[PATCH] mfd: twl6040: Fix device init errors for ACCCTL register

[Tony Lindgren <tony@atomide.com>]

I noticed that we can get a -EREMOTEIO errors on at least omap4 duovero:

twl6040 0-004b: Failed to write 2d = 19: -121

And then any following register access will produce errors.

There 2d offset above is register ACCCTL that gets written on twl6040
powerup. With error checking added to the related regcache_sync() call,
the -EREMOTEIO error is reproducable on twl6040 powerup at least
duovero.

To fix the error, we need to wait until twl6040 is accessible after the
powerup. Based on tests on omap4 duovero, we need to wait over 8ms after
powerup before register write will complete without failures. Let's also
make sure we warn about possible errors too.

Note that we have twl6040_patch[] reg_sequence with the ACCCTL register
configuration and regcache_sync() will write the new value to ACCCTL.

Cc: Florian Vaussard <florian.vaussard@epfl.ch>
Cc: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
---
 drivers/mfd/twl6040.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/mfd/twl6040.c b/drivers/mfd/twl6040.c
--- a/drivers/mfd/twl6040.c
+++ b/drivers/mfd/twl6040.c
@@ -322,8 +322,19 @@ int twl6040_power(struct twl6040 *twl6040, int on)
 			}
 		}
 
+		/*
+		 * Register access can produce errors after power-up unless we
+		 * wait at least 8ms based on measurements on duovero.
+		 */
+		usleep_range(10000, 12000);
+
 		/* Sync with the HW */
-		regcache_sync(twl6040->regmap);
+		ret = regcache_sync(twl6040->regmap);
+		if (ret) {
+			dev_err(twl6040->dev, "%s register write failed: %i\n",
+				__func__, ret);
+			goto out;
+		}
 
 		/* Default PLL configuration after power up */
 		twl6040->pll = TWL6040_SYSCLK_SEL_LPPLL;
-- 
2.20.1