.
> That would be awesome! Thank you very much!

Got something out with https://gitlab.freedesktop.org/libevdev/udev-hid-bpf/-/merge_requests/204
that seems to solve your case on the hid-recorder you provided.

> 
> > But if you want to play with it, feel free to do so :)
> > For a jump start on HID-BPF:
> > https://libevdev.pages.freedesktop.org/udev-hid-bpf/tutorial.html
> Thank you for the reference. I’m feeling quite fatigued from the
> debugging process, as it took me a considerable amount of time to
> identify the issue.
> It was surprisingly difficult to determine what exactly was wrong with
> the touchpad and to uncover the root cause; perhaps I didn’t search
> for the right terms.
> However, I’m glad I finally took the time to debug it, submit the bug
> report, and find out what was wrong. Thank you!
> 
> > The advantage of HID-BPF is that we can drop the file in the filesystem
> > and then forget about it.
> > My plan is to have the HID-BPF detect whether the quirk has been applied
> > or not and would gracefully remove itself once the kernel is fixed.
> That's cool, nice design.

Unfortunately I spoke too fast. The NSMU quirk is internal to the
hid-multitouch driver, and not exported in the struct hid_device. Thus
we don't have a direct access to the quirk. It should also be present in
the driver data of the hid_device or in the input_device, but in both
cases BPF prevents us to simply follow the pointers.

In the end, it shouldn't be an issue if you have both the fixed kernel
and the BPF because both fixes are not mutually exclusive.

> 
> > PS: please try to refrain from top-posting your reply. The usage is to
> > inline your reply or reply below,
> Sorry about that, hopefully I did it right this time.

Yep, much better this time. I didn't meant to be rude, but if you want
to interact with the LKML, some people gets angry about top posting and
would just send you a link without even reading your email. So better
having better habits :)

Cheers,
Benjamin