ahci: SATA driver + FAT32 persistent disk mounted at /disk

- ahci.rs: polling AHCI on PCI class 1/6/1, bus mastering + ABAR map,
  port 0 up, IDENTIFY + READ/WRITE DMA EXT through one 4KiB DMA buffer
- fat.rs: BPB parse, whole-FAT RAM cache with write-back to both copies,
  LFN read/write, dirs, multi-cluster files, FSInfo free-count sync,
  space-padded dot entries ('..' = cluster 0 under root), fsck-clean
- fs.rs: /disk mount node + 24-slot ephemeral node pool; ls/cat/hexdump/
  echo/mkdir/touch/rm/edit all work on disk paths unchanged
- QEMU: explicit bus assignment (disk=ide.0, ESP=ide.1); the implicit
  fat:rw:esp drive was stealing AHCI port 0
- Debugged against QEMU source: header DW0 is CFL in bits 4:0 / PRDTL in
  bits 31:16 (Linux libahci layout), FIS count at bytes 12-13, device at
  byte 7; QEMU rejects H2D FIS flags bits 6:4, so direction is the
  header W bit only
- Verified: full shell round-trip on /disk, reboot persistence (poweroff
  then mdir/mtype and a fresh boot), fsck.fat clean

1 files changed, 33 insertions, 0 deletions

diff --git a/rsnix/src/pci.rs b/rsnix/src/pci.rs
index 2d201f3..efdbc01 100644
--- a/rsnix/src/pci.rs
+++ b/rsnix/src/pci.rs
@@ -57,6 +57,39 @@ pub fn config_read(bus: u8, device: u8, func: u8, reg: u8) -> u32 {
     inl(CONFIG_DATA)
 }
 
+/// Write a dword to a device's config space (`reg` must be dword-aligned).
+pub fn config_write(bus: u8, device: u8, func: u8, reg: u8, val: u32) {
+    let addr = 0x8000_0000
+        | ((bus as u32) << 16)
+        | ((device as u32) << 11)
+        | ((func as u32) << 8)
+        | ((reg as u32) & 0xFC);
+    outl(CONFIG_ADDRESS, addr);
+    outl(CONFIG_DATA, val)
+}
+
+/// Read a memory BAR (index 0-5) as a 64-bit address, handling 32/64-bit
+/// BARs.  Returns None for I/O BARs or invalid values.
+pub fn bar64(bus: u8, device: u8, func: u8, bar_index: u8) -> Option<u64> {
+    let reg_off = 0x10 + bar_index * 4;
+    let lo = config_read(bus, device, func, reg_off);
+    if lo & 1 == 1 {
+        return None; // I/O BAR
+    }
+    let kind = (lo >> 1) & 0x3;
+    let hi = if kind == 2 {
+        config_read(bus, device, func, reg_off + 4)
+    } else {
+        0
+    };
+    let base = ((hi as u64) << 32) | (lo as u64 & 0xFFFF_FFF0);
+    if base == 0 {
+        None
+    } else {
+        Some(base)
+    }
+}
+
 /// Enumerate every device/function present on all buses.
 pub fn probe() -> alloc::vec::Vec<DeviceInfo> {
     let mut out = alloc::vec::Vec::new();