scsi: one bus layer for every controller, on top of BlockNode

CHANGELOG.md

@@ -51,6 +51,16 @@ hashes are given where a change is easiest to understand by reading the commit.
   cursor position fields are 12-bit signed; they were read as unsigned, so a
   cursor partly off the edge was drawn 4000 pixels away. (`src/dev/o2gfx.rs`)
 
+### SCSI
+
+- Every SCSI controller attaches its targets through one shared bus, and
+  disks report one emulated identity on every machine. The O2 and Octane now
+  say `IRIS EMUL DISK` and `IRIS EMUL CDROM` in INQUIRY; the Indy and Indigo2
+  keep their Sony CD-ROM.
+- A disc change loads a CHD CD as well as an ISO, and the O2, the Octane and
+  the Indigo2's second controller fold pending CHD diffs on a clean exit as the
+  first controller always did.
+
 ### Host efficiency on small machines
 
 - **Threads that have nothing to do now sleep in milliseconds, not

HACKING.md

@@ -70,16 +70,29 @@ reach the machine through one shared `MachineSlot` per machine, which the
 machine's `Drop` nulls, so a holder that outlives the machine (the monitor,
 after iris-gui's Stop) gets "machine is not running" instead of freed memory.
 
-SCSI storage follows the same pattern one level down. `src/scsi.rs` is the
-target (disk, CD-ROM, DaynaPort) any controller can hold, and
-`src/scsi_monitor.rs` implements the monitor's `scsi` and `cow` subcommands and
-the run-time disc changes once, over a controller's target table lent through
-its `ScsiTargets` trait. The WD33C93A (Indy, Indigo2) and the Adaptec 7880 (O2)
-lend theirs and keep only chip-specific subcommands (`regs`; the WD's `wdt`,
-`defer`, `debug`), and `DiscChanger` is implemented once for every
-`ScsiTargets`. Configuration is shared the same way: both machines attach
-their devices from the `[scsi.N]` tables, where the key is a slot
-(controller × 16 + id, see `config::scsi_slot`).
+SCSI storage follows the same pattern one level down. `ScsiTarget` is what
+varies on a bus (a `ScsiDevice` disk or CD-ROM, a `DaynaPort`): `request`,
+the lifecycle, and optional behaviour with do-nothing defaults (a
+copy-on-write layer, CHD sync, a disc changer), so callers ask for behaviour
+and never for a type. `ScsiBus` (`src/scsi/bus.rs`) is the mechanism: the
+id-to-target table with attach, request and overlay export and import, and
+the controller's INQUIRY identities. A disk or CD-ROM sits on a `BlockNode`
+(`src/block_node.rs`), built by `open_image` for everything that mounts an
+image. A CD-ROM's node is a `ChangerNode`: the queue of discs and the open
+active one, with the `Removable` trait (present, changed, eject, load)
+separate from `BlockNode` so that removable media which is not a block device,
+a tape, can use it too. The WD33C93A (Indy, Indigo2), the Adaptec
+7880 (O2) and the ISP1040 (Octane) each make an empty bus of their own kind
+(`new_bus`); the machine fills it from config and gives it to the controller's
+constructor, so a controller exists with its targets and keeps only its
+registers and DMA. `src/scsi_monitor.rs` implements the monitor's `scsi` and `cow`
+subcommands and the run-time disc changes once, over the bus a controller hands
+out through `ScsiTargets::scsi_bus`; chip-specific subcommands (`regs`; the
+WD's `wdt`, `defer`, `debug`) stay with the chip, and `DiscChanger` is
+implemented once for every `ScsiTargets`. Configuration is shared the same
+way: `scsi::attach_from_config` walks the `[scsi.N]` tables for every machine,
+where the key is a slot (controller × 16 + id, see `config::scsi_slot`), and
+the machine supplies only a route from controller number to bus.
 
 ## 2. Data path and endianness