[PATCH v2 0/5] auxdisplay: linedisp: support attribute attachment to auxdisplay devices

[PATCH v2 0/5] auxdisplay: linedisp: support attribute attachment to auxdisplay devices

[Jean-François Lessard]

This series modernizes the auxdisplay line display (linedisp) library to
enable seamless integration with auxdisplay parent devices while
maintaining backward compatibility.

The key improvement is adding attach/detach APIs that allow linedisp sysfs
attributes to be bound directly to their parent auxdisplay devices avoiding
child device proliferation and enabling a uniform 7-segment userspace
interface across different driver architectures.

This series introduces attachment infrastructure for linedisp devices.
The first consumer of this API will be the TM16XX driver series.
See the related patch series:
  auxdisplay: Add TM16xx 7-segment LED matrix display controllers driver

Changes include:
1. Encapsulate container_of() usage with to_linedisp() helper function for
   cleaner context retrieval
2. Improve message display behavior with static padding when message length
   is smaller than display width
3. Add 'num_chars' read-only attribute for userspace capability discovery
4. Add attach/detach API for sysfs attributes binding to parent devices
5. Document all linedisp sysfs attributes in ABI documentation

All existing linedisp_register() users remain unaffected. The new APIs
enable drivers like TM16XX to integrate 7-segment functionality within
their LED class device hierarchy while providing a uniform 7-segment API.

Thanks to Andy Shevchenko for early feedback and guidance.

V1 changelog:
- Remove should_scroll() check in linedisp_scroll()
- Keep attributes sorted alphabetically
- Document, invert logic and rename owns_device to direct
- Document linedisp_attach()/_register() must be freed by their
  respective APIs after use
- Change call order of display default message and add attributes
  to prevent racy userspace condition
- Separate delete_attachment() call from linedisp variable declaration
- Document existing attributes separately, in the first patch

RFC changelog:
- Replace scope_guard() with guard()() for synchronized list operations.
- Replace NULL assignments with proper list_entry_is_head() pattern.
- Clearly document why introducing the attach/detach APIs.
- Split in patch series, each patch containing a specific change.
- Implement static (non-scrolling) display for short messages.
- Document exisiting and new ABI sysfs attributes.

Jean-François Lessard (5):
  docs: ABI: auxdisplay: document linedisp library sysfs attributes
  auxdisplay: linedisp: encapsulate container_of usage within
    to_linedisp
  auxdisplay: linedisp: display static message when length <= display
    size
  auxdisplay: linedisp: add num_chars sysfs attribute
  auxdisplay: linedisp: support attribute attachment to auxdisplay
    devices

 .../ABI/testing/sysfs-auxdisplay-linedisp     |  90 +++++++
 drivers/auxdisplay/line-display.c             | 240 ++++++++++++++++--
 drivers/auxdisplay/line-display.h             |   4 +
 3 files changed, 312 insertions(+), 22 deletions(-)
 create mode 100644 Documentation/ABI/testing/sysfs-auxdisplay-linedisp

-- 
2.43.0

[PATCH v2 1/5] docs: ABI: auxdisplay: document linedisp library sysfs attributes

[Jean-François Lessard]

Add ABI documentation for sysfs attributes provided by the line-display
auxdisplay library module. These attributes enable text message display and
configuration on character-based auxdisplay devices.

Documents previously undocumented attributes:
- message, scroll_step_ms (introduced in v5.16)
- map_seg7, map_seg14 (introduced in v6.9)

The line-display library is used by multiple auxdisplay drivers and
can expose these attributes either on linedisp.N child devices or
directly on parent auxdisplay devices.

Signed-off-by: Jean-François Lessard <jefflessard3@gmail.com>
---
 .../ABI/testing/sysfs-auxdisplay-linedisp     | 79 +++++++++++++++++++
 1 file changed, 79 insertions(+)
 create mode 100644 Documentation/ABI/testing/sysfs-auxdisplay-linedisp

diff --git a/Documentation/ABI/testing/sysfs-auxdisplay-linedisp b/Documentation/ABI/testing/sysfs-auxdisplay-linedisp
new file mode 100644
index 000000000000..45cf4e5a2feb
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-auxdisplay-linedisp
@@ -0,0 +1,79 @@
+What:		/sys/.../message
+Date:		October 2021
+KernelVersion:	5.16
+Description:
+		Controls the text message displayed on character line displays.
+
+		Reading returns the current message with a trailing newline.
+		Writing updates the displayed message. Messages longer than the
+		display width will automatically scroll. Trailing newlines in
+		input are automatically trimmed.
+
+		Writing an empty string clears the display.
+
+		Example:
+		  echo "Hello World" > message
+		  cat message			# Returns "Hello World\n"
+
+What:		/sys/.../scroll_step_ms
+Date:		October 2021
+KernelVersion:	5.16
+Description:
+		Controls the scrolling speed for messages longer than the display
+		width, specified in milliseconds per scroll step.
+
+		Setting to 0 disables scrolling. Default is 500ms.
+
+		Example:
+		  echo "250" > scroll_step_ms	# 4Hz scrolling
+		  cat scroll_step_ms		# Returns "250\n"
+
+What:		/sys/.../map_seg7
+Date:		January 2024
+KernelVersion:	6.9
+Description:
+		Read/write binary blob representing the ASCII-to-7-segment
+		display conversion table used by the linedisp driver, as defined
+		by struct seg7_conversion_map in <linux/map_to_7segment.h>.
+
+		Only visible on displays with 7-segment capability.
+
+		This attribute is not human-readable. Writes must match the
+		struct size exactly, else -EINVAL is returned; reads return the
+		entire mapping as a binary blob.
+
+		This interface and its implementation match existing conventions
+		used in segment-mapped display drivers since 2005.
+
+		ABI note: This style of binary sysfs attribute *is an exception*
+		to current "one value per file, text only" sysfs rules, for
+		historical compatibility and driver uniformity. New drivers are
+		discouraged from introducing additional binary sysfs ABIs.
+
+		Reference interface guidance:
+		- include/uapi/linux/map_to_7segment.h
+
+What:		/sys/.../map_seg14
+Date:		January 2024
+KernelVersion:	6.9
+Description:
+		Read/write binary blob representing the ASCII-to-14-segment
+		display conversion table used by the linedisp driver, as defined
+		by struct seg14_conversion_map in <linux/map_to_14segment.h>.
+
+		Only visible on displays with 14-segment capability.
+
+		This attribute is not human-readable. Writes must match the
+		struct size exactly, else -EINVAL is returned; reads return the
+		entire mapping as a binary blob.
+
+		This interface and its implementation match existing conventions
+		used by segment-mapped display drivers since 2005.
+
+		ABI note: This style of binary sysfs attribute *is an exception*
+		to current "one value per file, text only" sysfs rules, for
+		historical compatibility and driver uniformity. New drivers are
+		discouraged from introducing additional binary sysfs ABIs.
+
+		Reference interface guidance:
+		- include/uapi/linux/map_to_14segment.h
-- 
2.43.0

[PATCH v2 2/5] auxdisplay: linedisp: encapsulate container_of usage within to_linedisp

[Jean-François Lessard]

Replace direct container_of() calls with a to_linedisp() helper function
throughout the line-display auxdisplay library module. This abstraction
prepares for upcoming dual-mode support where linedisp context retrieval
will need to handle both dedicated child devices and attached parent
auxdisplay devices.

No functional changes in this patch.

Signed-off-by: Jean-François Lessard <jefflessard3@gmail.com>
---
 drivers/auxdisplay/line-display.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/drivers/auxdisplay/line-display.c b/drivers/auxdisplay/line-display.c
index 8590a4cd21e0..e44341b1ea22 100644
--- a/drivers/auxdisplay/line-display.c
+++ b/drivers/auxdisplay/line-display.c
@@ -31,6 +31,11 @@
 
 #define DEFAULT_SCROLL_RATE	(HZ / 2)
 
+static struct linedisp *to_linedisp(struct device *dev)
+{
+	return container_of(dev, struct linedisp, dev);
+}
+
 /**
  * linedisp_scroll() - scroll the display by a character
  * @t: really a pointer to the private data structure
@@ -133,7 +138,7 @@ static int linedisp_display(struct linedisp *linedisp, const char *msg,
 static ssize_t message_show(struct device *dev, struct device_attribute *attr,
 			    char *buf)
 {
-	struct linedisp *linedisp = container_of(dev, struct linedisp, dev);
+	struct linedisp *linedisp = to_linedisp(dev);
 
 	return sysfs_emit(buf, "%s\n", linedisp->message);
 }
@@ -152,7 +157,7 @@ static ssize_t message_show(struct device *dev, struct device_attribute *attr,
 static ssize_t message_store(struct device *dev, struct device_attribute *attr,
 			     const char *buf, size_t count)
 {
-	struct linedisp *linedisp = container_of(dev, struct linedisp, dev);
+	struct linedisp *linedisp = to_linedisp(dev);
 	int err;
 
 	err = linedisp_display(linedisp, buf, count);
@@ -164,7 +169,7 @@ static DEVICE_ATTR_RW(message);
 static ssize_t scroll_step_ms_show(struct device *dev,
 				   struct device_attribute *attr, char *buf)
 {
-	struct linedisp *linedisp = container_of(dev, struct linedisp, dev);
+	struct linedisp *linedisp = to_linedisp(dev);
 
 	return sysfs_emit(buf, "%u\n", jiffies_to_msecs(linedisp->scroll_rate));
 }
@@ -173,7 +178,7 @@ static ssize_t scroll_step_ms_store(struct device *dev,
 				    struct device_attribute *attr,
 				    const char *buf, size_t count)
 {
-	struct linedisp *linedisp = container_of(dev, struct linedisp, dev);
+	struct linedisp *linedisp = to_linedisp(dev);
 	unsigned int ms;
 	int err;
 
@@ -195,7 +200,7 @@ static DEVICE_ATTR_RW(scroll_step_ms);
 
 static ssize_t map_seg_show(struct device *dev, struct device_attribute *attr, char *buf)
 {
-	struct linedisp *linedisp = container_of(dev, struct linedisp, dev);
+	struct linedisp *linedisp = to_linedisp(dev);
 	struct linedisp_map *map = linedisp->map;
 
 	memcpy(buf, &map->map, map->size);
@@ -205,7 +210,7 @@ static ssize_t map_seg_show(struct device *dev, struct device_attribute *attr, c
 static ssize_t map_seg_store(struct device *dev, struct device_attribute *attr,
 			     const char *buf, size_t count)
 {
-	struct linedisp *linedisp = container_of(dev, struct linedisp, dev);
+	struct linedisp *linedisp = to_linedisp(dev);
 	struct linedisp_map *map = linedisp->map;
 
 	if (count != map->size)
@@ -232,7 +237,7 @@ static struct attribute *linedisp_attrs[] = {
 static umode_t linedisp_attr_is_visible(struct kobject *kobj, struct attribute *attr, int n)
 {
 	struct device *dev = kobj_to_dev(kobj);
-	struct linedisp *linedisp = container_of(dev, struct linedisp, dev);
+	struct linedisp *linedisp = to_linedisp(dev);
 	struct linedisp_map *map = linedisp->map;
 	umode_t mode = attr->mode;
 
@@ -263,7 +268,7 @@ static DEFINE_IDA(linedisp_id);
 
 static void linedisp_release(struct device *dev)
 {
-	struct linedisp *linedisp = container_of(dev, struct linedisp, dev);
+	struct linedisp *linedisp = to_linedisp(dev);
 
 	kfree(linedisp->map);
 	kfree(linedisp->message);
-- 
2.43.0

[PATCH v2 3/5] auxdisplay: linedisp: display static message when length <= display size

[Jean-François Lessard]

Currently, when a message shorter than the display size is written, the
content wraps around (e.g., "123" on a 4-digit display shows "1231")
without scrolling, which is confusing and unintuitive.

Change behavior to display short messages statically with space padding
(e.g. "123 ") while only scrolling messages longer than the display width.
This provides more natural behavior that aligns with user expectations
and current linedisp_display() kernel-doc.

The scroll logic is also consolidated into a helper function for clarity.

No API changes are introduced.

Signed-off-by: Jean-François Lessard <jefflessard3@gmail.com>
---
 drivers/auxdisplay/line-display.c | 30 +++++++++++++++++++++---------
 1 file changed, 21 insertions(+), 9 deletions(-)

diff --git a/drivers/auxdisplay/line-display.c b/drivers/auxdisplay/line-display.c
index e44341b1ea22..73e4e77ea4f9 100644
--- a/drivers/auxdisplay/line-display.c
+++ b/drivers/auxdisplay/line-display.c
@@ -36,6 +36,11 @@ static struct linedisp *to_linedisp(struct device *dev)
 	return container_of(dev, struct linedisp, dev);
 }
 
+static inline bool should_scroll(struct linedisp *linedisp)
+{
+	return linedisp->message_len > linedisp->num_chars && linedisp->scroll_rate;
+}
+
 /**
  * linedisp_scroll() - scroll the display by a character
  * @t: really a pointer to the private data structure
@@ -67,8 +72,7 @@ static void linedisp_scroll(struct timer_list *t)
 	linedisp->scroll_pos %= linedisp->message_len;
 
 	/* rearm the timer */
-	if (linedisp->message_len > num_chars && linedisp->scroll_rate)
-		mod_timer(&linedisp->timer, jiffies + linedisp->scroll_rate);
+	mod_timer(&linedisp->timer, jiffies + linedisp->scroll_rate);
 }
 
 /**
@@ -118,8 +122,16 @@ static int linedisp_display(struct linedisp *linedisp, const char *msg,
 	linedisp->message_len = count;
 	linedisp->scroll_pos = 0;
 
-	/* update the display */
-	linedisp_scroll(&linedisp->timer);
+	if (should_scroll(linedisp)) {
+		/* display scrolling message */
+		linedisp_scroll(&linedisp->timer);
+	} else {
+		/* display static message */
+		memset(linedisp->buf, ' ', linedisp->num_chars);
+		memcpy(linedisp->buf, linedisp->message,
+		       umin(linedisp->num_chars, linedisp->message_len));
+		linedisp->ops->update(linedisp);
+	}
 
 	return 0;
 }
@@ -186,12 +198,12 @@ static ssize_t scroll_step_ms_store(struct device *dev,
 	if (err)
 		return err;
 
+	timer_delete_sync(&linedisp->timer);
+
 	linedisp->scroll_rate = msecs_to_jiffies(ms);
-	if (linedisp->message && linedisp->message_len > linedisp->num_chars) {
-		timer_delete_sync(&linedisp->timer);
-		if (linedisp->scroll_rate)
-			linedisp_scroll(&linedisp->timer);
-	}
+
+	if (should_scroll(linedisp))
+		linedisp_scroll(&linedisp->timer);
 
 	return count;
 }
-- 
2.43.0

[PATCH v2 4/5] auxdisplay: linedisp: add num_chars sysfs attribute

[Jean-François Lessard]

Add a read-only 'num_chars' sysfs attribute to report display digit count.

The num_chars attribute provides essential capability information to
userspace applications that need to know display dimensions before writing
messages, complementing the existing message and scroll controls.

No functional changes to existing behavior.

Signed-off-by: Jean-François Lessard <jefflessard3@gmail.com>
---
 Documentation/ABI/testing/sysfs-auxdisplay-linedisp | 11 +++++++++++
 drivers/auxdisplay/line-display.c                   | 11 +++++++++++
 2 files changed, 22 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-auxdisplay-linedisp b/Documentation/ABI/testing/sysfs-auxdisplay-linedisp
index 45cf4e5a2feb..55f1b559e84e 100644
--- a/Documentation/ABI/testing/sysfs-auxdisplay-linedisp
+++ b/Documentation/ABI/testing/sysfs-auxdisplay-linedisp
@@ -15,6 +15,17 @@ Description:
 		  echo "Hello World" > message
 		  cat message			# Returns "Hello World\n"
 
+What:		/sys/.../num_chars
+Date:		November 2025
+KernelVersion:	6.18
+Contact:	Jean-François Lessard <jefflessard3@gmail.com>
+Description:
+		Read-only attribute showing the character width capacity of
+		the line display device. Messages longer than this will scroll.
+
+		Example:
+		  cat num_chars		# Returns "16\n" for 16-char display
+
 What:		/sys/.../scroll_step_ms
 Date:		October 2021
 KernelVersion:	5.16
diff --git a/drivers/auxdisplay/line-display.c b/drivers/auxdisplay/line-display.c
index 73e4e77ea4f9..7f3e53e2847b 100644
--- a/drivers/auxdisplay/line-display.c
+++ b/drivers/auxdisplay/line-display.c
@@ -178,6 +178,16 @@ static ssize_t message_store(struct device *dev, struct device_attribute *attr,
 
 static DEVICE_ATTR_RW(message);
 
+static ssize_t num_chars_show(struct device *dev, struct device_attribute *attr,
+			      char *buf)
+{
+	struct linedisp *linedisp = to_linedisp(dev);
+
+	return sysfs_emit(buf, "%u\n", linedisp->num_chars);
+}
+
+static DEVICE_ATTR_RO(num_chars);
+
 static ssize_t scroll_step_ms_show(struct device *dev,
 				   struct device_attribute *attr, char *buf)
 {
@@ -240,6 +250,7 @@ static DEVICE_ATTR(map_seg14, 0644, map_seg_show, map_seg_store);
 
 static struct attribute *linedisp_attrs[] = {
 	&dev_attr_message.attr,
+	&dev_attr_num_chars.attr,
 	&dev_attr_scroll_step_ms.attr,
 	&dev_attr_map_seg7.attr,
 	&dev_attr_map_seg14.attr,
-- 
2.43.0

[PATCH v2 5/5] auxdisplay: linedisp: support attribute attachment to auxdisplay devices

[Jean-François Lessard]

Enable linedisp library integration into existing kernel devices (like LED
class) to provide a uniform 7-segment userspace API without creating
separate child devices, meeting the consistent interface while maintaining
coherent device hierarchies.

This allows uniform 7-segment API across all drivers while solving device
proliferation and fragmented userspace interfaces.

The sysfs attributes appear in one of the two locations depending on usage:
  1. On linedisp.N child devices (legacy linedisp_register())
  2. On the parent auxdisplay device (new linedisp_attach())
Functionality is identical in both modes.

Existing consumers of linedisp_register() are unaffected. The new API
enables drivers like TM16XX to integrate 7-segment display functionality
seamlessly within their LED class device hierarchy.

Signed-off-by: Jean-François Lessard <jefflessard3@gmail.com>
---

Notes:
    The attachment list operations are extremely lightweight (just
    adding/removing list entries), making spinlock the optimal choice because:
    - Very short critical sections: Only list traversal and pointer
      assignments; avoids context switching overhead for brief operations
    - No sleeping: No memory allocation or I/O within locked sections

 drivers/auxdisplay/line-display.c | 180 +++++++++++++++++++++++++++++-
 drivers/auxdisplay/line-display.h |   4 +
 2 files changed, 178 insertions(+), 6 deletions(-)

diff --git a/drivers/auxdisplay/line-display.c b/drivers/auxdisplay/line-display.c
index 7f3e53e2847b..4e22373fcc1a 100644
--- a/drivers/auxdisplay/line-display.c
+++ b/drivers/auxdisplay/line-display.c
@@ -6,20 +6,23 @@
  * Author: Paul Burton <paul.burton@mips.com>
  *
  * Copyright (C) 2021 Glider bv
+ * Copyright (C) 2025 Jean-François Lessard
  */
 
 #ifndef CONFIG_PANEL_BOOT_MESSAGE
 #include <generated/utsrelease.h>
 #endif
 
-#include <linux/container_of.h>
+#include <linux/cleanup.h>
 #include <linux/device.h>
 #include <linux/export.h>
 #include <linux/idr.h>
 #include <linux/jiffies.h>
 #include <linux/kstrtox.h>
+#include <linux/list.h>
 #include <linux/module.h>
 #include <linux/slab.h>
+#include <linux/spinlock.h>
 #include <linux/string.h>
 #include <linux/sysfs.h>
 #include <linux/timer.h>
@@ -31,9 +34,80 @@
 
 #define DEFAULT_SCROLL_RATE	(HZ / 2)
 
+/**
+ * struct linedisp_attachment - Holds the device to linedisp mapping
+ * @list: List entry for the linedisp_attachments list
+ * @device: Pointer to the device where linedisp attributes are added
+ * @linedisp: Pointer to the linedisp mapped to the device
+ * @direct: true for directly attached device using linedisp_attach(),
+ *	    false for child registered device using linedisp_register()
+ */
+struct linedisp_attachment {
+	struct list_head list;
+	struct device *device;
+	struct linedisp *linedisp;
+	bool direct;
+};
+
+static LIST_HEAD(linedisp_attachments);
+static DEFINE_SPINLOCK(linedisp_attachments_lock);
+
+static int create_attachment(struct device *dev, struct linedisp *linedisp, bool direct)
+{
+	struct linedisp_attachment *attachment;
+
+	attachment = kzalloc(sizeof(*attachment), GFP_KERNEL);
+	if (!attachment)
+		return -ENOMEM;
+
+	attachment->device = dev;
+	attachment->linedisp = linedisp;
+	attachment->direct = direct;
+
+	guard(spinlock)(&linedisp_attachments_lock);
+	list_add(&attachment->list, &linedisp_attachments);
+
+	return 0;
+}
+
+static struct linedisp *delete_attachment(struct device *dev, bool direct)
+{
+	struct linedisp_attachment *attachment;
+	struct linedisp *linedisp;
+
+	guard(spinlock)(&linedisp_attachments_lock);
+
+	list_for_each_entry(attachment, &linedisp_attachments, list) {
+		if (attachment->device == dev &&
+		    attachment->direct == direct)
+			break;
+	}
+
+	if (list_entry_is_head(attachment, &linedisp_attachments, list))
+		return NULL;
+
+	linedisp = attachment->linedisp;
+	list_del(&attachment->list);
+	kfree(attachment);
+
+	return linedisp;
+}
+
 static struct linedisp *to_linedisp(struct device *dev)
 {
-	return container_of(dev, struct linedisp, dev);
+	struct linedisp_attachment *attachment;
+
+	guard(spinlock)(&linedisp_attachments_lock);
+
+	list_for_each_entry(attachment, &linedisp_attachments, list) {
+		if (attachment->device == dev)
+			break;
+	}
+
+	if (list_entry_is_head(attachment, &linedisp_attachments, list))
+		return NULL;
+
+	return attachment->linedisp;
 }
 
 static inline bool should_scroll(struct linedisp *linedisp)
@@ -348,6 +422,90 @@ static int linedisp_init_map(struct linedisp *linedisp)
 #define LINEDISP_INIT_TEXT "Linux " UTS_RELEASE "       "
 #endif
 
+/**
+ * linedisp_attach - attach a character line display
+ * @linedisp: pointer to character line display structure
+ * @dev: pointer of the device to attach to
+ * @num_chars: the number of characters that can be displayed
+ * @ops: character line display operations
+ *
+ * Directly attach the line-display sysfs attributes to the passed device.
+ * The caller is responsible for calling linedisp_detach() to release resources
+ * after use.
+ *
+ * Return: zero on success, else a negative error code.
+ */
+int linedisp_attach(struct linedisp *linedisp, struct device *dev,
+		    unsigned int num_chars, const struct linedisp_ops *ops)
+{
+	int err;
+
+	memset(linedisp, 0, sizeof(*linedisp));
+	linedisp->ops = ops;
+	linedisp->num_chars = num_chars;
+	linedisp->scroll_rate = DEFAULT_SCROLL_RATE;
+
+	linedisp->buf = kzalloc(linedisp->num_chars, GFP_KERNEL);
+	if (!linedisp->buf)
+		return -ENOMEM;
+
+	/* initialise a character mapping, if required */
+	err = linedisp_init_map(linedisp);
+	if (err)
+		goto out_free_buf;
+
+	/* initialise a timer for scrolling the message */
+	timer_setup(&linedisp->timer, linedisp_scroll, 0);
+
+	err = create_attachment(dev, linedisp, true);
+	if (err)
+		goto out_del_timer;
+
+	/* display a default message */
+	err = linedisp_display(linedisp, LINEDISP_INIT_TEXT, -1);
+	if (err)
+		goto out_del_attach;
+
+	/* add attribute groups to target device */
+	err = device_add_groups(dev, linedisp_groups);
+	if (err)
+		goto out_del_attach;
+
+	return 0;
+
+out_del_attach:
+	delete_attachment(dev, true);
+out_del_timer:
+	timer_delete_sync(&linedisp->timer);
+out_free_buf:
+	kfree(linedisp->buf);
+	return err;
+}
+EXPORT_SYMBOL_NS_GPL(linedisp_attach, "LINEDISP");
+
+/**
+ * linedisp_detach - detach a character line display
+ * @dev: pointer of the device to detach from, that was previously
+ *	 attached with linedisp_attach()
+ */
+void linedisp_detach(struct device *dev)
+{
+	struct linedisp *linedisp;
+
+	linedisp = delete_attachment(dev, true);
+	if (!linedisp)
+		return;
+
+	timer_delete_sync(&linedisp->timer);
+
+	device_remove_groups(dev, linedisp_groups);
+
+	kfree(linedisp->map);
+	kfree(linedisp->message);
+	kfree(linedisp->buf);
+}
+EXPORT_SYMBOL_NS_GPL(linedisp_detach, "LINEDISP");
+
 /**
  * linedisp_register - register a character line display
  * @linedisp: pointer to character line display structure
@@ -355,6 +513,11 @@ static int linedisp_init_map(struct linedisp *linedisp)
  * @num_chars: the number of characters that can be displayed
  * @ops: character line display operations
  *
+ * Register the line-display sysfs attributes to a new device named
+ * "linedisp.N" added to the passed parent device.
+ * The caller is responsible for calling linedisp_unregister() to release
+ * resources after use.
+ *
  * Return: zero on success, else a negative error code.
  */
 int linedisp_register(struct linedisp *linedisp, struct device *parent,
@@ -390,19 +553,23 @@ int linedisp_register(struct linedisp *linedisp, struct device *parent,
 	/* initialise a timer for scrolling the message */
 	timer_setup(&linedisp->timer, linedisp_scroll, 0);
 
-	err = device_add(&linedisp->dev);
+	err = create_attachment(&linedisp->dev, linedisp, false);
 	if (err)
 		goto out_del_timer;
 
 	/* display a default message */
 	err = linedisp_display(linedisp, LINEDISP_INIT_TEXT, -1);
 	if (err)
-		goto out_del_dev;
+		goto out_del_attach;
+
+	err = device_add(&linedisp->dev);
+	if (err)
+		goto out_del_attach;
 
 	return 0;
 
-out_del_dev:
-	device_del(&linedisp->dev);
+out_del_attach:
+	delete_attachment(&linedisp->dev, false);
 out_del_timer:
 	timer_delete_sync(&linedisp->timer);
 out_put_device:
@@ -419,6 +586,7 @@ EXPORT_SYMBOL_NS_GPL(linedisp_register, "LINEDISP");
 void linedisp_unregister(struct linedisp *linedisp)
 {
 	device_del(&linedisp->dev);
+	delete_attachment(&linedisp->dev, false);
 	timer_delete_sync(&linedisp->timer);
 	put_device(&linedisp->dev);
 }
diff --git a/drivers/auxdisplay/line-display.h b/drivers/auxdisplay/line-display.h
index 4348d7a2f69a..36853b639711 100644
--- a/drivers/auxdisplay/line-display.h
+++ b/drivers/auxdisplay/line-display.h
@@ -6,6 +6,7 @@
  * Author: Paul Burton <paul.burton@mips.com>
  *
  * Copyright (C) 2021 Glider bv
+ * Copyright (C) 2025 Jean-François Lessard
  */
 
 #ifndef _LINEDISP_H
@@ -81,6 +82,9 @@ struct linedisp {
 	unsigned int id;
 };
 
+int linedisp_attach(struct linedisp *linedisp, struct device *dev,
+		    unsigned int num_chars, const struct linedisp_ops *ops);
+void linedisp_detach(struct device *dev);
 int linedisp_register(struct linedisp *linedisp, struct device *parent,
 		      unsigned int num_chars, const struct linedisp_ops *ops);
 void linedisp_unregister(struct linedisp *linedisp);
-- 
2.43.0

Re: [PATCH v2 0/5] auxdisplay: linedisp: support attribute attachment to auxdisplay devices

[Jean-François Lessard]

Le 18 septembre 2025 08 h 13 min 10 s HAE, "Jean-François Lessard" <jefflessard3@gmail.com> a écrit :
>This series modernizes the auxdisplay line display (linedisp) library to
>enable seamless integration with auxdisplay parent devices while
>maintaining backward compatibility.
>
>The key improvement is adding attach/detach APIs that allow linedisp sysfs
>attributes to be bound directly to their parent auxdisplay devices avoiding
>child device proliferation and enabling a uniform 7-segment userspace
>interface across different driver architectures.
>
>This series introduces attachment infrastructure for linedisp devices.
>The first consumer of this API will be the TM16XX driver series.
>See the related patch series:
>  auxdisplay: Add TM16xx 7-segment LED matrix display controllers driver
>
>Changes include:
>1. Encapsulate container_of() usage with to_linedisp() helper function for
>   cleaner context retrieval
>2. Improve message display behavior with static padding when message length
>   is smaller than display width
>3. Add 'num_chars' read-only attribute for userspace capability discovery
>4. Add attach/detach API for sysfs attributes binding to parent devices
>5. Document all linedisp sysfs attributes in ABI documentation
>
>All existing linedisp_register() users remain unaffected. The new APIs
>enable drivers like TM16XX to integrate 7-segment functionality within
>their LED class device hierarchy while providing a uniform 7-segment API.
>
>Thanks to Andy Shevchenko for early feedback and guidance.
>
>V1 changelog:
>- Remove should_scroll() check in linedisp_scroll()
>- Keep attributes sorted alphabetically
>- Document, invert logic and rename owns_device to direct
>- Document linedisp_attach()/_register() must be freed by their
>  respective APIs after use
>- Change call order of display default message and add attributes
>  to prevent racy userspace condition
>- Separate delete_attachment() call from linedisp variable declaration
>- Document existing attributes separately, in the first patch
>
>RFC changelog:
>- Replace scope_guard() with guard()() for synchronized list operations.
>- Replace NULL assignments with proper list_entry_is_head() pattern.
>- Clearly document why introducing the attach/detach APIs.
>- Split in patch series, each patch containing a specific change.
>- Implement static (non-scrolling) display for short messages.
>- Document exisiting and new ABI sysfs attributes.
>
>Jean-François Lessard (5):
>  docs: ABI: auxdisplay: document linedisp library sysfs attributes
>  auxdisplay: linedisp: encapsulate container_of usage within
>    to_linedisp
>  auxdisplay: linedisp: display static message when length <= display
>    size
>  auxdisplay: linedisp: add num_chars sysfs attribute
>  auxdisplay: linedisp: support attribute attachment to auxdisplay
>    devices
>
> .../ABI/testing/sysfs-auxdisplay-linedisp     |  90 +++++++
> drivers/auxdisplay/line-display.c             | 240 ++++++++++++++++--
> drivers/auxdisplay/line-display.h             |   4 +
> 3 files changed, 312 insertions(+), 22 deletions(-)
> create mode 100644 Documentation/ABI/testing/sysfs-auxdisplay-linedisp
>

Hi Andy and Geert,

This is a gentle ping on the v2 line-display enhancement patch series submitted
over a week ago. I have just submitted v5 of the TM16xx driver series:
https://lore.kernel.org/linux-devicetree/20250926141913.25919-1-jefflessard3@gmail.com/T/

The TM16xx driver depends on the line-display enhancements for proper
auxdisplay subsystem integration, as discussed in our previous exchanges. It
would be great if we could move forward with both patch series together.

Thanks in advance for your time and consideration.

Best regards,
Jean-François Lessard

Re: [PATCH v2 0/5] auxdisplay: linedisp: support attribute attachment to auxdisplay devices

[Jean-François Lessard]

Hi Geert,

I hope this message finds you well. I wanted to follow up on this v2
line-display enhancement patch series submitted three weeks ago.

Le 26 septembre 2025 10 h 31 min 16 s HAE, "Jean-François Lessard" <jefflessard3@gmail.com> a écrit :
>Le 18 septembre 2025 08 h 13 min 10 s HAE, "Jean-François Lessard" <jefflessard3@gmail.com> a écrit :
>>This series modernizes the auxdisplay line display (linedisp) library to
>>enable seamless integration with auxdisplay parent devices while
>>maintaining backward compatibility.
>>
>>The key improvement is adding attach/detach APIs that allow linedisp sysfs
>>attributes to be bound directly to their parent auxdisplay devices avoiding
>>child device proliferation and enabling a uniform 7-segment userspace
>>interface across different driver architectures.
>>
>>This series introduces attachment infrastructure for linedisp devices.
>>The first consumer of this API will be the TM16XX driver series.
>>See the related patch series:
>>  auxdisplay: Add TM16xx 7-segment LED matrix display controllers driver
>>
>>Changes include:
>>1. Encapsulate container_of() usage with to_linedisp() helper function for
>>   cleaner context retrieval
>>2. Improve message display behavior with static padding when message length
>>   is smaller than display width
>>3. Add 'num_chars' read-only attribute for userspace capability discovery
>>4. Add attach/detach API for sysfs attributes binding to parent devices
>>5. Document all linedisp sysfs attributes in ABI documentation
>>
>>All existing linedisp_register() users remain unaffected. The new APIs
>>enable drivers like TM16XX to integrate 7-segment functionality within
>>their LED class device hierarchy while providing a uniform 7-segment API.
>>
>>Thanks to Andy Shevchenko for early feedback and guidance.
>>
>>V1 changelog:
>>- Remove should_scroll() check in linedisp_scroll()
>>- Keep attributes sorted alphabetically
>>- Document, invert logic and rename owns_device to direct
>>- Document linedisp_attach()/_register() must be freed by their
>>  respective APIs after use
>>- Change call order of display default message and add attributes
>>  to prevent racy userspace condition
>>- Separate delete_attachment() call from linedisp variable declaration
>>- Document existing attributes separately, in the first patch
>>
>>RFC changelog:
>>- Replace scope_guard() with guard()() for synchronized list operations.
>>- Replace NULL assignments with proper list_entry_is_head() pattern.
>>- Clearly document why introducing the attach/detach APIs.
>>- Split in patch series, each patch containing a specific change.
>>- Implement static (non-scrolling) display for short messages.
>>- Document exisiting and new ABI sysfs attributes.
>>
>>Jean-François Lessard (5):
>>  docs: ABI: auxdisplay: document linedisp library sysfs attributes
>>  auxdisplay: linedisp: encapsulate container_of usage within
>>    to_linedisp
>>  auxdisplay: linedisp: display static message when length <= display
>>    size
>>  auxdisplay: linedisp: add num_chars sysfs attribute
>>  auxdisplay: linedisp: support attribute attachment to auxdisplay
>>    devices
>>
>> .../ABI/testing/sysfs-auxdisplay-linedisp     |  90 +++++++
>> drivers/auxdisplay/line-display.c             | 240 ++++++++++++++++--
>> drivers/auxdisplay/line-display.h             |   4 +
>> 3 files changed, 312 insertions(+), 22 deletions(-)
>> create mode 100644 Documentation/ABI/testing/sysfs-auxdisplay-linedisp
>>
>
>Hi Andy and Geert,
>
>This is a gentle ping on the v2 line-display enhancement patch series submitted
>over a week ago. I have just submitted v5 of the TM16xx driver series:
>https://lore.kernel.org/linux-devicetree/20250926141913.25919-1-jefflessard3@gmail.com/T/
>
>The TM16xx driver depends on the line-display enhancements for proper
>auxdisplay subsystem integration, as discussed in our previous exchanges. It
>would be great if we could move forward with both patch series together.
>

In the v1 submission, Andy mentioned that the changes look good to him but that
he would wait for your review/Acks before proceeding. The TM16xx driver (v5
recently submitted) depends on these line-display enhancements for proper
auxdisplay subsystem integration.

Would you be able to review the v2 series when you have a chance? Your feedback
would be greatly appreciated so we can move both series forward.

Thank you for your time and consideration.

Best regards,
Jean-François Lessard

Re: [PATCH v2 0/5] auxdisplay: linedisp: support attribute attachment to auxdisplay devices

[Andy Shevchenko]

On Thu, Oct 09, 2025 at 07:19:11AM -0400, Jean-François Lessard wrote:
> Hi Geert,
> 
> I hope this message finds you well. I wanted to follow up on this v2
> line-display enhancement patch series submitted three weeks ago.

Taking no answer as silent agreement, pushed to my review and testing queue, thanks!

If anything, we have time to address / drop / etc.

> Le 26 septembre 2025 10 h 31 min 16 s HAE, "Jean-François Lessard" <jefflessard3@gmail.com> a écrit :
> >Le 18 septembre 2025 08 h 13 min 10 s HAE, "Jean-François Lessard" <jefflessard3@gmail.com> a écrit :
> >>This series modernizes the auxdisplay line display (linedisp) library to
> >>enable seamless integration with auxdisplay parent devices while
> >>maintaining backward compatibility.
> >>
> >>The key improvement is adding attach/detach APIs that allow linedisp sysfs
> >>attributes to be bound directly to their parent auxdisplay devices avoiding
> >>child device proliferation and enabling a uniform 7-segment userspace
> >>interface across different driver architectures.
> >>
> >>This series introduces attachment infrastructure for linedisp devices.
> >>The first consumer of this API will be the TM16XX driver series.
> >>See the related patch series:
> >>  auxdisplay: Add TM16xx 7-segment LED matrix display controllers driver
> >>
> >>Changes include:
> >>1. Encapsulate container_of() usage with to_linedisp() helper function for
> >>   cleaner context retrieval
> >>2. Improve message display behavior with static padding when message length
> >>   is smaller than display width
> >>3. Add 'num_chars' read-only attribute for userspace capability discovery
> >>4. Add attach/detach API for sysfs attributes binding to parent devices
> >>5. Document all linedisp sysfs attributes in ABI documentation
> >>
> >>All existing linedisp_register() users remain unaffected. The new APIs
> >>enable drivers like TM16XX to integrate 7-segment functionality within
> >>their LED class device hierarchy while providing a uniform 7-segment API.
> >>
> >>Thanks to Andy Shevchenko for early feedback and guidance.
> >>
> >>V1 changelog:
> >>- Remove should_scroll() check in linedisp_scroll()
> >>- Keep attributes sorted alphabetically
> >>- Document, invert logic and rename owns_device to direct
> >>- Document linedisp_attach()/_register() must be freed by their
> >>  respective APIs after use
> >>- Change call order of display default message and add attributes
> >>  to prevent racy userspace condition
> >>- Separate delete_attachment() call from linedisp variable declaration
> >>- Document existing attributes separately, in the first patch
> >>
> >>RFC changelog:
> >>- Replace scope_guard() with guard()() for synchronized list operations.
> >>- Replace NULL assignments with proper list_entry_is_head() pattern.
> >>- Clearly document why introducing the attach/detach APIs.
> >>- Split in patch series, each patch containing a specific change.
> >>- Implement static (non-scrolling) display for short messages.
> >>- Document exisiting and new ABI sysfs attributes.

> >This is a gentle ping on the v2 line-display enhancement patch series submitted
> >over a week ago. I have just submitted v5 of the TM16xx driver series:
> >https://lore.kernel.org/linux-devicetree/20250926141913.25919-1-jefflessard3@gmail.com/T/
> >
> >The TM16xx driver depends on the line-display enhancements for proper
> >auxdisplay subsystem integration, as discussed in our previous exchanges. It
> >would be great if we could move forward with both patch series together.
> >
> 
> In the v1 submission, Andy mentioned that the changes look good to him but that
> he would wait for your review/Acks before proceeding. The TM16xx driver (v5
> recently submitted) depends on these line-display enhancements for proper
> auxdisplay subsystem integration.
> 
> Would you be able to review the v2 series when you have a chance? Your feedback
> would be greatly appreciated so we can move both series forward.
> 
> Thank you for your time and consideration.

-- 
With Best Regards,
Andy Shevchenko