drive: disable FUA on READ(10) — the ~10x sequential-read regression

Forcing FUA (Force Unit Access) on EVERY READ(10) bypassed the drive's
readahead/streaming cache on the bulk Pass-1 sweep, collapsing sequential
throughput ~10x (UHD 15-25 → ~2 MB/s, DVD → ~0.5 MB/s), disc-type-
agnostic — the cache IS the streaming throughput. This was the real speed
regression (not unlock/riplock, which the earlier chase suspected). Clear
byte-1 bit 0x08. FUA will return as a dedicated Pass-N recovery handler
that sets/clears it per marginal-sector re-read, where cache-masking of a
stochastic sector actually matters — never blanket on the bulk path (#55).

src/drive/mod.rs

@@ -719,22 +719,23 @@ impl Drive {
         count: u16,
         buf: &mut [u8],
         timeout_ms: u32,
-        // `recovery` only gates the Linux /dev/sr0 pread fallback below; on
+        // `recovery` gates only the Linux /dev/sr0 pread fallback below; on
         // other platforms it is intentionally unused.
         #[cfg_attr(not(target_os = "linux"), allow(unused_variables))] recovery: bool,
     ) -> Result<usize> {
+        // FUA (Force Unit Access) is DISABLED for now — byte-1 bit 0x08 cleared.
+        // Unconditionally forcing every READ(10) past the drive cache disabled
+        // the drive's readahead/streaming cache on the bulk sequential sweep and
+        // collapsed throughput ~10x (UHD 15-25 → ~2 MB/s, DVD → ~0.5 MB/s),
+        // disc-type-agnostic — the cache IS the streaming throughput.
+        //
+        // TODO(#55): reintroduce FUA as a dedicated Pass-N recovery HANDLER that
+        // sets/clears it per marginal-sector re-read (where cache-masking of a
+        // stochastic sector actually matters), never blanket-applied to the bulk
+        // read path.
         let cdb = [
             crate::scsi::SCSI_READ_10,
-            // FUA (Force Unit Access, bit 3 = 0x08): every read comes from the
-            // physical MEDIA, never the drive's cache. A recovery tool must not
-            // trust the cache — the BU40N caches aggressively (a re-read of a
-            // good sector returns in ~4 ms vs ~250 ms from media), so without
-            // FUA a retry of a marginal sector could return a cached result
-            // instead of giving the surface a fresh physical attempt. A
-            // stochastic sector that would read on a real media hit is otherwise
-            // masked by a cached miss. Cache-bypass is validated on the
-            // BU40N/Initio bridge (FUA reads execute, media 40 ms vs cache 4 ms).
-            0x08,
+            0x00,
             (lba >> 24) as u8,
             (lba >> 16) as u8,
             (lba >> 8) as u8,
@@ -1414,9 +1415,10 @@ mod command_tests {
     #[test]
     fn read_builds_read10_cdb_with_be_lba_and_count() {
         // Drive::read issues READ(10) (0x28). LBA bytes 2..5 big-endian,
-        // transfer length bytes 7..8 big-endian (MMC-6). FUA is set (byte 1 ==
-        // 0x08) so every read bypasses the drive cache and hits physical media.
-        // Distinct nibbles catch a swapped shift.
+        // transfer length bytes 7..8 big-endian (MMC-6). FUA is DISABLED (byte 1
+        // == 0x00) so the drive cache/readahead is allowed — forcing FUA on the
+        // bulk sweep collapsed throughput ~10x. Distinct nibbles catch a swapped
+        // shift.
         let RecordingHarness {
             drive: mut d,
             cdb,
@@ -1428,8 +1430,8 @@ mod command_tests {
         let c = cdb.lock().unwrap();
         assert_eq!(c[0], crate::scsi::SCSI_READ_10);
         assert_eq!(
-            c[1], 0x08,
-            "Drive::read sets FUA (force media, bypass cache)"
+            c[1], 0x00,
+            "FUA disabled — cache/readahead allowed on the bulk read path"
         );
         assert_eq!(&c[2..6], &[0x00, 0xAB, 0xCD, 0xEF], "LBA big-endian");
         assert_eq!(&c[7..9], &[0x00, 0x02], "transfer length big-endian");