[PATCH V2] tty: serial: fsl_lpuart: don't break the on-going transfer when global reset

[Sherry Sun]

lpuart_global_reset() shouldn't break the on-going transmit engin, need
to recover the on-going data transfer after reset.

This can help earlycon here, since commit 60f361722ad2 ("serial:
fsl_lpuart: Reset prior to registration") moved lpuart_global_reset()
before uart_add_one_port(), earlycon is writing during global reset,
as global reset will disable the TX and clear the baud rate register,
which caused the earlycon cannot work any more after reset, needs to
restore the baud rate and re-enable the transmitter to recover the
earlycon write.

Fixes: bd5305dcabbc ("tty: serial: fsl_lpuart: do software reset for imx7ulp and imx8qxp")
Signed-off-by: Sherry Sun <sherry.sun@xxxxxxx>
---
Changes in V2:
1. The while loop may never exit as the stat and sfifo are not re-read inside
the loop, fix that.
---
 drivers/tty/serial/fsl_lpuart.c | 22 +++++++++++++++++++---
 1 file changed, 19 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/serial/fsl_lpuart.c b/drivers/tty/serial/fsl_lpuart.c
index 67fa113f77d4..9a0781395b1f 100644
--- a/drivers/tty/serial/fsl_lpuart.c
+++ b/drivers/tty/serial/fsl_lpuart.c
@@ -408,11 +408,9 @@ static int lpuart_global_reset(struct lpuart_port *sport)
 {
 	struct uart_port *port = &sport->port;
 	void __iomem *global_addr;
+	unsigned long tx_enable, bd;
 	int ret;
 
-	if (uart_console(port))
-		return 0;
-
 	ret = clk_prepare_enable(sport->ipg_clk);
 	if (ret) {
 		dev_err(sport->port.dev, "failed to enable uart ipg clk: %d\n", ret);
@@ -420,11 +418,29 @@ static int lpuart_global_reset(struct lpuart_port *sport)
 	}
 
 	if (is_imx7ulp_lpuart(sport) || is_imx8qxp_lpuart(sport)) {
+		/*
+		 * If the transmitter is used by earlycon, wait transmit engin complete
+		 * and then reset
+		 */
+		tx_enable = lpuart32_read(port, UARTCTRL) & UARTCTRL_TE;
+		if (tx_enable) {
+			bd = lpuart32_read(&sport->port, UARTBAUD);
+			while (!(lpuart32_read(port, UARTSTAT) & UARTSTAT_TC &&
+				 lpuart32_read(port, UARTFIFO) & UARTFIFO_TXEMPT))
+				cpu_relax();
+		}
+
 		global_addr = port->membase + UART_GLOBAL - IMX_REG_OFF;
 		writel(UART_GLOBAL_RST, global_addr);
 		usleep_range(GLOBAL_RST_MIN_US, GLOBAL_RST_MAX_US);
 		writel(0, global_addr);
 		usleep_range(GLOBAL_RST_MIN_US, GLOBAL_RST_MAX_US);
+
+		/* Recover the transmitter for earlycon */
+		if (tx_enable) {
+			lpuart32_write(port, bd, UARTBAUD);
+			lpuart32_write(port, UARTCTRL_TE, UARTCTRL);
+		}
 	}
 
 	clk_disable_unprepare(sport->ipg_clk);
-- 
2.17.1