psm(4): Disabled IMEX side-buttons by default.

[b-aaz]

This is a workaround.
At least two hypervisors (QEMU and VBox) are detected to be using this mouse model by default and they send superfluous side button key events when using the scroll wheel.

Added a runtime rw sysctl that disables the side buttons on this specific model to workaround this issue.

This issue was also mentioned by users in discussions.
Links to some of other mentions of this issue I have found:
https://forums.freebsd.org/threads/gnome3-weird-scroll-wheel-behaviour.78532/
https://www.reddit.com/r/freebsd/comments/o8k71l/anyone_else_having_weird_behaviour_with_scrolling/

Summary by Sourcery

Disable Intellimouse Explorer side buttons by default via a runtime sysctl to avoid spurious button events from this mouse model.

Bug Fixes:

• Work around hypervisors sending spurious Intellimouse Explorer side button events when using the scroll wheel.

Enhancements:

• Add a runtime rw sysctl to allow enabling Intellimouse Explorer side buttons when desired.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Adds a tunable/sysctl to control Intellimouse Explorer side-button handling and defaults it to disabled to work around spurious side-button events from some hypervisors, altering the packet-processing path for that mouse model.

Sequence diagram for runtime control of explorer_sidebuttons via sysctl

sequenceDiagram
  actor Admin
  participant SysctlInterface
  participant Kernel_psm as Kernel_psm

  Admin->>SysctlInterface: sysctl hw.psm.explorer_sidebuttons=1
  SysctlInterface->>Kernel_psm: update explorer_sidebuttons
  Admin->>Kernel_psm: mouse input arrives
  Kernel_psm->>Kernel_psm: psmsoftintr processes Explorer packet
  Kernel_psm-->>Admin: side button events generated (if explorer_sidebuttons != 0)

Loading

File-Level Changes

Change	Details	Files
Introduce a runtime sysctl to enable/disable Intellimouse Explorer side buttons and gate side-button event processing on it, defaulting to disabled.	Add a new global flag variable for Intellimouse Explorer side-button support, initialized to disabled Expose the flag as a writable hw.psm.explorer_sidebuttons sysctl so behavior can be toggled at runtime Short-circuit the Intellimouse Explorer packet parsing path so that side-button events are not generated when the sysctl is disabled	sys/dev/atkbdc/psm.c

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[github-actions]

Thank you for taking the time to contribute to FreeBSD!
There is an issue that needs to be fixed:

• Real email address is needed^18d8a1f

Please review CONTRIBUTING.md, then update and push your branch again.

[sourcery-ai]

Hey - I've left some high level feedback:

• To match nearby sysctl usage and keep this tunable controllable both at boot and at runtime, consider using CTLFLAG_RWTUN for explorer_sidebuttons instead of plain CTLFLAG_RW.
• Align the if (!explorer_sidebuttons) brace style with the file’s KNF convention (opening brace on the same line as the if) for consistency with surrounding code.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- To match nearby sysctl usage and keep this tunable controllable both at boot and at runtime, consider using CTLFLAG_RWTUN for `explorer_sidebuttons` instead of plain CTLFLAG_RW.
- Align the `if (!explorer_sidebuttons)` brace style with the file’s KNF convention (opening brace on the same line as the `if`) for consistency with surrounding code.

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[ericbsd]

This will need to be tested before back-porting to releng/15.1.