Input: elo - fix checksum calculation

Fix 10-byte protocol checksum calculation and do not discard packet early
unless it is missing lead in byte.
Signed-off-by: Shaun Jackman <sjackman@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

drivers/input/touchscreen/elo.c

@@ -36,6 +36,10 @@ MODULE_LICENSE("GPL");
 
 #define ELO_MAX_LENGTH		10
 
+#define ELO10_LEAD_BYTE		'U'
+
+#define ELO10_TOUCH_PACKET	'T'
+
 /*
  * Per-touchscreen data.
  */
@@ -50,44 +54,43 @@ struct elo {
 	char phys[32];
 };
 
-static void elo_process_data_10(struct elo* elo, unsigned char data, struct pt_regs *regs)
+static void elo_process_data_10(struct elo *elo, unsigned char data, struct pt_regs *regs)
 {
 	struct input_dev *dev = elo->dev;
 
-	elo->csum += elo->data[elo->idx] = data;
-
+	elo->data[elo->idx] = data;
 	switch (elo->idx++) {
-
 		case 0:
-			if (data != 'U') {
+			elo->csum = 0xaa;
+			if (data != ELO10_LEAD_BYTE) {
+				pr_debug("elo: unsynchronized data: 0x%02x\n", data);
 				elo->idx = 0;
-				elo->csum = 0;
 			}
 			break;
 
-		case 1:
-			if (data != 'T') {
+		case 9:
 			elo->idx = 0;
-				elo->csum = 0;
+			if (data != elo->csum) {
+				pr_debug("elo: bad checksum: 0x%02x, expected 0x%02x\n",
+					 data, elo->csum);
+				break;
 			}
+			if (elo->data[1] != ELO10_TOUCH_PACKET) {
+				pr_debug(elo: "unexpected packet: 0x%02x\n", elo->data[1]);
 				break;
-
-		case 9:
-			if (elo->csum) {
+			}
 			input_regs(dev, regs);
 			input_report_abs(dev, ABS_X, (elo->data[4] << 8) | elo->data[3]);
 			input_report_abs(dev, ABS_Y, (elo->data[6] << 8) | elo->data[5]);
 			input_report_abs(dev, ABS_PRESSURE, (elo->data[8] << 8) | elo->data[7]);
 			input_report_key(dev, BTN_TOUCH, elo->data[8] || elo->data[7]);
 			input_sync(dev);
-			}
-			elo->idx = 0;
-			elo->csum = 0;
 			break;
 	}
+	elo->csum += data;
 }
 
-static void elo_process_data_6(struct elo* elo, unsigned char data, struct pt_regs *regs)
+static void elo_process_data_6(struct elo *elo, unsigned char data, struct pt_regs *regs)
 {
 	struct input_dev *dev = elo->dev;
 
@@ -135,7 +138,7 @@ static void elo_process_data_6(struct elo* elo, unsigned char data, struct pt_re
 	}
 }
 
-static void elo_process_data_3(struct elo* elo, unsigned char data, struct pt_regs *regs)
+static void elo_process_data_3(struct elo *elo, unsigned char data, struct pt_regs *regs)
 {
 	struct input_dev *dev = elo->dev;
 
@@ -161,7 +164,7 @@ static void elo_process_data_3(struct elo* elo, unsigned char data, struct pt_re
 static irqreturn_t elo_interrupt(struct serio *serio,
 		unsigned char data, unsigned int flags, struct pt_regs *regs)
 {
-	struct elo* elo = serio_get_drvdata(serio);
+	struct elo *elo = serio_get_drvdata(serio);
 
 	switch(elo->id) {
 		case 0: