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35ea398334
The first piece of real-hardware work, and it is three bugs that QEMU cannot show you. The report was an ASUS laptop -- Ryzen 7000-series APU, Radeon integrated, RTX discrete -- booting to a white underscore on a black screen, and unable to be offered 1920x1080. **The framebuffer address was truncated.** The loader handed over `(UINT32)gop->Mode->FrameBufferBase` and the kernel read a 32-bit field out of a VBE mode-info block -- a *BIOS* structure the UEFI path had been filling in because the kernel already knew how to read one. A machine that puts its aperture above 4 GB, which is ordinary on a laptop with a discrete GPU, got a plausible-looking address pointing at nothing. The splash went into it, the kernel was handed it, and the firmware's own cursor kept blinking on an otherwise black screen. Every step in that sequence succeeded on a number that was wrong, which is why there is no error message anywhere in it. SCHBOOT_FB_BASE is the loader's own display block: 64-bit address, 64-bit size, 32-bit width, height and stride, and the pixel format. The VBE block is still written for the BIOS loader, but anything that does not fit is left at zero rather than wrapped -- a truncated address is this bug, and a zero is at least honest. The kernel then has a problem the loader does not: its direct map covers the low 4 GB, so a framebuffer above it needs ioremap(), which needs kpdpt, which kvmalloc() has not allocated when the early console starts. A low framebuffer is attached exactly as before; a high one is attached immediately after kvmalloc(), still thousands of lines before anything else. **There was more than one adapter, and the first one was taken.** LocateProtocol answers with *a* GOP and the specification says nothing about which. QEMU with a stdvga and a virtio-gpu has **three** handles and two of them are Blt-only with a framebuffer address of zero -- so this is not a hardware-only hazard, it was a one-in-three chance of drawing into address 0 that happened to keep coming up heads. Every handle is scored now: no aperture, no candidacy; a readable current mode beats one that needs changing; size breaks the tie. **mode_usable() rejected modes real machines have.** A padded scanline (every GPU that aligns its rows -- and very likely the reason 1920x1080 could not be offered), a width not divisible by eight (1366x768, which is half the laptops in the world), and anything below 800x600 (the only mode some firmware gives over a KVM). All three passed on QEMU, which is why they survived. The mode list now offers 1366x768 and 640x480 where it did not. The loader also prints what it found, to the firmware's text console, which is cleared before the splash: invisible on a boot that works, and the only thing on the screen on a boot that dies before the splash. One process note, recorded because it cost a wrong test: `make UEFI_KEEP=1` ships the loader that is already built. Its own comment says "a build that quietly ships a stale bootloader is how you spend an evening debugging code that is not running", and that is exactly what happened -- a regression run that looked clean had tested the old loader. Loader work gets a full build. Both supported configurations verified after a clean build: one boot each, no traps, no panics, socktest and memfdtest green, and the adapter report shows the right one being chosen out of three. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> |
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| .. | ||
| efi.h | ||
| main.c | ||
| README.md | ||
| startup.nsh | ||
UEFI loader
The loader. This directory once existed only so the BIOS path could be moved
into boot/bios/ without pretending the split was hypothetical; it is now the
only path there is.
See ../README.md for the hand-off contract a loader has to satisfy. The
short version: the kernel wants long mode with paging on, its ELF segments
placed, a 24-byte-entry E820-shaped memory map, and framebuffer geometry — and
it does not care which firmware produced them.
What this will need, in rough order:
- A PE/COFF target. clang can emit one directly
(
--target=x86_64-unknown-windows -fuse-ld=lld-link), so no new toolchain: the same LLVM install already builds every other target here. - The handful of UEFI structures actually used — the system table, boot
services,
LOADED_IMAGE,SIMPLE_FILE_SYSTEM,GRAPHICS_OUTPUT. Written from the specification rather than by importing gnu-efi or EDK II, which is both a licence question and a large dependency for something this small. GetMemoryMaptranslated into the same 24-byte entriese820.calready parses. Not a second format for the kernel to learn.ExitBootServicesbefore touching the memory it described.- Graphics Output Protocol for the framebuffer, filling the same
VBE_INFO_ADDRstructure the kernel reads.
The image goes at EFI/BOOT/BOOTX64.EFI on a FAT ESP, which also means the
ISO carries a single El Torito EFI entry.