[PATCH 0/2] tty: serial: add and use a managed variant of uart_add_one_port()

[PATCH 0/2] tty: serial: add and use a managed variant of uart_add_one_port()

[Bartosz Golaszewski]

From: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

This series adds a managed variant of uart_add_one_port() and uses it in the
qcom-geni-serial driver.

I've been asked by Greg to send it separately and he didn't seem to be
impressed by the proposition of adding devres interfaces to the tty layer
in general. I can only assume it has something to do with the ongoing
discussion about the supposed danger of using devres interfaces in conjunction
with exporting character devices to user-space.

The bug in question can be triggered by opening a device file, unbinding the
driver that exported it and then calling any of the system calls on the
associated file descriptor.

After some testing I noticed that many subsystems are indeed either crashing
or deadlocking in the above situation. I've sent patches that attempt to fix
the GPIO and I2C subsystems[1][2]. Neither of these issues have anything to
do with devres and all to do with the fact that certain resources are freed
on driver unbind and others need to live for as long as the character device
exists. More details on that in the cover letters and commit messages in the
links.

I'd like to point out that the serial code is immune to this issue as before
every operation, the serial core takes the port lock and checks the uart
state. If the device no longer exists (when the uart port is removed, the
pointer to uart_port inside uart_state is to NULL), it gracefully returns
-ENODEV to user-space.

Please consider applying the patches in the series as devres is the easiest
way to lessen the burden on driver developers when dealing with complex error
paths and resource leaks. The general rule for devres is: if it can be freed
in .remove() then it can be managed by devres, which is the case for this new
helper.

Bart

[1] https://lkml.org/lkml/2022/12/8/826
[2] https://lkml.org/lkml/2022/12/5/414

Bartosz Golaszewski (2):
  tty: serial: provide devm_uart_add_one_port()
  tty: serial: qcom-geni-serial: use devres for uart port management

 .../driver-api/driver-model/devres.rst        |  3 ++
 drivers/tty/serial/qcom_geni_serial.c         |  8 +---
 drivers/tty/serial/serial_core.c              | 48 +++++++++++++++++++
 include/linux/serial_core.h                   |  6 +++
 4 files changed, 58 insertions(+), 7 deletions(-)

-- 
2.37.2

[PATCH 1/2] tty: serial: provide devm_uart_add_one_port()

[Bartosz Golaszewski]

From: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

Provide a devres variant of uart_add_one_port() that removes the managed
port at device detach.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
---
 .../driver-api/driver-model/devres.rst        |  3 ++
 drivers/tty/serial/serial_core.c              | 48 +++++++++++++++++++
 include/linux/serial_core.h                   |  6 +++
 3 files changed, 57 insertions(+)

diff --git a/Documentation/driver-api/driver-model/devres.rst b/Documentation/driver-api/driver-model/devres.rst
index 56082265e8e5..5d07a8c1eadb 100644
--- a/Documentation/driver-api/driver-model/devres.rst
+++ b/Documentation/driver-api/driver-model/devres.rst
@@ -436,6 +436,9 @@ RTC
 SERDEV
   devm_serdev_device_open()
 
+SERIAL
+  devm_uart_add_one_port()
+
 SLAVE DMA ENGINE
   devm_acpi_dma_controller_register()
 
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 179ee199df34..44cdb8aa80a4 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -3217,6 +3217,54 @@ int uart_remove_one_port(struct uart_driver *drv, struct uart_port *uport)
 }
 EXPORT_SYMBOL(uart_remove_one_port);
 
+struct uart_port_devres {
+	struct uart_driver *drv;
+	struct uart_port *port;
+};
+
+static void devm_uart_remove_one_port(struct device *dev, void *data)
+{
+	struct uart_port_devres *res = data;
+
+	uart_remove_one_port(res->drv, res->port);
+}
+
+/**
+ * devm_uart_add_one_port - managed variant of uart_register_driver()
+ * @dev: managed device
+ * @drv: pointer to the uart low level driver structure for this port
+ * @uport: uart port structure to use for this port.
+ *
+ * Context: task context, might sleep
+ *
+ * This is a devres wrapper around uart_add_one_port(). It allows the driver
+ * @drv to register its own uart_port structure with the core driver. The port
+ * will be unregistered on driver detach.
+ */
+int devm_uart_add_one_port(struct device *dev,
+			   struct uart_driver *drv, struct uart_port *port)
+{
+	struct uart_port_devres *res;
+	int ret;
+
+	res = devres_alloc(devm_uart_remove_one_port, sizeof(*res), GFP_KERNEL);
+	if (!res)
+		return -1;
+
+	ret = uart_add_one_port(drv, port);
+	if (ret) {
+		devres_free(res);
+		return -1;
+	}
+
+	res->drv = drv;
+	res->port = port;
+	devres_add(dev, res);
+
+	return 0;
+}
+EXPORT_SYMBOL(devm_uart_add_one_port);
+
 /**
  * uart_match_port - are the two ports equivalent?
  * @port1: first port
diff --git a/include/linux/serial_core.h b/include/linux/serial_core.h
index d657f2a42a7b..d0911f04706e 100644
--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -771,6 +771,12 @@ int uart_remove_one_port(struct uart_driver *reg, struct uart_port *port);
 bool uart_match_port(const struct uart_port *port1,
 		const struct uart_port *port2);
 
+/*
+ * UART devres
+ */
+int devm_uart_add_one_port(struct device *dev,
+			   struct uart_driver *drv, struct uart_port *port);
+
 /*
  * Power Management
  */
-- 
2.37.2

[PATCH 2/2] tty: serial: qcom-geni-serial: use devres for uart port management

[Bartosz Golaszewski]

From: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

Shrink and simplify the probe() and remove() code by using the managed
variant of uart_add_one_port().

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
---
 drivers/tty/serial/qcom_geni_serial.c | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
index 83b66b73303a..16532cb64465 100644
--- a/drivers/tty/serial/qcom_geni_serial.c
+++ b/drivers/tty/serial/qcom_geni_serial.c
@@ -1469,7 +1469,7 @@ static int qcom_geni_serial_probe(struct platform_device *pdev)
 	platform_set_drvdata(pdev, port);
 	port->handle_rx = console ? handle_rx_console : handle_rx_uart;
 
-	ret = uart_add_one_port(drv, uport);
+	ret = devm_uart_add_one_port(&pdev->dev, drv, uport);
 	if (ret)
 		return ret;
 
@@ -1478,7 +1478,6 @@ static int qcom_geni_serial_probe(struct platform_device *pdev)
 			IRQF_TRIGGER_HIGH, port->name, uport);
 	if (ret) {
 		dev_err(uport->dev, "Failed to get IRQ ret %d\n", ret);
-		uart_remove_one_port(drv, uport);
 		return ret;
 	}
 
@@ -1495,7 +1494,6 @@ static int qcom_geni_serial_probe(struct platform_device *pdev)
 						port->wakeup_irq);
 		if (ret) {
 			device_init_wakeup(&pdev->dev, false);
-			uart_remove_one_port(drv, uport);
 			return ret;
 		}
 	}
@@ -1505,12 +1503,8 @@ static int qcom_geni_serial_probe(struct platform_device *pdev)
 
 static int qcom_geni_serial_remove(struct platform_device *pdev)
 {
-	struct qcom_geni_serial_port *port = platform_get_drvdata(pdev);
-	struct uart_driver *drv = port->private_data.drv;
-
 	dev_pm_clear_wake_irq(&pdev->dev);
 	device_init_wakeup(&pdev->dev, false);
-	uart_remove_one_port(drv, &port->uport);
 
 	return 0;
 }
-- 
2.37.2