linux-kernel - Re: [PATCH] HID: multitouch: fix calculation of last slot field in multi-touch reports

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Date:   Tue, 29 May 2018 16:45:34 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Ben Chan <benchan@...omium.org>
Cc:     linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jiri Kosina <jikos@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        dtor@...omium.org, drinkcat@...omium.org
Subject: Re: [PATCH] HID: multitouch: fix calculation of last slot field in
 multi-touch reports

On Tue, May 29, 2018 at 03:56:55PM -0700, Ben Chan wrote:
> According to [1] and also seemingly agreed by [2], the Scan Time usage
> (0x0D 0x56) is a report level usage, not a contact level usage.
> 
> However, the hid-multitouch driver currently includes HID_DG_SCANTIME
> when calculating `td->last_slot_field', which may lead to
> mt_complete_slot() being prematurely called in certain cases (e.g. when
> each touch input report includes more than one contact and the Scan Time
> usage appears before any contact logical collection).
> 
> This patch fixes the issue by skipping mt_store_field() on
> HID_DG_SCANTIME, similar to how HID_DG_CONTACTCOUNT and
> HID_DG_CONTACTMAX are handled.
> 
> [1] https://docs.microsoft.com/en-us/windows-hardware/design/component-guidelines/windows-precision-touchpad-required-hid-top-level-collections#windows-precision-touchpad-input-reports
> [2] https://patchwork.kernel.org/patch/1742181/
> 
> Fixes: 29cc309d8bf19 ("HID: hid-multitouch: forward MSC_TIMESTAMP")
> Signed-off-by: Ben Chan <benchan@...omium.org>

This looks reasonable to me.

Reviewed-by: Dmitry Torokhov <dmitry.torokhov@...il.com>

> ---
>  drivers/hid/hid-multitouch.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/hid/hid-multitouch.c b/drivers/hid/hid-multitouch.c
> index dad2fbb0e3f8..161551aab496 100644
> --- a/drivers/hid/hid-multitouch.c
> +++ b/drivers/hid/hid-multitouch.c
> @@ -620,13 +620,16 @@ static int mt_touch_input_mapping(struct hid_device *hdev, struct hid_input *hi,
>  			hid_map_usage(hi, usage, bit, max,
>  				EV_MSC, MSC_TIMESTAMP);
>  			input_set_capability(hi->input, EV_MSC, MSC_TIMESTAMP);
> -			mt_store_field(usage, td, hi);
>  			/* Ignore if indexes are out of bounds. */
>  			if (field->index >= field->report->maxfield ||
>  			    usage->usage_index >= field->report_count)
>  				return 1;
>  			td->scantime_index = field->index;
>  			td->scantime_val_index = usage->usage_index;
> +			/*
> +			 * We don't set td->last_slot_field as scan time is
> +			 * global to the report.
> +			 */
>  			return 1;
>  		case HID_DG_CONTACTCOUNT:
>  			/* Ignore if indexes are out of bounds. */
> -- 
> 2.17.0.921.gf22659ad46-goog
> 
> --
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-- 
Dmitry