60d06658f0
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
21 lines
1.1 KiB
C
21 lines
1.1 KiB
C
#pragma once
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// Kernel mappings that are neither the direct map nor plain device registers.
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// Both take one 1 GB slot of the device window ioremap() hands out, so each is
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// limited to a gigabyte and meant to be called a handful of times at boot.
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// Program this CPU's PAT so that a mapping can ask for write-combining. Every
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// CPU runs it before it touches a write-combined mapping -- the PAT is per
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// processor, and one left at its reset value reads the same bits as
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// write-back, which on device memory the MTRRs then turn back into uncached.
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void pat_init(void);
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// Device memory, write-combined: stores are gathered into bursts rather than
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// sent one at a time, and reads are as slow as uncached. For a framebuffer,
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// never for registers. Returns 0 if there is no window left.
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void *ioremap_wc(uint64 pa, uint64 size);
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// `size` bytes of ordinary RAM, contiguous in the kernel's address space and
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// not physically: kalloc() pages behind a page table of their own. Zeroed.
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// For buffers too big for the DMA arena that no device reads. Never freed.
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void *vmap_alloc(uint64 size);
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