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

[Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>]

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?


Thanks
Neeraj

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