[PATCH v3] serial: mxs-auart: keep the AUART unit in reset state when not in use

[jbe@pengutronix.de (Juergen Borleis)]

Whenever the UART device driver gets closed from userland, the driver
disables the UART unit and then stops its clock to save power.

The bit which disabled the UART unit is described as:

  "UART Enable. If this bit is set to 1, the UART is enabled. Data
  transmission and reception occurs for the UART signals. When the
  UART is disabled in the middle of transmission or reception, it
  completes the current character before stopping."

The important part is the "it completes the current character". Whenever
a reception is ongoing when the UART gets disabled (including the clock
off) the statemachine freezes and "remembers" this state on the next
open() and re-enabling of the unit's clock.

In this case we end up receiving an additional bogus character
immediately.

The solution in this change is to move the AUART unit into its reset
state on close() and only release it from its reset state on the next
open().

Signed-off-by: Juergen Borleis <jbe@pengutronix.de>
---

Notes:
    v3:
     - missing newline added to an error message
    
    v2:
     - rename mxs_auart_do_reset() to mxs_auart_keep_reset() to better reflect
       what it really does
     - adapt the delay in mxs_auart_keep_reset() to wait for the reset of the
       AURAT unit to what is used in mxs_auart_reset()
     - typo fixed

 drivers/tty/serial/mxs-auart.c | 38 ++++++++++++++++++++++++++++++--------
 1 file changed, 30 insertions(+), 8 deletions(-)

diff --git a/drivers/tty/serial/mxs-auart.c b/drivers/tty/serial/mxs-auart.c
index 13cf773..135ee6c 100644
--- a/drivers/tty/serial/mxs-auart.c
+++ b/drivers/tty/serial/mxs-auart.c
@@ -858,6 +858,30 @@ static void mxs_auart_reset(struct uart_port *u)
 	writel(AUART_CTRL0_CLKGATE, u->membase + AUART_CTRL0_CLR);
 }
 
+static void mxs_auart_keep_reset(struct uart_port *u)
+{
+	int i;
+	u32 reg;
+
+	reg = readl(u->membase + AUART_CTRL0);
+	/* if already in reset state, keep it untouched */
+	if (reg & AUART_CTRL0_SFTRST)
+		return;
+
+	writel(AUART_CTRL0_CLKGATE, u->membase + AUART_CTRL0_CLR);
+	writel(AUART_CTRL0_SFTRST, u->membase + AUART_CTRL0_SET);
+
+	for (i = 0; i < 10000; i++) {
+		reg = readl(u->membase + AUART_CTRL0);
+		/* reset is finished when the clock is gated */
+		if (reg & AUART_CTRL0_CLKGATE)
+			return;
+		udelay(3);
+	}
+
+	dev_err(u->dev, "Failed to reset the unit.\n");
+}
+
 static int mxs_auart_startup(struct uart_port *u)
 {
 	int ret;
@@ -867,7 +891,10 @@ static int mxs_auart_startup(struct uart_port *u)
 	if (ret)
 		return ret;
 
-	writel(AUART_CTRL0_CLKGATE, u->membase + AUART_CTRL0_CLR);
+	/* reset the unit if not aleady done */
+	mxs_auart_keep_reset(u);
+	/* bring it out of reset now */
+	mxs_auart_reset(u);
 
 	writel(AUART_CTRL2_UARTEN, u->membase + AUART_CTRL2_SET);
 
@@ -899,13 +926,8 @@ static void mxs_auart_shutdown(struct uart_port *u)
 	if (auart_dma_enabled(s))
 		mxs_auart_dma_exit(s);
 
-	writel(AUART_CTRL2_UARTEN, u->membase + AUART_CTRL2_CLR);
-
-	writel(AUART_INTR_RXIEN | AUART_INTR_RTIEN | AUART_INTR_CTSMIEN,
-			u->membase + AUART_INTR_CLR);
-
-	writel(AUART_CTRL0_CLKGATE, u->membase + AUART_CTRL0_SET);
-
+	/* reset the unit and keep it in reset state */
+	mxs_auart_keep_reset(u);
 	clk_disable_unprepare(s->clk);
 }
 
-- 
2.1.4