arm64: add support for 8250/16550 earlyprintk

This patch adds support for using earlyprintk with 8250/16550 UART
ports. The 8250/16550 UART can either have 8-bit or 32-bit aligned
registers which is HW vendor dependent.

Kernel args for 8-bit aligned regs: earlyprintk=uart8250-8bit,<phys_address>

Kernel args for 32-bit aligned regs: earlyprintk=uart8250-32bit,<phys_address>
Signed-off-by: Anup Patel <anup.patel@linaro.org>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/early_printk.c

@@ -24,6 +24,7 @@
 #include <linux/io.h>
 
 #include <linux/amba/serial.h>
+#include <linux/serial_reg.h>
 
 static void __iomem *early_base;
 static void (*printch)(char ch);
@@ -51,6 +52,26 @@ static void smh_printch(char ch)
 		     : : "r" (&ch) : "x0", "x1", "memory");
 }
 
+/*
+ * 8250/16550 (8-bit aligned registers) single character TX.
+ */
+static void uart8250_8bit_printch(char ch)
+{
+	while (!(readb_relaxed(early_base + UART_LSR) & UART_LSR_THRE))
+		;
+	writeb_relaxed(ch, early_base + UART_TX);
+}
+
+/*
+ * 8250/16550 (32-bit aligned registers) single character TX.
+ */
+static void uart8250_32bit_printch(char ch)
+{
+	while (!(readl_relaxed(early_base + (UART_LSR << 2)) & UART_LSR_THRE))
+		;
+	writel_relaxed(ch, early_base + (UART_TX << 2));
+}
+
 struct earlycon_match {
 	const char *name;
 	void (*printch)(char ch);
@@ -59,6 +80,8 @@ struct earlycon_match {
 static const struct earlycon_match earlycon_match[] __initconst = {
 	{ .name = "pl011", .printch = pl011_printch, },
 	{ .name = "smh", .printch = smh_printch, },
+	{ .name = "uart8250-8bit", .printch = uart8250_8bit_printch, },
+	{ .name = "uart8250-32bit", .printch = uart8250_32bit_printch, },
 	{}
 };