On the first real machine with a discrete card (an RTX 3060, 800x600 from
the UEFI GOP) the console was slow enough to photograph mid-scroll, and the
boot died on CPU 1's "cpu1: starting": CPU 0 spun 200M times on the console
lock while CPU 1 held it, scrolling, and called it a deadlock.
The framebuffer is device memory in the uncached 3-4 GB page of the direct
map. QEMU's stdvga is RAM underneath, so nothing showed there; on a card,
gfx_scroll_up's memmove read the whole screen back across the bus one byte
at a time, and every anti-aliased glyph read back each pixel it blended.
So the console now draws into a copy of the screen in RAM (vmap_alloc: kalloc
pages behind a page table of their own, since nothing needs it contiguous),
and gfx_flush() copies the changed rectangle onto the card through an
ioremap_wc() mapping -- PAT entry 4 set to write-combining on every CPU.
Flushes happen where drawing ends: a message, a terminal write, an echo, a
terminal switch. Scrolling moves eight bytes at a time instead of one.
/dev/fb0 reads and writes the card itself, as before. The early console's
fb_info now carries the physical address it always should have.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4
60d06658f0