nexoniarz/SchOS
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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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11
kernel/core/main.c
View file

@ -12,6 +12,7 @@
#include "klog.h"
#include "usb.h"
#include "acpi.h"
#include "vmap.h"
static void *klog_base; // the persistent log, just above the kernel image
@ -137,7 +138,9 @@ kmain(uint32 fb_addr)
bi.stride = *(uint16*)P2V(SCHBOOT_VBE_STRIDE);
bi.bpp = *(uchar *)P2V(SCHBOOT_VBE_BPP);
}
bi.ptr = (volatile uchar *)P2V(fb_phys);
bi.ptr = (volatile uchar *)P2V(fb_phys);
bi.phys = fb_phys;
bi.size = (uint64)bi.stride * bi.height;
// Only if the loader actually described something, and only if the direct
// map reaches it. A framebuffer above 4 GB needs ioremap(), and ioremap()
@ -201,6 +204,7 @@ kmain(uint32 fb_addr)
dma_init(P2V(lowtop - DMA_ARENA_BYTES), P2V(lowtop));
if (boot_verbose) cprintf("verbose boot: dma_init done\n");
kvmalloc();
pat_init();
if (boot_verbose) cprintf("verbose boot: kvmalloc done\n");
// Now that the direct map reaches it, the rest of RAM can be allocated from.
@ -231,6 +235,10 @@ kmain(uint32 fb_addr)
(void *)fb_early);
}
}
// The console has been drawing straight onto the card. From here it draws
// into RAM and copies out. See gfx_flush().
gfx_accelerate();
// ACPI first, the legacy MP table only if it finds nothing. Modern firmware
// publishes a complete MADT and an MP table with one processor in it, so
// asking the older interface first means never seeing more than one CPU.
@ -409,6 +417,7 @@ mpenter(void)
// AP keeps whatever mode the firmware left its APIC in across INIT, so the
// boot processor having switched itself back says nothing about this one.
lapic_xapic_mode();
pat_init();
switchkvm();
seginit();
lapicinit();