Fix rendering and VT management issues in fbdev

[jserv]

This fixes critical bugs in the Linux framebuffer backend:

1. BPP crash prevention

• Return false instead of break for unsupported bits per pixel
• Prevents array out-of-bounds access in put_span table

2. Damage callback logic

• Fix inverted condition that prevented screen updates
• !tx->vt_active && MAP_FAILED → tx->vt_active && != MAP_FAILED
• Enables all rendering when VT is active

3. VT restoration in error path

• Restore VT mode before closing fd on init failure
• Prevents blank TTY after failed initialization

4. Poll function registration (lines 350-353, 376)

• Add twin_fbdev_poll() stub for dispatch compatibility

Tested-on: lima ARM64 Linux (virtio_gpudrmfb 1280x800@32bpp)

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Summary by cubic

Fixes fbdev rendering and VT handling to prevent crashes and blank TTYs. Ensures screen updates when the VT is active and registers a no-op poll for dispatcher compatibility.

• Bug Fixes
  • Return false for unsupported bpp to avoid out-of-bounds writes.
  • Fix damage check to update only when VT is active and framebuffer is mapped.
  • Restore VT mode on init failure and on exit to prevent a blank TTY.
  • Add and register twin_fbdev_poll() to keep the main loop running.

[cubic-dev-ai]

1 issue found across 1 file

Prompt for AI agents (all 1 issues)

Understand the root cause of the following 1 issues and fix them.


<file name="backend/fbdev.c">

<violation number="1" location="backend/fbdev.c:192">
This condition is still inverted: tx-&gt;vt_active becomes true when the VT is inactive (see linux_vt.h), so the draw loop never runs and the screen stays stale.</violation>
</file>

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