[PATCH] serial: 8250_dw: Handle no_console_suspend when uart loses state

[Doug Anderson <dianders@chromium.org>]

In certain suspend modes on certain boards the 8250 UART may lose
state when the device goes to suspend.  If we're using
no_console_suspend this can cause lots of problems during resume.

Let's cache the basic UART config registers at suspend time and if we
notice that the UART loses state (by looking at a canary stored in the
scratch register) we'll restore it.

Signed-off-by: Doug Anderson <dianders@chromium.org>
---
Note that I don't have a board that properly suspends and resumes from
the deepest sleep state that runs atop mainline, so this is tested on
a local tree that is based on 3.14 with backports (specifically on
rk3288-pinky).  It is compile tested atop mainline.

 drivers/tty/serial/8250/8250_dw.c | 67 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 67 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index 0bfdccc..444ff540 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -38,6 +38,9 @@
 #define DW_UART_CPR	0xf4 /* Component Parameter Register */
 #define DW_UART_UCV	0xf8 /* UART Component Version */
 
+/* We'll place this canary in SCR while suspending; if gone we've lost state */
+#define DW_UART_SCR_STATE		0x22
+
 /* Component Parameter Register bits */
 #define DW_UART_CPR_ABP_DATA_WIDTH	(3 << 0)
 #define DW_UART_CPR_AFCE_MODE		(1 << 4)
@@ -63,6 +66,13 @@ struct dw8250_data {
 	struct clk		*pclk;
 	struct reset_control	*rst;
 	struct uart_8250_dma	dma;
+
+	int			suspending;
+	int			saved_lcr;
+	int			saved_dll;
+	int			saved_dlm;
+	int			saved_ier;
+	int			saved_fcr;
 };
 
 #define BYT_PRV_CLK			0x800
@@ -92,10 +102,35 @@ static void dw8250_force_idle(struct uart_port *p)
 	(void)p->serial_in(p, UART_RX);
 }
 
+static void dw8250_serial_restore(struct uart_port *p)
+{
+	struct dw8250_data *data = p->private_data;
+	struct uart_8250_port *port8250 = serial8250_get_port(data->line);
+
+	data->suspending = 0;
+
+	serial_out(port8250, UART_LCR, data->saved_lcr | UART_LCR_DLAB);
+	serial_out(port8250, UART_DLL, data->saved_dll);
+	serial_out(port8250, UART_DLM, data->saved_dlm);
+	serial_out(port8250, UART_LCR, data->saved_lcr);
+
+	serial_out(port8250, UART_IER, data->saved_ier);
+	serial_out(port8250, UART_FCR, data->saved_fcr);
+	serial_out(port8250, UART_MCR, data->last_mcr);
+}
+
 static void dw8250_serial_out(struct uart_port *p, int offset, int value)
 {
 	struct dw8250_data *d = p->private_data;
 
+	/*
+	 * If we started suspending and we see SCR went back to 0, assume we've
+	 * suspended and resumed and lost state.  Restore it now.
+	 */
+	if (d->suspending &&
+	    readb(p->membase + (UART_SCR << p->regshift)) != DW_UART_SCR_STATE)
+		dw8250_serial_restore(p);
+
 	if (offset == UART_MCR)
 		d->last_mcr = value;
 
@@ -133,6 +168,14 @@ static void dw8250_serial_out32(struct uart_port *p, int offset, int value)
 {
 	struct dw8250_data *d = p->private_data;
 
+	/*
+	 * If we started suspending and we see SCR went back to 0, assume we've
+	 * suspended and resumed and lost state.  Restore it now.
+	 */
+	if (d->suspending &&
+	    readb(p->membase + (UART_SCR << p->regshift)) != DW_UART_SCR_STATE)
+		dw8250_serial_restore(p);
+
 	if (offset == UART_MCR)
 		d->last_mcr = value;
 
@@ -487,9 +530,29 @@ static int dw8250_remove(struct platform_device *pdev)
 static int dw8250_suspend(struct device *dev)
 {
 	struct dw8250_data *data = dev_get_drvdata(dev);
+	struct uart_8250_port *port8250 = serial8250_get_port(data->line);
+	struct uart_port *port = &port8250->port;
 
 	serial8250_suspend_port(data->line);
 
+	/* We only deal with ports that were left on (no_console_suspend) */
+	if (port->suspended)
+		return 0;
+
+	/* We'll save our registers in case we lose state in suspend */
+	data->saved_fcr = serial_in(port8250, UART_FCR);
+	data->saved_ier = serial_in(port8250, UART_IER);
+	data->saved_lcr = serial_in(port8250, UART_LCR);
+
+	serial_out(port8250, UART_LCR, data->saved_lcr | UART_LCR_DLAB);
+	data->saved_dlm = serial_in(port8250, UART_DLM);
+	data->saved_dll = serial_in(port8250, UART_DLL);
+	serial_out(port8250, UART_LCR, data->saved_lcr);
+
+	/* Put a special canary in the scratch so we tell when state is lost */
+	serial_out(port8250, UART_SCR, DW_UART_SCR_STATE);
+	data->suspending = 1;
+
 	return 0;
 }
 
@@ -497,6 +560,10 @@ static int dw8250_resume(struct device *dev)
 {
 	struct dw8250_data *data = dev_get_drvdata(dev);
 
+	/* We never lost state; stop checking for canary */
+	if (data->suspending)
+		data->suspending = 0;
+
 	serial8250_resume_port(data->line);
 
 	return 0;
-- 
2.1.0.rc2.206.gedb03e5