[PATCH v1 2/2] tty: serial: 8250_dw: Keep init pinctrl state until first open

["Michał Kardaś" <mkmkl@google.com>]

When a UART port is connected to an external peripheral whose power
domain is kept powered off until first use, transitioning the UART
pins from the "init" state to the "default" state prematurely at probe
completion can cause parasitic back-powering into the unpowered
peripheral.

Ensure that on boards specifying an "init" pinctrl state for the port,
the "init" state is preserved until the port is first opened:

1. In dw8250_probe(), call pinctrl_keep_init_state(dev) to opt out of the
   automatic "init" -> "default" transition at probe completion and record
   this in data->in_init_state.
2. In dw8250_do_pm(), when the port is opened (state == 0), transition
   pins from "init" to "default" if data->in_init_state is set, and clear
   the flag.

Suggested-by: Douglas Anderson <dianders@chromium.org>
Co-developed-by: Vic Huang <vich@google.com>
Signed-off-by: Vic Huang <vich@google.com>
Signed-off-by: Michał Kardaś <mkmkl@google.com>
---
 drivers/tty/serial/8250/8250_dw.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index 5fba913f3301..74e568e2a0d3 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -18,6 +18,7 @@
 #include <linux/io.h>
 #include <linux/lockdep.h>
 #include <linux/module.h>
+#include <linux/pinctrl/consumer.h>
 #include <linux/platform_device.h>
 #include <linux/pm_runtime.h>
 #include <linux/property.h>
@@ -77,6 +78,7 @@ struct dw8250_data {
 	unsigned int		skip_autocfg:1;
 	unsigned int		uart_16550_compatible:1;
 	unsigned int		in_idle:1;
+	unsigned int		in_init_state:1;
 
 	u8			no_int_count;
 };
@@ -461,8 +463,15 @@ static int dw8250_handle_irq(struct uart_port *p)
 static void
 dw8250_do_pm(struct uart_port *port, unsigned int state, unsigned int old)
 {
-	if (!state)
+	struct dw8250_data *d = to_dw8250_data(port->private_data);
+
+	if (!state) {
 		pm_runtime_get_sync(port->dev);
+		if (d->in_init_state) {
+			d->in_init_state = false;
+			pinctrl_pm_select_default_state(port->dev);
+		}
+	}
 
 	serial8250_do_pm(port, state, old);
 
@@ -770,6 +779,8 @@ static int dw8250_probe(struct platform_device *pdev)
 	if (data->data.line < 0)
 		return data->data.line;
 
+	data->in_init_state = pinctrl_keep_init_state(dev);
+
 	platform_set_drvdata(pdev, data);
 
 	pm_runtime_enable(dev);
-- 
2.55.0.654.g21b8a5bc05-goog