Re: [PATCH 1/5] HID: hid-multitouch: fix wrong protocol detection

["Drews, Paul" <paul.drews@xxxxxxxxx>]

>>> The previous implementation introduced a randomness in the splitting
>>> of the different touches reported by the device. This version is more
>>> robust as we don't rely on hi->input->absbit, but on our own structure.
>>>
>>> This also prepares hid-multitouch to better support Win8 devices.
>>>
>>> Signed-off-by: Benjamin Tissoires <benjamin.tissoires@xxxxxxx>
>
>>>  struct mt_device {
>>>       struct mt_slot curdata; /* placeholder of incoming data */
>>>       struct mt_class mtclass;        /* our mt device class */
>>> +     struct mt_fields *fields;       /* temporary placeholder for storing the
>>> +                                        multitouch fields */
>>
>> Why not skip the pointer here?
>
>well, the idea was to keep the memory footprint low. As these values
>are only needed at init, then I freed them once I finished using them.
>I can of course skip the pointer, but in that case, wouldn't the
>struct declaration be worthless?
>
>
>>> +static void mt_post_parse(struct mt_device *td)
>>> +{
>>> +     struct mt_fields *f = td->fields;
>>> +
>>> +     if (td->touches_by_report > 0) {
>>> +             int field_count_per_touch = f->length / td->touches_by_report;
>>> +             td->last_slot_field = f->usages[field_count_per_touch - 1]->hid;
>>> +     }
>>> +}
>>> +
>

The patch as it stands more-or-less depends on the group of usage->hid
values repeating for each touch in the multi-touch report, and choosing
the last usage->hid seen in the first group as the ultimate last_slot_field
value.  A suggestion: as long as we're relying on this group repetition
anyway, why not take advantage of the repetition wrap-around to
detect the last_slot_field without having to allocate memory and store
everything?  I've been using the following patch that does it this way
with an Atmel MaXTouch Digitizer (3EB:211C).

Prior to this patch I was getting a MTBF of about 1 failure in 10 boots
due to the out-of-range bitmap lookup coming up with an unlucky
result and making the wrong last_slot_field conclusion.  Symptom:
touch events get reported to user-space with previous x,y coordinates.
Also confirmed using a printk to instrument the kernel for this.

With this patch, I have tested beyond 10X the MTBF on 3.4-rc7 with no failures.
I don't have a touchscreen other than that Atmel to test with.  Will this
method work with the buggy touchscreen that the original patch was
intended to fix?

Patch follows:
========================================================
>From 9ff29221247f6a3531f4b7939898fe708aa96830 Mon Sep 17 00:00:00 2001
From: Paul Drews <paul.drews@xxxxxxxxx>
Date: Wed, 16 May 2012 11:15:00 -0700
Subject: [PATCH] Repair detection of last slot in multitouch reports

The logic for detecting the last per-touch slot in a
multitouch report erroneously used hid usage values (large
numbers such as 0xd0032) as indices into the smaller absbit
bitmap (with bit indexes up to 0x3f).  This caused
intermittent failures in the configuration of the last-slot
value leading to stale x,y coordinates being reported in
multi-touch input events.  It also carried the risk of a
segmentation fault due to the out-of-range bitmap index.

This patch takes a different approach of detecting the last
per-touch slot:  when the hid usage value wraps around to
the first hid usage value we have seen already, we must be
looking at the slots for the next touch of a multi-touch
report, so the last hid usage value we have seen so far must
be the last per-touch value.

Signed-off-by: Paul Drews <paul.drews@xxxxxxxxx>
---
 drivers/hid/hid-multitouch.c |   39 ++++++++++++++++++++++++++-------------
 1 files changed, 26 insertions(+), 13 deletions(-)

diff --git a/drivers/hid/hid-multitouch.c b/drivers/hid/hid-multitouch.c
index 2e6d187..226f828 100644
--- a/drivers/hid/hid-multitouch.c
+++ b/drivers/hid/hid-multitouch.c
@@ -75,6 +75,9 @@ struct mt_device {
 	struct mt_class mtclass;	/* our mt device class */
 	unsigned last_field_index;	/* last field index of the report */
 	unsigned last_slot_field;	/* the last field of a slot */
+	bool last_slot_field_found;	/* last_slot_field has full init */
+	unsigned first_slot_field;
+	bool first_slot_field_found;	/* for detecting wrap to next touch */
 	__s8 inputmode;		/* InputMode HID feature, -1 if non-existent */
 	__s8 maxcontact_report_id;	/* Maximum Contact Number HID feature,
 				   -1 if non-existent */
@@ -275,11 +278,21 @@ static void set_abs(struct input_dev *input, unsigned int code,
 	input_set_abs_params(input, code, fmin, fmax, fuzz, 0);
 }
 
-static void set_last_slot_field(struct hid_usage *usage, struct mt_device *td,
-		struct hid_input *hi)
+static void update_last_slot_field(struct hid_usage *usage,
+		struct mt_device *td)
 {
-	if (!test_bit(usage->hid, hi->input->absbit))
-		td->last_slot_field = usage->hid;
+	if (!td->last_slot_field_found) {
+		if (td->first_slot_field_found) {
+			if (td->last_slot_field == usage->hid)
+				td->last_slot_field_found = true; /* wrapped */
+			else
+				td->last_slot_field = usage->hid;
+		} else {
+			td->first_slot_field = usage->hid;
+			td->first_slot_field_found = true;
+			td->last_slot_field = usage->hid;
+		}
+	}
 }
 
 static int mt_input_mapping(struct hid_device *hdev, struct hid_input *hi,
@@ -340,7 +353,7 @@ static int mt_input_mapping(struct hid_device *hdev, struct hid_input *hi,
 				cls->sn_move);
 			/* touchscreen emulation */
 			set_abs(hi->input, ABS_X, field, cls->sn_move);
-			set_last_slot_field(usage, td, hi);
+			update_last_slot_field(usage, td);
 			td->last_field_index = field->index;
 			return 1;
 		case HID_GD_Y:
@@ -350,7 +363,7 @@ static int mt_input_mapping(struct hid_device *hdev, struct hid_input *hi,
 				cls->sn_move);
 			/* touchscreen emulation */
 			set_abs(hi->input, ABS_Y, field, cls->sn_move);
-			set_last_slot_field(usage, td, hi);
+			update_last_slot_field(usage, td);
 			td->last_field_index = field->index;
 			return 1;
 		}
@@ -359,24 +372,24 @@ static int mt_input_mapping(struct hid_device *hdev, struct hid_input *hi,
 	case HID_UP_DIGITIZER:
 		switch (usage->hid) {
 		case HID_DG_INRANGE:
-			set_last_slot_field(usage, td, hi);
+			update_last_slot_field(usage, td);
 			td->last_field_index = field->index;
 			return 1;
 		case HID_DG_CONFIDENCE:
-			set_last_slot_field(usage, td, hi);
+			update_last_slot_field(usage, td);
 			td->last_field_index = field->index;
 			return 1;
 		case HID_DG_TIPSWITCH:
 			hid_map_usage(hi, usage, bit, max, EV_KEY, BTN_TOUCH);
 			input_set_capability(hi->input, EV_KEY, BTN_TOUCH);
-			set_last_slot_field(usage, td, hi);
+			update_last_slot_field(usage, td);
 			td->last_field_index = field->index;
 			return 1;
 		case HID_DG_CONTACTID:
 			if (!td->maxcontacts)
 				td->maxcontacts = MT_DEFAULT_MAXCONTACT;
 			input_mt_init_slots(hi->input, td->maxcontacts);
-			td->last_slot_field = usage->hid;
+			update_last_slot_field(usage, td);
 			td->last_field_index = field->index;
 			td->touches_by_report++;
 			return 1;
@@ -385,7 +398,7 @@ static int mt_input_mapping(struct hid_device *hdev, struct hid_input *hi,
 					EV_ABS, ABS_MT_TOUCH_MAJOR);
 			set_abs(hi->input, ABS_MT_TOUCH_MAJOR, field,
 				cls->sn_width);
-			set_last_slot_field(usage, td, hi);
+			update_last_slot_field(usage, td);
 			td->last_field_index = field->index;
 			return 1;
 		case HID_DG_HEIGHT:
@@ -395,7 +408,7 @@ static int mt_input_mapping(struct hid_device *hdev, struct hid_input *hi,
 				cls->sn_height);
 			input_set_abs_params(hi->input,
 					ABS_MT_ORIENTATION, 0, 1, 0, 0);
-			set_last_slot_field(usage, td, hi);
+			update_last_slot_field(usage, td);
 			td->last_field_index = field->index;
 			return 1;
 		case HID_DG_TIPPRESSURE:
@@ -406,7 +419,7 @@ static int mt_input_mapping(struct hid_device *hdev, struct hid_input *hi,
 			/* touchscreen emulation */
 			set_abs(hi->input, ABS_PRESSURE, field,
 				cls->sn_pressure);
-			set_last_slot_field(usage, td, hi);
+			update_last_slot_field(usage, td);
 			td->last_field_index = field->index;
 			return 1;
 		case HID_DG_CONTACTCOUNT:
-- 
1.7.3.4
========================================================
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