serial: 8250_dw: Limit dw8250_tx_wait_empty quirk to armada-38x devices

The previous implementation has had a detrimental effect on devices using
high bitrates (bluetooth), as the fifo being non-empty for a single check
would result in a 10 µs delay.

Limit the change to devices with the new "marvell,armada-38x-uart"
compatible string. Also update the code to allow the first 1000 retries
to not perform a delay.

The maximum duration of retries has been increased to cover a worst-case
seen on the Armada 385 SoC. "dmesg ; resize", will fill the buffer with
text to output before doing a resize. At 9600 baud this took up to 13 ms
to flush all characters and avoid some getting lost.

Signed-off-by: Joshua Scott <joshua.scott@alliedtelesis.co.nz>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/arm/boot/dts/armada-38x.dtsi

@@ -163,7 +163,7 @@
 			};
 
 			uart0: serial@12000 {
-				compatible = "snps,dw-apb-uart";
+				compatible = "marvell,armada-38x-uart";
 				reg = <0x12000 0x100>;
 				reg-shift = <2>;
 				interrupts = <GIC_SPI 12 IRQ_TYPE_LEVEL_HIGH>;
@@ -173,7 +173,7 @@
 			};
 
 			uart1: serial@12100 {
-				compatible = "snps,dw-apb-uart";
+				compatible = "marvell,armada-38x-uart";
 				reg = <0x12100 0x100>;
 				reg-shift = <2>;
 				interrupts = <GIC_SPI 13 IRQ_TYPE_LEVEL_HIGH>;

drivers/tty/serial/8250/8250_dw.c

@@ -121,25 +121,44 @@ static void dw8250_check_lcr(struct uart_port *p, int value)
 }
 
 /* Returns once the transmitter is empty or we run out of retries */
-static void dw8250_tx_wait_empty(struct uart_port *p, int tries)
+static void dw8250_tx_wait_empty(struct uart_port *p)
 {
+	unsigned int tries = 20000;
+	unsigned int delay_threshold = tries - 1000;
 	unsigned int lsr;
 
 	while (tries--) {
 		lsr = readb (p->membase + (UART_LSR << p->regshift));
 		if (lsr & UART_LSR_TEMT)
 			break;
-		udelay (10);
+
+		/* The device is first given a chance to empty without delay,
+		 * to avoid slowdowns at high bitrates. If after 1000 tries
+		 * the buffer has still not emptied, allow more time for low-
+		 * speed links. */
+		if (tries < delay_threshold)
+			udelay (1);
 	}
 }
 
-static void dw8250_serial_out(struct uart_port *p, int offset, int value)
+static void dw8250_serial_out38x(struct uart_port *p, int offset, int value)
 {
 	struct dw8250_data *d = p->private_data;
 
 	/* Allow the TX to drain before we reconfigure */
 	if (offset == UART_LCR)
-		dw8250_tx_wait_empty(p, 1000);
+		dw8250_tx_wait_empty(p);
+
+	writeb(value, p->membase + (offset << p->regshift));
+
+	if (offset == UART_LCR && !d->uart_16550_compatible)
+		dw8250_check_lcr(p, value);
+}
+
+
+static void dw8250_serial_out(struct uart_port *p, int offset, int value)
+{
+	struct dw8250_data *d = p->private_data;
 
 	writeb(value, p->membase + (offset << p->regshift));
 
@@ -357,6 +376,9 @@ static void dw8250_quirks(struct uart_port *p, struct dw8250_data *data)
 			p->serial_in = dw8250_serial_in32be;
 			p->serial_out = dw8250_serial_out32be;
 		}
+		if (of_device_is_compatible(np, "marvell,armada-38x-uart"))
+			p->serial_out = dw8250_serial_out38x;
+
 	} else if (acpi_dev_present("APMC0D08", NULL, -1)) {
 		p->iotype = UPIO_MEM32;
 		p->regshift = 2;
@@ -666,6 +688,7 @@ static const struct dev_pm_ops dw8250_pm_ops = {
 static const struct of_device_id dw8250_of_match[] = {
 	{ .compatible = "snps,dw-apb-uart" },
 	{ .compatible = "cavium,octeon-3860-uart" },
+	{ .compatible = "marvell,armada-38x-uart" },
 	{ /* Sentinel */ }
 };
 MODULE_DEVICE_TABLE(of, dw8250_of_match);