[PATCH] input: multi-touch fix for ALPS touchpads ("SS4 plus" variant)

[Nir Perry]

Hi all,

I think a minor "typo" bug was accidentally introduced to ALPS
touchpad driver by a previous bug-fix (commit
4a646580f793d19717f7e034c8d473b509c27d49, "Input: ALPS - fix
two-finger scroll breakage in right side on ALPS touchpad").
It breaks how multi-touch events are decoded on some ALPS touchpads,
so for example tapping with three-fingers can no longer be used to
emulate middle-mouse-button (the kernel doesn't recognize this as the
proper event, and doesn't report it correctly to userspace).
This affects touchpads that use SS4 "plus" protocol variant, like
those found on Dell E7270 & E7470 laptops (tested on E7270).

The cause of the problem
------------------------------------------
First, probably due to a typo, the code in alps_decode_ss4_v2() for
case SS4_PACKET_ID_MULTI used inconsistent indices to "f->mt[]". You
can see 0 & 1 are used for the "if" part but 2 & 3 are used for the
"else" part, which I believe is a typo.
Second, in the previous patch, new macros were introduced to decode X
coordinates specific to the SS4 "plus" variant, but the macro to
define the maximum X value wasn't changed accordingly. The macros to
decode X values for "plus" variant are effectively shifted right by 1
bit, but the max wasn't shifted too. This causes the driver to
incorrectly handle "no data" cases, which also interfered with how
multi-touch was handled. To fix it - I created new SS4 "plus" macros
for the max value - SS4_PLUS_MFPACKET_NO_AX &
SS4_PLUS_MFPACKET_NO_AX_BL. To make the change a little more readable,
I moved also the Y-max lines so they are closer to the X-max lines.
To get three-finger tap to work both changes are required.

The included patch was generated against the mainline tree today, but
was also tested against the 4.14 kernel branch. I've included in this
e-mail the people involved with the old patch from August, plus Pali
RohÃr who is listed as the ALPS PS/2 touchpad driver reviewer (in the
maintainers file).

Fixes: 4a646580f793d19717f7e034c8d473b509c27d49 ("Input: ALPS - fix
two-finger scroll breakage in right side on ALPS touchpad")

Regards,
Nir

Signed-off-by: Nir Perry <nirperry@xxxxxxxxx>
diff --git a/drivers/input/mouse/alps.c b/drivers/input/mouse/alps.c
index 579b899..dbe57da 100644
--- a/drivers/input/mouse/alps.c
+++ b/drivers/input/mouse/alps.c
@@ -1250,29 +1250,32 @@ static int alps_decode_ss4_v2(struct alps_fields *f,
        case SS4_PACKET_ID_MULTI:
                if (priv->flags & ALPS_BUTTONPAD) {
                        if (IS_SS4PLUS_DEV(priv->dev_id)) {
-                               f->mt[0].x = SS4_PLUS_BTL_MF_X_V2(p, 0);
-                               f->mt[1].x = SS4_PLUS_BTL_MF_X_V2(p, 1);
+                               f->mt[2].x = SS4_PLUS_BTL_MF_X_V2(p, 0);
+                               f->mt[3].x = SS4_PLUS_BTL_MF_X_V2(p, 1);
+                               no_data_x = SS4_PLUS_MFPACKET_NO_AX_BL;
                        } else {
                                f->mt[2].x = SS4_BTL_MF_X_V2(p, 0);
                                f->mt[3].x = SS4_BTL_MF_X_V2(p, 1);
+                               no_data_x = SS4_MFPACKET_NO_AX_BL;
                        }
+                       no_data_y = SS4_MFPACKET_NO_AY_BL;

                        f->mt[2].y = SS4_BTL_MF_Y_V2(p, 0);
                        f->mt[3].y = SS4_BTL_MF_Y_V2(p, 1);
-                       no_data_x = SS4_MFPACKET_NO_AX_BL;
-                       no_data_y = SS4_MFPACKET_NO_AY_BL;
                } else {
                        if (IS_SS4PLUS_DEV(priv->dev_id)) {
-                               f->mt[0].x = SS4_PLUS_STD_MF_X_V2(p, 0);
-                               f->mt[1].x = SS4_PLUS_STD_MF_X_V2(p, 1);
+                               f->mt[2].x = SS4_PLUS_STD_MF_X_V2(p, 0);
+                               f->mt[3].x = SS4_PLUS_STD_MF_X_V2(p, 1);
+                               no_data_x = SS4_PLUS_MFPACKET_NO_AX;
                        } else {
-                               f->mt[0].x = SS4_STD_MF_X_V2(p, 0);
-                               f->mt[1].x = SS4_STD_MF_X_V2(p, 1);
+                               f->mt[2].x = SS4_STD_MF_X_V2(p, 0);
+                               f->mt[3].x = SS4_STD_MF_X_V2(p, 1);
+                               no_data_x = SS4_MFPACKET_NO_AX;
                        }
+                       no_data_y = SS4_MFPACKET_NO_AY;
+
                        f->mt[2].y = SS4_STD_MF_Y_V2(p, 0);
                        f->mt[3].y = SS4_STD_MF_Y_V2(p, 1);
-                       no_data_x = SS4_MFPACKET_NO_AX;
-                       no_data_y = SS4_MFPACKET_NO_AY;
                }

                f->first_mp = 0;
diff --git a/drivers/input/mouse/alps.h b/drivers/input/mouse/alps.h
index c80a7c7..3dfae83 100644
--- a/drivers/input/mouse/alps.h
+++ b/drivers/input/mouse/alps.h
@@ -145,6 +145,8 @@ enum SS4_PACKET_ID {
 #define SS4_MFPACKET_NO_AY     4080    /* Y-Coordinate value */
 #define SS4_MFPACKET_NO_AX_BL  8176    /* Buttonless X-Coordinate value */
 #define SS4_MFPACKET_NO_AY_BL  4088    /* Buttonless Y-Coordinate value */
+#define SS4_PLUS_MFPACKET_NO_AX        4080    /* SS4 PLUS, x  */
+#define SS4_PLUS_MFPACKET_NO_AX_BL     4088    /* Buttonless SS4 PLUS, x */

 /*
  * enum V7_PACKET_ID - defines the packet type for V7