Commit 1cdaf601 authored by Jean-François Lessard's avatar Jean-François Lessard Committed by Andy Shevchenko
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docs: ABI: auxdisplay: document linedisp library sysfs attributes 



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: default avatarJean-François Lessard <jefflessard3@gmail.com>
Signed-off-by: default avatarAndy Shevchenko <andriy.shevchenko@linux.intel.com>
parent 86cc796e

Documentation/ABI/testing/sysfs-auxdisplay-linedisp

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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