linux-kernel - Re: [PATCH] HID: Improve Windows Precision Touchpad detection.

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Date:   Thu, 7 Nov 2019 09:48:09 +0100
From:   Benjamin Tissoires <benjamin.tissoires@...hat.com>
To:     Blaž Hrastnik <blaz@...n.io>
Cc:     Jiri Kosina <jikos@...nel.org>,
        "open list:HID CORE LAYER" <linux-input@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] HID: Improve Windows Precision Touchpad detection.

Hi Blaž,

On Wed, Nov 6, 2019 at 12:03 PM Blaž Hrastnik <blaz@...n.io> wrote:
>
> Per Microsoft spec, usage 0xC5 (page 0xFF) returns a blob containing
> data used to verify the touchpad as a Windows Precision Touchpad.
>
>    0x85, REPORTID_PTPHQA,    //    REPORT_ID (PTPHQA)
>     0x09, 0xC5,              //    USAGE (Vendor Usage 0xC5)
>     0x15, 0x00,              //    LOGICAL_MINIMUM (0)
>     0x26, 0xff, 0x00,        //    LOGICAL_MAXIMUM (0xff)
>     0x75, 0x08,              //    REPORT_SIZE (8)
>     0x96, 0x00, 0x01,        //    REPORT_COUNT (0x100 (256))
>     0xb1, 0x02,              //    FEATURE (Data,Var,Abs)
>
> However, some devices, namely Microsoft's Surface line of products
> instead implement a "segmented device certification report" (usage 0xC6)
> which returns the same report, but in smaller chunks.
>
>     0x06, 0x00, 0xff,        //     USAGE_PAGE (Vendor Defined)
>     0x85, REPORTID_PTPHQA,   //     REPORT_ID (PTPHQA)
>     0x09, 0xC6,              //     USAGE (Vendor usage for segment #)
>     0x25, 0x08,              //     LOGICAL_MAXIMUM (8)
>     0x75, 0x08,              //     REPORT_SIZE (8)
>     0x95, 0x01,              //     REPORT_COUNT (1)
>     0xb1, 0x02,              //     FEATURE (Data,Var,Abs)
>     0x09, 0xC7,              //     USAGE (Vendor Usage)
>     0x26, 0xff, 0x00,        //     LOGICAL_MAXIMUM (0xff)
>     0x95, 0x20,              //     REPORT_COUNT (32)
>     0xb1, 0x02,              //     FEATURE (Data,Var,Abs)
>
> By expanding Win8 touchpad detection to also look for the segmented
> report, all Surface touchpads are now properly recognized by
> hid-multitouch.
>
> Signed-off-by: Blaž Hrastnik <blaz@...n.io>
> ---

This looks good to me.
We *could* shorten the ifs and make only one conditional, but I find
it this way more readable and future proof.

There is just one last step required before we merge this: add a
regression test so we ensure we do not break it in the future.

It should be merely a matter of sending a MR to
https://gitlab.freedesktop.org/libevdev/hid-tools.
It should consist in adding the report descriptor in the same way we
have https://gitlab.freedesktop.org/libevdev/hid-tools/blob/master/tests/test_multitouch.py#L1656-1658.
Then, make sure an unpatched kernel breaks the multitouch test (sudo
pytest-3 -k 'multitouch and TestPTP') and that a patched kernel is
fixed.

Cheers,
Benjamin

>  drivers/hid/hid-core.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/drivers/hid/hid-core.c b/drivers/hid/hid-core.c
> index 63fdbf09b044..2af597cd5d65 100644
> --- a/drivers/hid/hid-core.c
> +++ b/drivers/hid/hid-core.c
> @@ -742,6 +742,10 @@ static void hid_scan_feature_usage(struct hid_parser *parser, u32 usage)
>         if (usage == 0xff0000c5 && parser->global.report_count == 256 &&
>             parser->global.report_size == 8)
>                 parser->scan_flags |= HID_SCAN_FLAG_MT_WIN_8;
> +
> +       if (usage == 0xff0000c6 && parser->global.report_count == 1 &&
> +           parser->global.report_size == 8)
> +               parser->scan_flags |= HID_SCAN_FLAG_MT_WIN_8;
>  }
>
>  static void hid_scan_collection(struct hid_parser *parser, unsigned type)
> --
> 2.23.0
>