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e25063466d
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> Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
91 lines
3.0 KiB
Plaintext
91 lines
3.0 KiB
Plaintext
What: /sys/.../message
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Date: October 2021
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KernelVersion: 5.16
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Description:
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Controls the text message displayed on character line displays.
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Reading returns the current message with a trailing newline.
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Writing updates the displayed message. Messages longer than the
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display width will automatically scroll. Trailing newlines in
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input are automatically trimmed.
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Writing an empty string clears the display.
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Example:
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echo "Hello World" > message
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cat message # Returns "Hello World\n"
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What: /sys/.../num_chars
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Date: November 2025
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KernelVersion: 6.18
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Contact: Jean-François Lessard <jefflessard3@gmail.com>
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Description:
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Read-only attribute showing the character width capacity of
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the line display device. Messages longer than this will scroll.
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Example:
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cat num_chars # Returns "16\n" for 16-char display
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What: /sys/.../scroll_step_ms
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Date: October 2021
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KernelVersion: 5.16
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Description:
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Controls the scrolling speed for messages longer than the display
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width, specified in milliseconds per scroll step.
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Setting to 0 disables scrolling. Default is 500ms.
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Example:
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echo "250" > scroll_step_ms # 4Hz scrolling
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cat scroll_step_ms # Returns "250\n"
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What: /sys/.../map_seg7
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Date: January 2024
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KernelVersion: 6.9
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Description:
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Read/write binary blob representing the ASCII-to-7-segment
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display conversion table used by the linedisp driver, as defined
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by struct seg7_conversion_map in <linux/map_to_7segment.h>.
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Only visible on displays with 7-segment capability.
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This attribute is not human-readable. Writes must match the
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struct size exactly, else -EINVAL is returned; reads return the
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entire mapping as a binary blob.
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This interface and its implementation match existing conventions
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used in segment-mapped display drivers since 2005.
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ABI note: This style of binary sysfs attribute *is an exception*
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to current "one value per file, text only" sysfs rules, for
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historical compatibility and driver uniformity. New drivers are
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discouraged from introducing additional binary sysfs ABIs.
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Reference interface guidance:
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- include/uapi/linux/map_to_7segment.h
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What: /sys/.../map_seg14
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Date: January 2024
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KernelVersion: 6.9
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Description:
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Read/write binary blob representing the ASCII-to-14-segment
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display conversion table used by the linedisp driver, as defined
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by struct seg14_conversion_map in <linux/map_to_14segment.h>.
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Only visible on displays with 14-segment capability.
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This attribute is not human-readable. Writes must match the
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struct size exactly, else -EINVAL is returned; reads return the
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entire mapping as a binary blob.
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This interface and its implementation match existing conventions
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used by segment-mapped display drivers since 2005.
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ABI note: This style of binary sysfs attribute *is an exception*
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to current "one value per file, text only" sysfs rules, for
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historical compatibility and driver uniformity. New drivers are
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discouraged from introducing additional binary sysfs ABIs.
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Reference interface guidance:
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- include/uapi/linux/map_to_14segment.h
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