[FlareCoding]

Summary

gfxfb learns when an output (what shows one monitor's picture) lights, goes dark or relights at another mode, tells every open file of /dev/gfxfb through an output event (a record read returns naming the output), and moves the primary, the role of the output a one-screen program draws on, off an output that goes dark.

Changes

• Let a display report an output's change: gfxfb::output_changed() reads the output again, records a lit output's base buffer (the buffer it was lit with, which offset 0 maps on the primary output) and queues a GFXFB_EVENT_OUTPUT event naming the output in every open file, and output_state.relighting marks an output taken dark to be lit again at another mode. A failed display's reports change nothing, and neither the boot display nor virtio-gpu reports a change.
• Give each file at most one output event per output: queue_output_event() puts a newer output event at the tail of the file's queue in place of the one the file holds for that output, so the queue's 2 x GFXFB_OUTPUTS_MAX records always suffice and, since a display ends an output's present before reporting it dark, a file reads that present's GFXFB_PRESENT_DARK end before the output's event. The event wakes read and poll, and only an unread present event, never an output event, fails the file's next present on that output with EBUSY.
• Keep a primary output that one screen can draw on: the primary stays on an output that relights, moves from an output that goes dark to the lowest output with a recorded base buffer, which gets an output event too, and with no primary output goes to the first output lit with one. While there is no primary output or it relights, GFXFB_GET_INFO and GFXFB_FLUSH fail with EAGAIN and mmap at offset 0 is refused, each reading the primary under the node's outputs lock.
• Hand back what a dark output held: a dark output's base buffer and the buffer it showed go back to the display once no handle or mapping holds them, the stale mark a skipped flush leaves (which makes the base buffer's next present copy it whole) clears, and the output loses its cursor until the next picture, as gfxfb.h's display contract and gfxfb_abi.h's cursor paragraph state. A lit output keeps showing a buffer a program created, since its hardware still reads it.
• Keep stlxdm's present ends apart from output events: flip_presenter::take_end() returns at an event that is not a present event, leaving end_fd() set, so an output event never reads as a present that latched.
• Tests: on the fake display, an output event reaching every file behind the presenter's GFXFB_PRESENT_DARK end, a blocking read woken by a change, a close kept waiting through an output change, a queue holding a present event and an output event for each of 16 outputs, a replaced output event keeping a wrapped queue in order, the primary staying on a relighting output and moving off a dark one from output 1 to output 0 and back and to the lowest of several lit outputs, a lit output without a recorded base buffer counted as dark, a dark output's buffers handed back, a program's buffer shown before a lit report kept, a relit output's stale mark cleared and its cursor hidden, the stale mark kept through reports that keep the base buffer, a failed display's reports ignored, and reports for another display or a missing output dropped.
• Boots: in QEMU on x86_64 and aarch64, against pr/stlxdm-hw-cursor, the desktop came up on virtio-gpu and on the firmware's framebuffer with no faults or forbidden log lines. No boot reports an output change, so the boots show only that the paths the base runs are unchanged.

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Note

Medium Risk
Changes primary-output selection, buffer lifetime, and client event queues in the kernel graphics path; behavior is heavily tested but any future driver calling output_changed() must follow the new ordering and relighting contract.

Overview
Adds runtime hotplug-style output notifications for /dev/gfxfb: displays call new gfxfb::output_changed(), gfxfb re-reads the output, updates per-output base/shown buffer records, moves or retains the primary output (including relighting via output_state.relighting), and delivers GFXFB_EVENT_OUTPUT to every open client—replacing any prior output event for that output and waking read/poll.

Primary drawing path no longer uses a fixed m_primary_record; GFXFB_GET_INFO, GFXFB_FLUSH, and mmap offset 0 go through primary_base() and return EAGAIN while there is no primary or the primary is relighting. Present-event tracking is tightened so only GFXFB_EVENT_PRESENT blocks the next present on an output.

stlxdm ignores non-present events in take_end() so output events do not look like latched presents. virtio-gpu and boot display only set relighting = false (no reports yet). ABI/kernel docs and a large gfxfb test suite cover primary moves, buffer drops on dark, failed-display no-ops, and event ordering.

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