[PATCH] Input: synaptics - retry SMBus intertouch setup when companion is not ready

["Raphaël Larocque" <rlarocque@disroot.org>]

On some machines (here: Lenovo ThinkPad T440p 20AWS0H800),
the SMBus companion device is not ready
by the time psmouse probes the Synaptics touchpad
during early boot.  synaptics_setup_intertouch() returns -EAGAIN in
this situation, but synaptics_init() previously treated this the same
as any other error and fell through immediately to PS/2 mode.  As a
result, the touchpad and TrackPoint were unresponsive for several
minutes after a cold boot, until an internal reconnect cycle (which
already carries retry logic in synaptics_reconnect()) eventually
succeeded.

Fix this by adding an exponential back-off retry loop in synaptics_init()
when synaptics_setup_intertouch() returns -EAGAIN: clean up the failed
state, sleep for 500 ms on the first attempt doubling each time up to
4000 ms, then retry.  The worst-case added latency at boot is ~7.5 s
(500 + 1000 + 2000 + 4000 ms), and only for hardware that actually
reports SYN_CAP_INTERTOUCH.  The pattern mirrors what
synaptics_reconnect() already does after resume.

Tested-by: Raphaël Larocque <rlarocque@disroot.org>
Signed-off-by: Raphaël Larocque <rlarocque@disroot.org>
---
 drivers/input/mouse/synaptics.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/drivers/input/mouse/synaptics.c b/drivers/input/mouse/synaptics.c
index c70502e24031..5c1bfff2b16c 100644
--- a/drivers/input/mouse/synaptics.c
+++ b/drivers/input/mouse/synaptics.c
@@ -1918,6 +1918,23 @@ int synaptics_init(struct psmouse *psmouse)
 		}
 
 		error = synaptics_setup_intertouch(psmouse, &info, true);
+		if (error == -EAGAIN) {
+			/*
+			 * On some systems (e.g. ThinkPad T440p) the SMBus
+			 * companion device is not yet ready at boot time.
+			 * Clean up and retry with exponential back-off,
+			 * mirroring synaptics_reconnect().
+			 */
+			unsigned int delay = 500;
+
+			do {
+				psmouse_smbus_cleanup(psmouse);
+				msleep(delay);
+				error = synaptics_setup_intertouch(psmouse, &info, true);
+				delay *= 2;
+			} while (error == -EAGAIN && delay <= 4000);
+		}
+
 		if (!error)
 			return PSMOUSE_SYNAPTICS_SMBUS;
 	}
-- 
2.53.0