Input: elantech - detect new ICs and setup Host Notify for them

New ICs are using a different scheme for the alternate bus parameter.
Given that they are new and are only using either PS2 only or PS2 + SMBus
Host Notify, we force those new ICs to use the SMBus solution for enhanced
reporting.

This allows the touchpad found on the Lenovo T480s to report 5 fingers
every 8 ms, instead of having a limit of 2 every 8 ms.

Signed-off-by: Benjamin Tissoires <[e-mail]>
Acked-by: KT Liao <[e-mail]>
Signed-off-by: Dmitry Torokhov <[e-mail]>

drivers/input/mouse/elantech.c

···
 
 	if (elantech_smbus == ELANTECH_SMBUS_NOT_SET) {
 		/*
-		 * FIXME:
-		 * constraint the I2C capable devices by using FW version,
-		 * board version, or by using DMI matching
+		 * New ICs are enabled by default.
+		 * Old ICs are up to the user to decide.
 		 */
-		return -ENXIO;
+		if (!ETP_NEW_IC_SMBUS_HOST_NOTIFY(info->fw_version))
+			return -ENXIO;
 	}
 
 	psmouse_info(psmouse, "Trying to set up SMBus access\n");
···
 static bool elantech_use_host_notify(struct psmouse *psmouse,
 				     struct elantech_device_info *info)
 {
+	if (ETP_NEW_IC_SMBUS_HOST_NOTIFY(info->fw_version))
+		return true;
+
 	switch (info->bus) {
 	case ETP_BUS_PS2_ONLY:
 		/* expected case */

drivers/input/mouse/elantech.h

···
 #define ETP_BUS_PS2_SMB_HST_NTFY	4
 
 /*
+ * New ICs are either using SMBus Host Notify or just plain PS2.
+ *
+ * ETP_FW_VERSION_QUERY is:
+ * Byte 1:
+ *  - bit 0..3: IC BODY
+ * Byte 2:
+ *  - bit 4: HiddenButton
+ *  - bit 5: PS2_SMBUS_NOTIFY
+ *  - bit 6: PS2CRCCheck
+ */
+#define ETP_NEW_IC_SMBUS_HOST_NOTIFY(fw_version)	\
+		((((fw_version) & 0x0f2000) == 0x0f2000) && \
+		 ((fw_version) & 0x0000ff) > 0)
+
+/*
  * The base position for one finger, v4 hardware
  */
 struct finger_pos {