These were the awkward ones. Each began with an operation packed into a
single byte — two of them with a version wedged in beside it — so there was
no wrapping them: the layouts had to be rebuilt. The device manager's own
enumerate and subscribe become the reserved verbs that mean the same thing
everywhere, its replies lose three status structs the envelope already
carries, and a device id becomes the packet's target. Power drops the
version it repeated on every request, because describe is the handshake,
and stops claiming a 64-byte ceiling it never needed for calls. USB moves a
control transfer's data to the packet tail in both directions, which makes
the status length the transferred length and retires a field that had been
saying the same thing twice.
The danger in this one was not the protocols but their readers. Init
recognised a power button by two bytes at the head of a message, the ACPI
service dispatched on the first byte, the xHCI driver read its operation
with a raw integer load, and the HID drivers reinterpreted a report
wholesale — none of which would have failed to compile once the layouts
moved. They would simply have stopped: no shutdown on the power button, no
reports from the keyboard. Every one of them now reads through the
generated types, and the shutdown gate that answers only a subscriber is
the same code it was.
Two sizes were decided by measuring rather than assuming. The child-added
message is both a request and the event broadcast to subscribers, and
alignment rounds it to 48 bytes, which puts its packet exactly on the
64-byte push floor — a test pins that, because a field added carelessly
would now overflow it. The interrupt report gives up eight bytes of inline
room to make space for the header; the two drivers that produce reports
send eight and four.
Suite 110/110.
The folded header stops being a rule in a document and becomes the layout
on the wire. Verbs number from sixteen, leaving describe, enumerate,
subscribe and unsubscribe reserved and answered the same way by every
provider — none of them writes a line to do it. What each protocol used to
carry in a field of its own now travels in the header: a vfs node and a
display layer are the packet's target, and a reply opens with a status the
envelope stamps rather than one each protocol spelled for itself.
Display gains the most. One forty-byte request had served eleven verbs, so
attach_scanout smuggled stride through x, refresh through y and format
through colour, and every coordinate crossed as a bitcast. Per-operation
structs end all three: the fields have their own names and their own signs,
and the tile payload grows to 224 bytes because the prefix shrank. Scanout
loses a message maximum of 64 it had no business declaring — it answers
calls, and the floor for a call is 256 — and virtio-gpu stops hard-coding
that number at its harness.
Two changes are semantic rather than notational. A directory now ends at an
entry with no name, because the fixed part of a reply always travels and a
zero-length reply no longer exists to mean anything. And input joins the
service harness, the last loop in the tree that answered no ping and heard
no terminate; its subscriber table, its pruning and its fan-out are the
same code, and a shutdown now asks it to stop instead of killing it.
A new conformance case reads the registry's own listing and asks every
protocol it finds for its name, its version and its verb count, then offers
a verb nobody defines and requires -ENOSYS — the envelope's promise,
checked against providers rather than against itself. What it cannot reach
in that boot it names on the serial line instead of passing quietly.
Suite 110/110.
Daniel Samson