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gfx: draw the console in RAM, and write it to the card write-combined

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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
This commit is contained in:
Nexoniarz 2026-10-22 23:46:58 +02:00
commit 60d06658f0
9 changed files with 347 additions and 8 deletions
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7
include/gfx.h
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@ -74,6 +74,13 @@ void gfx_putc(int c, uint32 color);
void gfx_puts(int x, int y, char *s, uint32 color);
void gfx_set_bg(uint32 color);
void gfx_screen_reset(void);
// See kernel/dev/gfx.c: drawing goes to a copy of the screen in RAM once
// gfx_accelerate() has run, and reaches the display at gfx_flush().
void gfx_accelerate(void);
void gfx_flush(void);
void gfx_flush_all(void); // the crash screen: unconditionally
volatile uchar *gfx_hw_ptr(void);
int gfx_cols(void);
int gfx_rows(void);