serial: 8250_mtk: track busclk state to avoid bus error

Commit e32a83c ("serial: 8250-mtk: modify mtk uart power and
clock management") introduced polling a debug register to make sure
the UART is idle before disabling the bus clock. However, at least on
some MediaTek SoCs access to that very debug register requires the bus
clock being enabled. Hence calling the suspend function while already
in suspended state results in that register access triggering a bus
error. In order to avoid that, track the state of the bus clock and
only poll the debug register if not already in suspended state.

Fixes: e32a83c ("serial: 8250-mtk: modify mtk uart power and clock management")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>

drivers/tty/serial/8250/8250_mtk.c

@@ -32,7 +32,7 @@
 #define MTK_UART_RXTRI_AD	0x14	/* RX Trigger address */
 #define MTK_UART_FRACDIV_L	0x15	/* Fractional divider LSB address */
 #define MTK_UART_FRACDIV_M	0x16	/* Fractional divider MSB address */
-#define MTK_UART_DEBUG0	0x18
+#define MTK_UART_DEBUG0		0x18
 #define MTK_UART_IER_XOFFI	0x20	/* Enable XOFF character interrupt */
 #define MTK_UART_IER_RTSI	0x40	/* Enable RTS Modem status interrupt */
 #define MTK_UART_IER_CTSI	0x80	/* Enable CTS Modem status interrupt */
@@ -65,6 +65,7 @@ enum dma_rx_status {
 };
 #endif
 
+#define MTK_UART_STATE_BIT_BUSCLK_EN	BIT(0)
 struct mtk8250_data {
 	int			line;
 	unsigned int		rx_pos;
@@ -76,6 +77,7 @@ struct mtk8250_data {
 	enum dma_rx_status	rx_status;
 #endif
 	int			rx_wakeup_irq;
+	unsigned long		state;		/* MTK_UART_STATE_BIT_xxx */
 };
 
 /* flow control mode */
@@ -431,6 +433,9 @@ static int __maybe_unused mtk8250_runtime_suspend(struct device *dev)
 	struct mtk8250_data *data = dev_get_drvdata(dev);
 	struct uart_8250_port *up = serial8250_get_port(data->line);
 
+	if (!test_and_clear_bit(MTK_UART_STATE_BIT_BUSCLK_EN, &data->state))
+		return 0;
+
 	/* wait until UART in idle status */
 	while
 		(serial_in(up, MTK_UART_DEBUG0));
@@ -444,6 +449,9 @@ static int __maybe_unused mtk8250_runtime_resume(struct device *dev)
 {
 	struct mtk8250_data *data = dev_get_drvdata(dev);
 
+	if (test_and_set_bit(MTK_UART_STATE_BIT_BUSCLK_EN, &data->state))
+		return 0;
+
 	clk_prepare_enable(data->bus_clk);
 
 	return 0;