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nusb/kb: skip 0x01 lead byte hack, you dont wanna know

apparently, some mouse send constant 0x01 byte
before normal 4 byte mouse packet. this is known in
openbsd/freebsd as UQ_MS_LEADING_BYTE quirk.

--- a/sys/src/cmd/nusb/kb/kb.c

+++ b/sys/src/cmd/nusb/kb/kb.c

@@ -226,12 +226,13 @@

 ptrwork(void* a)

 {

 	static char maptab[] = {0x0, 0x1, 0x4, 0x5, 0x2, 0x3, 0x6, 0x7};

-	int x, y, b, c, nerrs;

+	int x, y, b, c, nerrs, skiplead;

 	char	err[ERRMAX], buf[64];

 	char	mbuf[80];

 	KDev*	f = a;

 	sethipri();

+	skiplead = -1;

 	nerrs = 0;

 	for(;;){

 		if(f->ep == nil)

@@ -256,6 +257,21 @@

 		nerrs = 0;

 		if(c < 3)

 			continue;

+

+		if(c > 4){

+			/*

+			 * horrible horrible hack. some mouse send a

+			 * constant 0x01 lead byte. stop the hack as

+			 * soon as buf[0] changes.

+			 */

+			if(skiplead == -1)

+				skiplead = buf[0] & 0xFF;

+			if(skiplead == 0x01 && skiplead == buf[0])

+				memmove(buf, buf+1, --c);

+			else

+				skiplead = 0;

+		}

+

 		if(f->accel){

 			x = scale(f, buf[1]);

 			y = scale(f, buf[2]);