Real-PC diagnose boot (photo + OCR): the boot stick connects at
SuperSpeed on port 21 and 'device setup failed' lands in the SAME
millisecond — an instant failure, not a timeout. setupDevice reset
every port unconditionally; that is required to enable USB2 ports, but
a SuperSpeed port that trained its link is ALREADY enabled (xHCI
advances USB3 ports to Enabled, no reset — spec 4.3), and driving a hot
reset into the live link drops PED mid-reset on real silicon. QEMU
tolerates the spurious reset, which is why the harness never saw it.
An enabled speed>=4 port now skips the reset (a not-yet-enabled SS link
still gets one). Every setup step names its failure — port reset with
the PORTSC value, Enable Slot, device-slot exhaustion, Address Device
with its completion code — so the on-screen transcript of the next
failure identifies the exact xHCI command instead of one blanket line.
block.open's give-up window drops 60 s -> 30 s (the slowest observed
healthy chain completed at ~24 s); a machine whose stick failed setup
should not sit a further minute pretending otherwise.
init writes /mnt/usb/DANOS.LOG at shutdown and then enters S5 — but the write sat in the USB flash controller's write cache and was lost when power was cut, because nothing issued SCSI SYNCHRONIZE CACHE. Invisible in QEMU (its backing file commits immediately); real on hardware. The boot-time flush survived only because the machine kept running afterward and the cache drained on its own.
- block protocol gains a flush op; usb-storage serves it with SYNCHRONIZE CACHE (10); runtime.block gains Device.flush()
- the FAT server tracks whether blocks were written and, on a file close, commits the device cache (durable-on-close — the right default for removable media, and it makes init's existing shutdown close() persist the log before S5, no init/VFS change needed)
- verified the SYNCHRONIZE CACHE actually reaches the driver on each dirty close; usb-storage/fat-mount/mutations/rename/mtime/log-flush all green
Add a FAT12/16/32 filesystem the VFS mounts at /mnt/usb, reading and writing a
USB stick through the block device. Verified end to end under QEMU: the fat
server mounts the volume, the VFS routes /mnt/usb to it, and a client lists the
root and reads a file (the ELF magic of /mnt/usb/system/kernel).
- engine.zig: the FAT engine over a BlockDevice interface — mount (a bare FAT or,
as QEMU's VVFAT and most real sticks present it, an MBR-partitioned disk), FAT
chain walk (12/16/32), cluster allocation, directory traversal with long-name
read, and file read / write / create. Host-tested against a RAM-backed FAT16
image (create, cluster-spanning write, mid-file overwrite, read-back, list).
- on-disk.zig: the align(1) boot-sector / directory / long-name / FSInfo structs
and the cluster-count FAT-type detection.
- fat.zig: the server — wraps the .block device (a DMA bounce buffer) in a
BlockDevice, mounts the FAT, serves the vfs-protocol as a backend, and mounts
itself into the VFS at /mnt/usb. Spawned by init as a boot service.
- runtime.block: the block-device client (geometry / read / write by physical
address, so whole sectors never cross IPC).
- Raise the kernel service-name registry (maximum_services) 8 -> 16: it is
indexed directly by ServiceId, and fat = 8 was being rejected, so the fat
server exited before registering.
- VFS: an absolute path with no matching mount is now not-found rather than
silently created in the flat ramfs — so /mnt/usb fails cleanly until mounted.
Tests: fat-mount (the full stack: block -> FAT -> VFS mount -> list + file read)
passes; host units cover the engine and on-disk structs; the vfs, shutdown, and
USB regression suite stays green (10/10).
Daniel Samson