arm64: Add simple earlyprintk support

This patch adds support for "earlyprintk=" parameter on the kernel
command line. The format is:

  earlyprintk=<name>[,<addr>][,<options>]

where <name> is the name of the (UART) device, e.g. "pl011", <addr> is
the I/O address. The <options> aren't currently used.

The mapping of the earlyprintk device is done very early during kernel
boot and there are restrictions on which functions it can call. A
special early_io_map() function is added which creates the mapping from
the pre-defined EARLY_IOBASE to the device I/O address passed via the
kernel parameter. The pgd entry corresponding to EARLY_IOBASE is
pre-populated in head.S during kernel boot.

Only PL011 is currently supported and it is assumed that the interface
is already initialised by the boot loader before the kernel is started.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>

1 files changed, 118 insertions, 0 deletions

diff --git a/arch/arm64/kernel/early_printk.c b/arch/arm64/kernel/early_printk.c
new file mode 100644
index 00000000000..7e320a2edb9
--- /dev/null
+++ b/arch/arm64/kernel/early_printk.c
@@ -0,0 +1,118 @@
+/*
+ * Earlyprintk support.
+ *
+ * Copyright (C) 2012 ARM Ltd.
+ * Author: Catalin Marinas <catalin.marinas@arm.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#include <linux/kernel.h>
+#include <linux/console.h>
+#include <linux/init.h>
+#include <linux/string.h>
+#include <linux/mm.h>
+#include <linux/io.h>
+
+#include <linux/amba/serial.h>
+
+static void __iomem *early_base;
+static void (*printch)(char ch);
+
+/*
+ * PL011 single character TX.
+ */
+static void pl011_printch(char ch)
+{
+	while (readl_relaxed(early_base + UART01x_FR) & UART01x_FR_TXFF)
+		;
+	writeb_relaxed(ch, early_base + UART01x_DR);
+	while (readl_relaxed(early_base + UART01x_FR) & UART01x_FR_BUSY)
+		;
+}
+
+struct earlycon_match {
+	const char *name;
+	void (*printch)(char ch);
+};
+
+static const struct earlycon_match earlycon_match[] __initconst = {
+	{ .name = "pl011", .printch = pl011_printch, },
+	{}
+};
+
+static void early_write(struct console *con, const char *s, unsigned n)
+{
+	while (n-- > 0) {
+		if (*s == '\n')
+			printch('\r');
+		printch(*s);
+		s++;
+	}
+}
+
+static struct console early_console = {
+	.name =		"earlycon",
+	.write =	early_write,
+	.flags =	CON_PRINTBUFFER | CON_BOOT,
+	.index =	-1,
+};
+
+/*
+ * Parse earlyprintk=... parameter in the format:
+ *
+ *   <name>[,<addr>][,<options>]
+ *
+ * and register the early console. It is assumed that the UART has been
+ * initialised by the bootloader already.
+ */
+static int __init setup_early_printk(char *buf)
+{
+	const struct earlycon_match *match = earlycon_match;
+	phys_addr_t paddr = 0;
+
+	if (!buf) {
+		pr_warning("No earlyprintk arguments passed.\n");
+		return 0;
+	}
+
+	while (match->name) {
+		size_t len = strlen(match->name);
+		if (!strncmp(buf, match->name, len)) {
+			buf += len;
+			break;
+		}
+		match++;
+	}
+	if (!match->name) {
+		pr_warning("Unknown earlyprintk arguments: %s\n", buf);
+		return 0;
+	}
+
+	/* I/O address */
+	if (!strncmp(buf, ",0x", 3)) {
+		char *e;
+		paddr = simple_strtoul(buf + 1, &e, 16);
+		buf = e;
+	}
+	/* no options parsing yet */
+
+	if (paddr)
+		early_base = early_io_map(paddr, EARLYCON_IOBASE);
+
+	printch = match->printch;
+	register_console(&early_console);
+
+	return 0;
+}
+
+early_param("earlyprintk", setup_early_printk);