3182da3965
Claude Code keeps per-project memory at <home>/.claude/projects/<key>/,
where <key> is the project's absolute path with every separator and the
drive colon turned into a dash. That makes it both rename-fragile and
machine-specific, and this project has hit each of those once:
- D:\DEV\Schunix -> D:\DEV\SchOS changed the key, and 43 memories were
invisible until moved across by hand.
- Development happens from Windows and from Linux, and each looks in a
different place for the same checkout.
Committing .claude/memory/ fixes both -- git carries it across renames
and between hosts -- and `import` puts it wherever the local machine
expects. .claude/settings.local.json stays ignored: those are per-machine
permissions.
The colon maps to a dash rather than being dropped. Deleting it gives
D-DEV-SchOS, which matches nothing and silently finds zero memories; that
was the first attempt. On Git Bash the key comes from `cygpath -w`,
because Claude Code is a Windows program and sees D:\DEV\SchOS, not
/d/DEV/SchOS.
60 lines
3.5 KiB
Markdown
60 lines
3.5 KiB
Markdown
---
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name: schunix-driver-infrastructure
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description: "Dynamic IRQ registration, a DMA arena and PCI capabilities exist now; PCI interrupts are level/active-low and the old ioapicenable only did edge/high"
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metadata:
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node_type: memory
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type: project
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originSessionId: a665e14b-86bd-4b41-a228-51377f3e9225
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modified: 2026-08-04T22:21:01.983Z
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---
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**Built 2026-08-04**, before USB, because xHCI, any NIC and virtio-gpu all
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need the same three things and none of them existed.
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- **`kernel/dev/irq.c` + `include/irq.h`** — `irq_register(irq, fn, arg,
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name, level_low)`. `trap.c` used to be a switch listing every interrupt by
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name, which cannot work for a PCI device: its line comes out of config
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space at probe time, so there is no constant to write a `case` label for.
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Chains of up to 4 handlers per line, because PCI INTx pins are shared and a
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handler must recognise that its own device is idle and return.
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Registration publishes an entry then bumps `n` with a barrier, so dispatch
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needs no lock. `/proc/interrupts` (minor 12) shows counts and trigger mode.
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- **`kernel/core/dma.c` + `include/dma.h`** — `dma_alloc(size, align)` /
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`dma_free(p, size)` (Linux's contract: caller remembers the size). Carved
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off the top of physical memory in `main.c` *before* `kinit1`, because
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kalloc's free list makes no contiguity promise. x86 PCI is cache coherent
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so no PAT games are needed — but ordering is still your problem.
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- **`pci_find_cap()` and `pci_msi_enable()`** in `kernel/dev/pci.c`.
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**Four traps worth remembering:**
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1. **PCI interrupts are level-triggered, active low.** `ioapicenable()`
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hardcoded edge/active-high, which is right for every ISA-era device and
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wrong for every PCI one — and it fails *silently*: you get exactly one
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interrupt, then the device holds the line asserted, no further edge ever
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comes, and it is never heard from again. `ioapicroute(irq, cpu,
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level_low)` is the real call now.
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2. **`IRQ_ERROR` (19) and `IRQ_SPURIOUS` (31) are LAPIC vectors that xv6
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numbered as if they were IOAPIC pins**, so `T_IRQ0+19` is already taken. A
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PCI device routed to pin 19 would collide. `irq_register` refuses them.
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3. **`cprintf` has no field widths.** `%04x` is not a directive to it: it
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prints `%` and `0` literally, consumes no argument, and every argument
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after it lands in the wrong place. It understands only
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`%d %u %l %x %p %b %s %%`.
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4. **Unaligned MMIO reads return garbage.** Reading a uint32 at BAR offset 2
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to get xHCI's HCIVERSION returned zero. Read the aligned dword and shift.
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5. **Never let a device raise an interrupt before its line is claimed.** This
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one cost real time. xHCI transfers completed, completion events sat in the
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event ring, `USBSTS.EINT` and `IMAN.IP` were both set -- and the interrupt
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count stayed at zero. Cause: the controller asserted INTx during
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enumeration, while the IOAPIC entry was still masked. **An interrupt that
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arrives at a masked pin is dropped, not deferred** (the 82093AA manual says
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so, and QEMU implements it), and INTx is a *level* -- so the line then sits
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high forever with no further transition for anyone to notice. Unmasking
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afterwards finds nothing pending. The fix is ordering: enumerate with
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controller interrupts off, then `irq_register`, then clear the device's
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pending status so it drives the line low, and only then enable. The next
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event is a genuine low-to-high transition.
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Verify with `pwsh -File tools/bootcheck.ps1 -Usb`. See [[schunix-usb-xhci]],
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[[schunix-pci-drivers]].
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