Re: Query regarding hid-multitouch.c driver in 4.14/4.19

[Benjamin Tissoires <benjamin.tissoires@redhat.com>]

Hi Neeraj,

On Wed, Nov 13, 2019 at 4:11 AM Neeraj Upadhyay <neeraju@codeaurora.org> wrote:
>
> Hi,
>
> I have one query regarding hid-multitouch.c driver and need your guidance on
> how hid-multitouchc can restore/support the original behaviour, where, for
> devices, for which application is not
> HID_DG_TOUCHSCREEN/HID_DG_TOUCHPAD, and has
> HID_DG_CONTACTID usage in its report, can still use generic input mappings.
>
> We are using kernel versions 4.14 , 4.19 respectively in 2 different
> projects:
>
> 4.14:
> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/drivers/hid/hid-multitouch.c?h=v4.14.153
> 4.19:
> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/drivers/hid/hid-multitouch.c?h=v4.19.83
>
> I checked the application for our hid device, it's HID_DG_PEN, device
> also has a HID_DG_CONTACTID usage defined in
>
> its report.
>
> In 4.19, is_mt_collection is set to 'true'. All multitouch code paths or
> input mapping is configured
>
> mt_allocate_report_data()
>          ...
>          for (n = 0; n < field->report_count; n++) {
>                          if (field->usage[n].hid == HID_DG_CONTACTID)
>                                  rdata->is_mt_collection = true;   //
> is_mt_collection is set to 'true'
>                  }
>          }
>
> mt_input_mapping()
>          ...
>          if (rdata->is_mt_collection)
>              return mt_touch_input_mapping(...)  //
> mt_touch_input_mapping() is called
>
> mt_event()
>          if (rdata && rdata->is_mt_collection)
>              return mt_touch_event();  // mt_touch_event() is called
>
> However, in 4.14, the behaviour was different, mt input mapping was done
> only
> for HID_DG_TOUCHSCREEN/HID_DG_TOUCHPAD , and because our hid device is
> HID_DG_PEN, generic mappings were applied for it; with these settings,
> device
> responds to events.
>
> static int mt_input_mapping()
>          if (field->application == HID_DG_TOUCHSCREEN ||
>              field->application == HID_DG_TOUCHPAD)
>              return mt_touch_input_mapping();  // This is not called.
>
>
> mt_touch_input_mapping()
>          case HID_DG_CONTACTID:
>                          mt_store_field(usage, td, hi);
>                          td->touches_by_report++;
>                          td->mt_report_id = field->report->id; //
> mt_report_id is not set.
>                          return 1;
>
>
> Looks like this behaviour changed, with below commits:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/drivers/hid/hid-multitouch.c?h=v4.19.83&id=8dfe14b3b47ff832cb638731f9fc696a3a84f804
> 8dfe14b3b47f    HID: multitouch: ditch mt_report_id
> https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/drivers/hid/hid-multitouch.c?h=v4.19.83&id=ba6b055e0f3b4ff4942e4ab273260affcfad9bff
> ba6b055e0f3b     HID: input: enable Totem on the Dell Canvas 27
>
> Can you please suggest on how we can support/preserve the original
> behaviour?

Hmm, I would initially say that a firmware that exports Contact ID for
a Pen is definitely wrong. The Contact ID usage has been introduced in
https://www.usb.org/sites/default/files/hutrr34.pdf and is
specifically for multi-touch, not multi pen.

Anyway, couple of questions:
- does the device supports multi-pen?
- can you share the report descriptor and a few events when triggering
this particular report (ideally with hid-recorder from
https://gitlab.freedesktop.org/libevdev/hid-tools/

Cheers,
Benjamin

>
>
> Thanks
> Neeraj
>
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