ahci: scan all ports; fat: mount first valid FAT32 disk

The kernel previously started only AHCI port 0, so any launch where the
firmware/QEMU put something else on port 0 (e.g. the ESP from an implicit
-drive file=fat:rw:esp) left the real disk unnoticed: 'unsupported sector
size 0' and no mount.

- ahci.rs: enumerate CAP.NP/PI ports, start every port that links up
  (empty ports settle at DET=0 and are skipped quickly), keep up to 8
  disks, and expose disk_count()/select() for the filesystem
- fat.rs: try each disk in port order and mount the first valid FAT32
  volume; non-FAT disks are logged and skipped
- scripts/boot.sh: attach the data disk on ide.0 and the ESP on ide.1
  (DISK_IMG env var, default /tmp/disk.img), matching the Makefile
- Verified with the disk on port 0 (Makefile style), port 5 (old-style
  command line), and with the ESP on port 0; fsck.fat stays clean

1 files changed, 23 insertions, 2 deletions

diff --git a/rsnix/src/fat.rs b/rsnix/src/fat.rs
index 70fa688..616a08f 100644
--- a/rsnix/src/fat.rs
+++ b/rsnix/src/fat.rs
@@ -838,9 +838,30 @@ pub fn remove(parent_dir_cluster: u32, name: &str) -> Result<(), ()> {
 
 // --- mount -------------------------------------------------------------------
 
-/// Parse the BPB, cache the FAT and validate the volume.  Returns true if
-/// a usable FAT32 volume is mounted.
+/// Try to mount a FAT32 volume on every attached disk, in port order, and
+/// keep the first one that validates.  Returns true if a usable FAT32
+/// volume is mounted.
 pub fn init() -> bool {
+    let disks = crate::ahci::disk_count();
+    if disks == 0 {
+        crate::serial_println!("fat: no disk");
+        return false;
+    }
+    for d in 0..disks {
+        if !crate::ahci::select(d) {
+            continue;
+        }
+        if try_mount() {
+            return true;
+        }
+        crate::serial_println!("fat: no FAT32 volume on disk {}", d);
+    }
+    false
+}
+
+/// Parse the BPB of the selected disk, cache the FAT and validate the
+/// volume.
+fn try_mount() -> bool {
     let mut bpb = [0u8; SECTOR];
     if read_sector(0, &mut bpb).is_err() {
         crate::serial_println!("fat: no disk");