[PATCH v2 2/3] serial: core: Add support for DEVNAME:0.0 style naming for kernel console

[Tony Lindgren]

We can now add hardware based addressing to serial ports. Starting with
commit 84a9582fd203 ("serial: core: Start managing serial controllers to
enable runtime PM"), and all the related fixes to this commit, the serial
core now knows to which serial port controller the ports are connected.

The serial ports can be addressed with DEVNAME:0.0 style naming. The names
are something like 00:04:0.0 for a serial port on qemu, and something like
2800000.serial:0.0 on platform device using systems like ARM64 for example.

The DEVNAME is the unique serial port hardware controller device name, AKA
the name for port->dev. The 0.0 are the serial core controller id and port
id.

Typically 0.0 are used for each controller and port instance unless the
serial port hardware controller has multiple controllers or ports.

Using DEVNAME:0.0 style naming actually solves two long term issues for
addressing the serial ports:

1. According to Andy Shevchenko, using DEVNAME:0.0 style naming fixes an
   issue where depending on the BIOS settings, the kernel serial port ttyS
   instance number may change if HSUART is enabled

2. Device tree using architectures no longer necessarily need to specify
   aliases to find a specific serial port, and we can just allocate the
   ttyS instance numbers dynamically in whatever probe order

To do this, we need a custom init time parser for the console= command
line option as printk already handles parsing it with console_setup().
Also early_param() gets handled by console_setup() if "console" and
"earlycon" are used.

Signed-off-by: Tony Lindgren <tony@xxxxxxxxxxx>
---
 drivers/tty/serial/Makefile          |   3 +
 drivers/tty/serial/serial_base.h     |  11 +++
 drivers/tty/serial/serial_base_con.c | 133 +++++++++++++++++++++++++++
 drivers/tty/serial/serial_core.c     |   4 +
 4 files changed, 151 insertions(+)
 create mode 100644 drivers/tty/serial/serial_base_con.c

diff --git a/drivers/tty/serial/Makefile b/drivers/tty/serial/Makefile
--- a/drivers/tty/serial/Makefile
+++ b/drivers/tty/serial/Makefile
@@ -3,6 +3,9 @@
 # Makefile for the kernel serial device drivers.
 #
 
+# Parse kernel command line consoles before the serial drivers probe
+obj-$(CONFIG_SERIAL_CORE_CONSOLE) += serial_base_con.o
+
 obj-$(CONFIG_SERIAL_CORE) += serial_base.o
 serial_base-y := serial_core.o serial_base_bus.o serial_ctrl.o serial_port.o
 
diff --git a/drivers/tty/serial/serial_base.h b/drivers/tty/serial/serial_base.h
--- a/drivers/tty/serial/serial_base.h
+++ b/drivers/tty/serial/serial_base.h
@@ -45,3 +45,14 @@ void serial_ctrl_unregister_port(struct uart_driver *drv, struct uart_port *port
 
 int serial_core_register_port(struct uart_driver *drv, struct uart_port *port);
 void serial_core_unregister_port(struct uart_driver *drv, struct uart_port *port);
+
+#ifdef CONFIG_SERIAL_CORE_CONSOLE
+int serial_base_add_preferred_console(struct uart_driver *drv,
+				      struct uart_port *port);
+#else
+static inline int serial_base_add_preferred_console(struct uart_driver *drv,
+						    struct uart_port *port)
+{
+	return 0;
+}
+#endif
diff --git a/drivers/tty/serial/serial_base_con.c b/drivers/tty/serial/serial_base_con.c
new file mode 100644
--- /dev/null
+++ b/drivers/tty/serial/serial_base_con.c
@@ -0,0 +1,133 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Serial base console options handling
+ *
+ * Copyright (C) 2023 Texas Instruments Incorporated - https://www.ti.com/
+ * Author: Tony Lindgren <tony@xxxxxxxxxxx>
+ */
+
+#include <linux/init.h>
+#include <linux/list.h>
+#include <linux/kernel.h>
+#include <linux/serial_core.h>
+#include <linux/slab.h>
+
+#include "serial_base.h"
+
+static LIST_HEAD(serial_base_consoles);
+
+struct serial_base_console {
+	struct list_head node;
+	char *name;
+	char *opt;
+};
+
+/*
+ * Adds a preferred console for a serial port if console=DEVNAME:0.0
+ * style addressing is used for the kernel command line. Translates
+ * from DEVNAME:0.0 to port->dev_name such as ttyS. Duplicates are
+ * ignored by add_preferred_console().
+ */
+int serial_base_add_preferred_console(struct uart_driver *drv,
+				      struct uart_port *port)
+{
+	struct serial_base_console *entry;
+	char *port_match;
+
+	port_match = kasprintf(GFP_KERNEL, "%s:%i.%i", dev_name(port->dev),
+			       port->ctrl_id, port->port_id);
+	if (!port_match)
+		return -ENOMEM;
+
+	list_for_each_entry(entry, &serial_base_consoles, node) {
+		if (!strcmp(port_match, entry->name)) {
+			add_preferred_console(drv->dev_name, port->line,
+					      entry->opt);
+			break;
+		}
+	}
+
+	kfree(port_match);
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(serial_base_add_preferred_console);
+
+/* Adds a command line console to the list of consoles for driver probe time */
+static int __init serial_base_add_con(char *name, char *opt)
+{
+	struct serial_base_console *con;
+
+	con = kzalloc(sizeof(*con), GFP_KERNEL);
+	if (!con)
+		return -ENOMEM;
+
+	con->name = kstrdup(name, GFP_KERNEL);
+	if (!con->name)
+		goto free_con;
+
+	if (opt) {
+		con->opt = kstrdup(opt, GFP_KERNEL);
+		if (!con->name)
+			goto free_name;
+	}
+
+	list_add_tail(&con->node, &serial_base_consoles);
+
+	return 0;
+
+free_name:
+	kfree(con->name);
+
+free_con:
+	kfree(con);
+
+	return -ENOMEM;
+}
+
+/* Parse console name and options */
+static int __init serial_base_parse_one(char *param, char *val,
+					const char *unused, void *arg)
+{
+	char *opt;
+
+	if (strcmp(param, "console"))
+		return 0;
+
+	if (!val)
+		return 0;
+
+	opt = strchr(val, ',');
+	if (opt) {
+		opt[0] = '\0';
+		opt++;
+	}
+
+	if (!strlen(val))
+		return 0;
+
+	return serial_base_add_con(val, opt);
+}
+
+/*
+ * The "console=" option is handled by console_setup() in printk. We can't use
+ * early_param() as do_early_param() checks for "console" and "earlycon" options
+ * so console_setup() potentially handles console also early. Use parse_args().
+ */
+static int __init serial_base_opts_init(void)
+{
+	char *command_line;
+
+	command_line = kstrdup(boot_command_line, GFP_KERNEL);
+	if (!command_line)
+		return -ENOMEM;
+
+	parse_args("Setting serial core console", command_line,
+		   NULL, 0, -1, -1, NULL, serial_base_parse_one);
+
+	kfree(command_line);
+
+	return 0;
+}
+
+arch_initcall(serial_base_opts_init);
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -3358,6 +3358,10 @@ int serial_core_register_port(struct uart_driver *drv, struct uart_port *port)
 	if (ret)
 		goto err_unregister_ctrl_dev;
 
+	ret = serial_base_add_preferred_console(drv, port);
+	if (ret)
+		goto err_unregister_port_dev;
+
 	ret = serial_core_add_one_port(drv, port);
 	if (ret)
 		goto err_unregister_port_dev;
-- 
2.42.0