Input: alps - only Dell laptops have separate button bits for v2 dualpoint sticks

It turns out that only Dell laptops have the separate button bits for
v2 dualpoint sticks and that commit 92bac83dd79e ("Input: alps - non
interleaved V2 dualpoint has separate stick button bits") causes
regressions on Toshiba laptops.

This commit adds a check for Dell laptops to the code for handling these
extra button bits, fixing this regression.

This patch has been tested on a Dell Latitude D620 to make sure that it
does not reintroduce the original problem.

Reported-and-tested-by: Douglas Christman <[e-mail]>
Cc: [e-mail]
Signed-off-by: Hans de Goede <[e-mail]>
Signed-off-by: Dmitry Torokhov <[e-mail]>

Documentation/input/alps.txt

···
  byte 5:  0   z6   z5   z4   z3   z2   z1   z0
 
 Protocol Version 2 DualPoint devices send standard PS/2 mouse packets for
-the DualPoint Stick. For non interleaved dualpoint devices the pointingstick
-buttons get reported separately in the PSM, PSR and PSL bits.
+the DualPoint Stick. The M, R and L bits signal the combined status of both
+the pointingstick and touchpad buttons, except for Dell dualpoint devices
+where the pointingstick buttons get reported separately in the PSM, PSR
+and PSL bits.
 
 Dualpoint device -- interleaved packet format
 ---------------------------------------------

drivers/input/mouse/alps.c

···
 #include <linux/input/mt.h>
 #include <linux/serio.h>
 #include <linux/libps2.h>
+#include <linux/dmi.h>
 
 #include "psmouse.h"
 #include "alps.h"
···
 #define ALPS_FOUR_BUTTONS	0x40	/* 4 direction button present */
 #define ALPS_PS2_INTERLEAVED	0x80	/* 3-byte PS/2 packet interleaved with
 					   6-byte ALPS packet */
+#define ALPS_DELL		0x100	/* device is a Dell laptop */
 #define ALPS_BUTTONPAD		0x200	/* device is a clickpad */
 
 static const struct alps_model_info alps_model_data[] = {
···
 		return;
 	}
 
-	/* Non interleaved V2 dualpoint has separate stick button bits */
+	/* Dell non interleaved V2 dualpoint has separate stick button bits */
 	if (priv->proto_version == ALPS_PROTO_V2 &&
-	    priv->flags == (ALPS_PASS | ALPS_DUALPOINT)) {
+	    priv->flags == (ALPS_DELL | ALPS_PASS | ALPS_DUALPOINT)) {
 		left |= packet[0] & 1;
 		right |= packet[0] & 2;
 		middle |= packet[0] & 4;
···
 	priv->byte0 = protocol->byte0;
 	priv->mask0 = protocol->mask0;
 	priv->flags = protocol->flags;
+	if (dmi_name_in_vendors("Dell"))
+		priv->flags |= ALPS_DELL;
 
 	priv->x_max = 2000;
 	priv->y_max = 1400;