Merge patch series "auxdisplay: linedisp: support attribute attachment to auxdisplay devices"

Jean-François Lessard <jefflessard3@gmail.com> says:

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.

Link: https://lore.kernel.org/r/20250918121321.116248-1-jefflessard3@gmail.com
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

Author: andy-shev

Documentation/ABI/testing/sysfs-auxdisplay-linedisp

@@ -0,0 +1,90 @@
+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/.../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
+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