Re: [RFC] Adding device-initiated haptic feedback knobs for pressurepads

[Dmitry Torokhov]

Hi Rong,

On Tue, Jun 16, 2026 at 11:34:45PM +0800, Rong Zhang wrote:
> Hi all,
> 
> A pressurepad is a MT touchpad that simulates haptic feedback. The
> haptic feedback may be either device-initiated or host-initiated. While
> drivers/hid/hid-haptic.c [1] provides comprehensive support for host-
> initiated ones, device-initiated ones are currently not configurable at
> all.
> 
> According to Microsoft's Input Device Haptics Implementation Guide [2],
> pressurepads with device-initiated haptic feedback can support
> SET_FEATURE reports to allow users to customize the intensity of the
> haptic feedback (usage page 0x0e Haptics, usage 0x23 Intensity) and/or
> the force required to trigger a button press (usage page 0x0d
> Digitizers, usage 0xb0 Button Press Threshold). Each knob should occupy
> a dedicated report ID.
> 
> The first knob maps to "ClickForceSensitivity" in Windows registry and
> "Touchpad feedback" => "Intensity" in Windows Settings [3]. The second
> knob maps to "ClickForceSensitivity" in Windows registry. 
> 
> The pressurepad on my laptop supports the first knob:
> 
>    # 0x05, 0x0e,                    //  Usage Page (Haptic)                595
>    # 0x09, 0x01,                    //  Usage (Simple Haptic Controller)   597
>    # 0xa1, 0x02,                    //  Collection (Logical)               599
>    # 0x09, 0x23,                    //   Usage (Intensity)                 601
>    # 0x85, 0x09,                    //   Report ID (9)                     603
>    # 0x15, 0x00,                    //   Logical Minimum (0)               605
>    # 0x25, 0x64,                    //   Logical Maximum (100)             607
>    # 0x75, 0x08,                    //   Report Size (8)                   609
>    # 0x95, 0x01,                    //   Report Count (1)                  611
>    # 0xb1, 0x02,                    //   Feature (Data,Var,Abs)            613
>    # 0xc0,                          //  End Collection                     615
> 
> I hardly use Windows but the last time I booted it I did see "Touchpad
> feedback" => "Intensity" in Windows Settings.
> 
> On Linux, I can tune the knob by setting the corresponding feature
> report via HIDRAW. The pressurepad remembers the knob value until it
> loses power. However, getting the feature report returns garbage data
> (probably from the last input report in the buffer), which makes some
> sense as the implementation guide only requires SET_FEATURE support.
> 
> I'd like to add device-initiated haptic feedback configurations for
> Linux. For now, I only focus on the first knob, i.e., Haptics Intensity,
> since it's supported on my device. If I somehow get a device with the
> second knob I may work on it too.

The question is why would we want to support device-initiated feedback
instead of enhancing userspace to fully control it. For the record on
ChromeOS we tried implementing support for device-initiated feedback but
it made the kernel code very cumbersome and left a lot of gaps/rough
edges when combining device- and host-initiated haptic effects. 

I'd prefer it all being driven from userspace.

Thanks.

-- 
Dmitry