[PATCH] serdev: handle devices without an ACPI or device tree node

[PATCH] serdev: handle devices without an ACPI or device tree node

[ChunAn Wu]

Some drivers, such as dell-uart-backlight, manually instantiate serdev
clients without an ACPI companion or device tree node.

For these devices, both modalias helpers return -ENODEV. This makes the
modalias sysfs read fail and suppresses the device add uevent, even
though the device is registered and bound to its driver.

Return success with no modalias when neither metadata source exists.
Devices with ACPI or device tree metadata keep their existing alias
generation and error handling.

Signed-off-by: ChunAn Wu <an.wu@canonical.com>
---
 drivers/tty/serdev/core.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/drivers/tty/serdev/core.c b/drivers/tty/serdev/core.c
index 7500efcdfc21..366c6d906233 100644
--- a/drivers/tty/serdev/core.c
+++ b/drivers/tty/serdev/core.c
@@ -26,11 +26,26 @@
 static bool is_registered;
 static DEFINE_IDA(ctrl_ida);
 
+static bool serdev_device_has_modalias_node(const struct device *dev)
+{
+	if (ACPI_COMPANION(dev))
+		return true;
+
+	return dev->of_node && !dev_of_node_reused(dev);
+}
+
 static ssize_t modalias_show(struct device *dev,
 			     struct device_attribute *attr, char *buf)
 {
 	int len;
 
+	/*
+	 * Serdev devices instantiated by a driver instead of by firmware have
+	 * no node to derive a modalias from, which is not an error.
+	 */
+	if (!serdev_device_has_modalias_node(dev))
+		return 0;
+
 	len = acpi_device_modalias(dev, buf, PAGE_SIZE - 1);
 	if (len != -ENODEV)
 		return len;
@@ -51,6 +66,9 @@ static int serdev_device_uevent(const struct device *dev, struct kobj_uevent_env
 
 	/* TODO: platform modalias */
 
+	if (!serdev_device_has_modalias_node(dev))
+		return 0;
+
 	rc = acpi_device_uevent_modalias(dev, env);
 	if (rc != -ENODEV)
 		return rc;

base-commit: 24a19b953e2a3d0ea5a8c4951a0ab8f32720c907
-- 
2.34.1

Re: [PATCH] serdev: handle devices without an ACPI or device tree node

[sashiko-bot]

> Some drivers, such as dell-uart-backlight, manually instantiate serdev
> clients without an ACPI companion or device tree node.
> 
> For these devices, both modalias helpers return -ENODEV. This makes the
> modalias sysfs read fail and suppresses the device add uevent, even
> though the device is registered and bound to its driver.
> 
> Return success with no modalias when neither metadata source exists.
> Devices with ACPI or device tree metadata keep their existing alias
> generation and error handling.
> 
> Signed-off-by: ChunAn Wu <an.wu@canonical.com>

Sashiko has reviewed this patch and found no issues.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261019054513.1966382-1-an.wu@canonical.com?part=1

Re: [PATCH] serdev: handle devices without an ACPI or device tree node

[An Wu]

cc linux-kernel@vger.kernel.org and jirislaby@kernel.org

On Mon, Oct 19, 2026 at 1:45 PM ChunAn Wu <an.wu@canonical.com> wrote:
>
> Some drivers, such as dell-uart-backlight, manually instantiate serdev
> clients without an ACPI companion or device tree node.
>
> For these devices, both modalias helpers return -ENODEV. This makes the
> modalias sysfs read fail and suppresses the device add uevent, even
> though the device is registered and bound to its driver.
>
> Return success with no modalias when neither metadata source exists.
> Devices with ACPI or device tree metadata keep their existing alias
> generation and error handling.
>
> Signed-off-by: ChunAn Wu <an.wu@canonical.com>
> ---
>  drivers/tty/serdev/core.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
>
> diff --git a/drivers/tty/serdev/core.c b/drivers/tty/serdev/core.c
> index 7500efcdfc21..366c6d906233 100644
> --- a/drivers/tty/serdev/core.c
> +++ b/drivers/tty/serdev/core.c
> @@ -26,11 +26,26 @@
>  static bool is_registered;
>  static DEFINE_IDA(ctrl_ida);
>
> +static bool serdev_device_has_modalias_node(const struct device *dev)
> +{
> +       if (ACPI_COMPANION(dev))
> +               return true;
> +
> +       return dev->of_node && !dev_of_node_reused(dev);
> +}
> +
>  static ssize_t modalias_show(struct device *dev,
>                              struct device_attribute *attr, char *buf)
>  {
>         int len;
>
> +       /*
> +        * Serdev devices instantiated by a driver instead of by firmware have
> +        * no node to derive a modalias from, which is not an error.
> +        */
> +       if (!serdev_device_has_modalias_node(dev))
> +               return 0;
> +
>         len = acpi_device_modalias(dev, buf, PAGE_SIZE - 1);
>         if (len != -ENODEV)
>                 return len;
> @@ -51,6 +66,9 @@ static int serdev_device_uevent(const struct device *dev, struct kobj_uevent_env
>
>         /* TODO: platform modalias */
>
> +       if (!serdev_device_has_modalias_node(dev))
> +               return 0;
> +
>         rc = acpi_device_uevent_modalias(dev, env);
>         if (rc != -ENODEV)
>                 return rc;
>
> base-commit: 24a19b953e2a3d0ea5a8c4951a0ab8f32720c907
> --
> 2.34.1
>

Re: [PATCH] serdev: handle devices without an ACPI or device tree node

[Markus Probst]

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On Mon, 2026-10-19 at 13:45 +0800, ChunAn Wu wrote:
> Some drivers, such as dell-uart-backlight, manually instantiate serdev
> clients without an ACPI companion or device tree node.
> 
> For these devices, both modalias helpers return -ENODEV. This makes the
> modalias sysfs read fail and suppresses the device add uevent, even
> though the device is registered and bound to its driver.
> 
> Return success with no modalias when neither metadata source exists.
> Devices with ACPI or device tree metadata keep their existing alias
> generation and error handling.
> 
> Signed-off-by: ChunAn Wu <an.wu@canonical.com>
> ---
>  drivers/tty/serdev/core.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/drivers/tty/serdev/core.c b/drivers/tty/serdev/core.c
> index 7500efcdfc21..366c6d906233 100644
> --- a/drivers/tty/serdev/core.c
> +++ b/drivers/tty/serdev/core.c
> @@ -26,11 +26,26 @@
>  static bool is_registered;
>  static DEFINE_IDA(ctrl_ida);
>  
> +static bool serdev_device_has_modalias_node(const struct device *dev)
> +{
> +	if (ACPI_COMPANION(dev))
> +		return true;
> +
> +	return dev->of_node && !dev_of_node_reused(dev);
> +}
> +
>  static ssize_t modalias_show(struct device *dev,
>  			     struct device_attribute *attr, char *buf)
>  {
>  	int len;
>  
> +	/*
> +	 * Serdev devices instantiated by a driver instead of by firmware have
> +	 * no node to derive a modalias from, which is not an error.
> +	 */
> +	if (!serdev_device_has_modalias_node(dev))
> +		return 0;
> +
>  	len = acpi_device_modalias(dev, buf, PAGE_SIZE - 1);
>  	if (len != -ENODEV)
>  		return len;
> @@ -51,6 +66,9 @@ static int serdev_device_uevent(const struct device *dev, struct kobj_uevent_env
>  
>  	/* TODO: platform modalias */
>  
> +	if (!serdev_device_has_modalias_node(dev))
> +		return 0;
> +
>  	rc = acpi_device_uevent_modalias(dev, env);
>  	if (rc != -ENODEV)
>  		return rc;
> 
> base-commit: 24a19b953e2a3d0ea5a8c4951a0ab8f32720c907

There is a todo comment:
/* TODO: platform modalias */

Resolving this should also fix your issue.

Also keep it closer to
- platform_uevent in drivers/base/platform.c
- modalias_show in drivers/base/platform.c

and avoid the extra `serdev_device_has_modalias_node` function.

Please also address the similar todo comment in `serdev_device_match`
in the same process.

Thanks
- Markus Probst


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