[PATCH] serial: 8250_of: Fix regression in reset support

[Linus Walleij <linus.walleij@linaro.org>]

commit e2860e1f62f2 ("serial: 8250_of: Add reset support")
introduced reset support for the 8250_of driver.

However it unconditionally uses the assert/deassert pair to
deassert reset on the device at probe and assert it at
remove. This does not work with systems that have a
self-deasserting reset controller, such as Gemini, that
recently added a reset controller.

As a result, the console will not probe on the Gemini with
this message:

Serial: 8250/16550 driver, 1 ports, IRQ sharing disabled
of_serial: probe of 42000000.serial failed with error -524

This (-ENOTSUPP) is the error code returned by the
deassert() operation on self-deasserting reset controllers.

Add some code and comments that will:

- In probe() avoid to bail out if -ENOTSUPP is returned
  from the reset_deassert() call.

- If reset_assert() bails out with -ENOTSUPP on remove(),
  try the self-deasserting method as a fallback.

Cc: Joel Stanley <joel@jms.id.au>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Fixes: e2860e1f62f2 ("serial: 8250_of: Add reset support")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
Philipp: please comment on this or ACK if it is the right
approach. It sort of sets a precedent for handling
different reset controllers from the consumer side.

Another possibility is to modify the reset core such
that .deassert() bails out silently if the controller only
has .reset(), and .assert() just calls .reset() if the
controller does not have .assert().

Actually I think the latter is more intuitive but it is
also more intrusive.
---
 drivers/tty/serial/8250/8250_of.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_of.c b/drivers/tty/serial/8250/8250_of.c
index 0cf95fddccfc..927ee8561c8d 100644
--- a/drivers/tty/serial/8250/8250_of.c
+++ b/drivers/tty/serial/8250/8250_of.c
@@ -138,7 +138,12 @@ static int of_platform_serial_setup(struct platform_device *ofdev,
 	if (IS_ERR(info->rst))
 		goto out;
 	ret = reset_control_deassert(info->rst);
-	if (ret)
+	/*
+	 * If the deassert operation is not supported, this could be because
+	 * the reset controller is self-deasserting and onlt supports the
+	 * .reset() operation, so this is not a probe error.
+	 */
+	if (ret && (ret != -ENOTSUPP))
 		goto out;
 
 	port->type = type;
@@ -235,10 +240,14 @@ static int of_platform_serial_probe(struct platform_device *ofdev)
 static int of_platform_serial_remove(struct platform_device *ofdev)
 {
 	struct of_serial_info *info = platform_get_drvdata(ofdev);
+	int ret;
 
 	serial8250_unregister_port(info->line);
 
-	reset_control_assert(info->rst);
+	ret = reset_control_assert(info->rst);
+	/* If the assert operation is not supported, use pure reset() */
+	if (ret == -ENOTSUPP)
+		reset_control_reset(info->rst);
 	if (info->clk)
 		clk_disable_unprepare(info->clk);
 	kfree(info);
-- 
2.9.4