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

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Message-ID: <20250918121321.116248-1-jefflessard3@gmail.com>
Date: Thu, 18 Sep 2025 08:13:10 -0400
From: Jean-François Lessard <jefflessard3@...il.com>
To: Andy Shevchenko <andy@...nel.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: linux-kernel@...r.kernel.org
Subject: [PATCH v2 0/5] auxdisplay: linedisp: support attribute attachment to auxdisplay devices

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