[PATCH RFC] input: fix weird issue of synaptics psmouse sync lost after resume

All,

BugLink: https://bugs.launchpad.net/bugs/717970

So in summary, the symptom is intermittent key events lost after resume
on some machines with synaptics touchpad (seems this is synaptics _only_),
and key events loss is due to serio port reconnect after psmouse sync lost.
Removing psmouse and inserting it back during the suspend/resume process
is able to work around the issue, so the difference between psmouse_connect()
and psmouse_reconnect() is the key to the root cause of this problem.

After comparing the two different paths, synaptics driver has its own
implementation of synaptics_reconnect(), and the missing psmouse_probe()
seems significant, the patch below added psmouse_probe() to the reconnect
process, and has been verified many times that the issue could not be reliably
reproduced.

There are two PS/2 commands in psmouse_probe():

  1. PSMOUSE_CMD_GETID
  2. PSMOUSE_CMD_RESET_DIS

The weird thing is, the PSMOUSE_CMD_GETID seems to be significant, and the
PSMOUSE_CMD_RESET_DIS is irrelevant to this issue after trying several times.
Now it's rather difficult to form a sane theory. So this patch is
really for RFC.
Note it does fix a real problem though.

========= >8 =========

diff --git a/drivers/input/mouse/synaptics.c b/drivers/input/mouse/synaptics.c
index 8eac3b7..7a72d82 100644
--- a/drivers/input/mouse/synaptics.c
+++ b/drivers/input/mouse/synaptics.c

@@ -1218,6 +1218,7 @@ static int synaptics_reconnect(struct psmouse *psmouse)
 {
        struct synaptics_data *priv = psmouse->private;
        struct synaptics_data old_priv = *priv;
+       unsigned char param[2];
        int retry = 0;
        int error;

@@ -1233,6 +1234,7 @@ static int synaptics_reconnect(struct psmouse *psmouse)
                         */
                        ssleep(1);
                }
+               ps2_command(&psmouse->ps2dev, param, PSMOUSE_CMD_GETID);
                error = synaptics_detect(psmouse, 0);
        } while (error && ++retry < 3);