[PATCH] tty: serial: 8250: Passing attr_group to universal driver

[CrescentCY Hsieh]

Many low-level drivers in Linux kernel register their serial ports with
the help of universal driver (8250_core, 8250_port).

There is an attribute group called `serial8250_dev_attr_group` within
`8250_port.c` to handle the `rx_trig_bytes` attribute:
https://lore.kernel.org/all/20140716011931.31474.68825.stgit@yuno-kbuild.novalocal/

However, if a low-level driver has some HW specifications that need to
be set or retrieved using an attr_group, the universal driver
(8250_port) would overwrite the low-level driver's attr_group.

This patch allows the low-level driver's attr_group to be passed to the
universal driver (8250_port) and combines the two attr_groups. This
ensures that the corresponding system file will only be created if the
device is registered by such a low-level driver.

Signed-off-by: CrescentCY Hsieh <crescentcy.hsieh@xxxxxxxx>
---
 drivers/tty/serial/8250/8250_core.c |  9 +++++++++
 drivers/tty/serial/8250/8250_port.c | 26 ++++++++++++++++++++++++--
 include/linux/serial_core.h         |  1 +
 3 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_core.c b/drivers/tty/serial/8250/8250_core.c
index 43824a174a51..01d04f9d5192 100644
--- a/drivers/tty/serial/8250/8250_core.c
+++ b/drivers/tty/serial/8250/8250_core.c
@@ -1130,6 +1130,8 @@ int serial8250_register_8250_port(const struct uart_8250_port *up)
 			uart->port.pm = up->port.pm;
 		if (up->port.handle_break)
 			uart->port.handle_break = up->port.handle_break;
+		if (up->port.attr_group)
+			uart->port.attr_group = up->port.attr_group;
 		if (up->dl_read)
 			uart->dl_read = up->dl_read;
 		if (up->dl_write)
@@ -1210,6 +1212,13 @@ void serial8250_unregister_port(int line)
 		uart->port.type = PORT_UNKNOWN;
 		uart->port.dev = &serial8250_isa_devs->dev;
 		uart->port.port_id = line;
+
+		if (uart->port.attr_group_allocated) {
+			kfree(uart->port.attr_group->attrs);
+			kfree(uart->port.attr_group);
+			uart->port.attr_group_allocated = false;
+		}
+		uart->port.attr_group = NULL;
 		uart->capabilities = 0;
 		serial8250_init_port(uart);
 		serial8250_apply_quirks(uart);
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 893bc493f662..ddfa8b59e562 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -3135,9 +3135,31 @@ static struct attribute_group serial8250_dev_attr_group = {
 static void register_dev_spec_attr_grp(struct uart_8250_port *up)
 {
 	const struct serial8250_config *conf_type = &uart_config[up->port.type];
+	struct attribute **upp_attrs = NULL;
+	int upp_attrs_num = 0, i;
 
-	if (conf_type->rxtrig_bytes[0])
-		up->port.attr_group = &serial8250_dev_attr_group;
+	up->port.attr_group_allocated = false;
+
+	if (up->port.attr_group) {
+		upp_attrs = up->port.attr_group->attrs;
+
+		while (upp_attrs[upp_attrs_num])
+			upp_attrs_num++;
+
+		up->port.attr_group = kcalloc(1, sizeof(struct attribute_group), GFP_KERNEL);
+		up->port.attr_group->attrs = kcalloc(upp_attrs_num + 2, sizeof(struct attribute *), GFP_KERNEL);
+
+		for (i = 0; i < upp_attrs_num; ++i)
+			up->port.attr_group->attrs[i] = upp_attrs[i];
+
+		if (conf_type->rxtrig_bytes[0])
+			up->port.attr_group->attrs[upp_attrs_num] = &dev_attr_rx_trig_bytes.attr;
+
+		up->port.attr_group_allocated = true;
+	} else {
+		if (conf_type->rxtrig_bytes[0])
+			up->port.attr_group = &serial8250_dev_attr_group;
+	}
 }
 
 static void serial8250_config_port(struct uart_port *port, int flags)
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index 8cb65f50e830..3212d64c32c6 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -581,6 +581,7 @@ struct uart_port {
 	unsigned char		console_reinit;
 	const char		*name;			/* port name */
 	struct attribute_group	*attr_group;		/* port specific attributes */
+	bool			attr_group_allocated;	/* whether attr_group is dynamic allocated */
 	const struct attribute_group **tty_groups;	/* all attributes (serial core use only) */
 	struct serial_rs485     rs485;
 	struct serial_rs485	rs485_supported;	/* Supported mask for serial_rs485 */
-- 
2.34.1