[PATCH] elo: Support non-pressure-sensitive ELO touchscreens

* Use the touch status bit rather than the pressure bits to
  distinguish a BTN_TOUCH event. Non-pressure-sensitive touchscreens
  always report full pressure.
* Report ABS_PRESSURE information only if the touchscreen supports it.
* Implement the checksum calculation correctly, and verify that the
  transmitted checksum is correct.
* Use dev_dbg to log errors in the protocol.

Signed-off-by: Shaun Jackman <sjackman@gmail.com>

diff --git a/drivers/input/touchscreen/elo.c b/drivers/input/touchscreen/elo.c
index c86a2eb..b972184 100644
--- a/drivers/input/touchscreen/elo.c
+++ b/drivers/input/touchscreen/elo.c
@@ -35,6 +35,8 @@ MODULE_LICENSE("GPL");
  */
 
 #define	ELO_MAX_LENGTH	10
+#define ELO10_TOUCH 0x03
+#define ELO10_PRESSURE 0x80
 
 /*
  * Per-touchscreen data.
@@ -53,38 +55,40 @@ struct elo {
 static void elo_process_data_10(struct elo* elo, unsigned char data, struct pt_regs *regs)
 {
 	struct input_dev *dev = elo->dev;
+	struct device *dbg = &elo->serio->dev;
 
-	elo->csum += elo->data[elo->idx] = data;
-
+	elo->data[elo->idx] = data;
 	switch (elo->idx++) {
-
 		case 0:
+			elo->csum = 0xaa;
 			if (data != 'U') {
+				dev_dbg(dbg, "unsynchronized data: 0x%02x\n", data);
 				elo->idx = 0;
-				elo->csum = 0;
 			}
 			break;
-
-		case 1:
-			if (data != 'T') {
-				elo->idx = 0;
-				elo->csum = 0;
-			}
-			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;
+			if (elo->csum != elo->data[9]) {
+				dev_dbg(dbg, "bad checksum: 0x%02x, expected 0x%02x\n",
+						elo->data[9], elo->csum);
+				break;
+			}
+			if (elo->data[1] != 'T') {
+				dev_dbg(dbg, "unexpected packet: 0x%02x\n",
+						elo->data[1]);
+				break;
+			}
+			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]);
+			if (elo->data[2] & ELO10_PRESSURE)
+				input_report_abs(dev, ABS_PRESSURE,
+						(elo->data[8] << 8) | elo->data[7]);
+			input_report_key(dev, BTN_TOUCH, elo->data[2] & ELO10_TOUCH);
+			input_sync(dev);
 			break;
 	}
+	elo->csum += data;
 }
 
 static void elo_process_data_6(struct elo* elo, unsigned char data, struct pt_regs *regs)