Daniel Samson
d2dfbcabf8
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.
56 lines
2.3 KiB
Zig
56 lines
2.3 KiB
Zig
//! The scanout wire protocol — what the compositor says to a native scanout driver (e.g.
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//! virtio-gpu) over `/protocol/scanout` to put a composited frame on screen. The driver owns
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//! the panel and the shared scanout surface it handed the compositor (via the display
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//! service's `attach_scanout`); the compositor composites into that surface, then asks the
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//! driver to present a damaged rectangle. Tiny by design — one present request. Separate from
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//! the display protocol because the directions differ: clients call the compositor over
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//! `/protocol/display`; the compositor calls the driver over `/protocol/scanout`. See
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//! docs/display-v2.md.
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//!
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//! One scanout per driver instance, so `Header.target` is always 0.
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const envelope = @import("envelope");
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/// `present(rect)`: put the given rectangle of the shared scanout surface on the panel (on
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/// virtio-gpu: transfer-to-host of the region, then a fenced resource flush).
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pub const Present = extern struct {
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x: u32 = 0,
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y: u32 = 0,
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width: u32 = 0,
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height: u32 = 0,
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};
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/// `set_mode(width, height)`: change the scanout resolution — the shared surface is sized to
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/// the largest mode, so this just re-points the scanout rectangle; the surface is unchanged.
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pub const SetMode = extern struct { width: u32, height: u32 };
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/// One offered display mode.
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pub const Mode = extern struct { width: u32, height: u32 };
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pub const max_modes = 4;
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/// The answer to `get_modes`: a small fixed list of modes. The success/failure verdict is
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/// the reply's `Status`, so this carries only the modes.
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pub const Modes = extern struct {
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count: u32 = 0,
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_padding: u32 = 0,
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modes: [max_modes]Mode = @splat(.{ .width = 0, .height = 0 }),
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};
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pub const Protocol = envelope.Define(.{
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.name = "scanout",
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.version = 1,
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.operations = &.{
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.{ .name = "present", .request = Present },
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.{ .name = "get_modes", .reply = Modes },
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.{ .name = "set_mode", .request = SetMode },
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},
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});
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pub const Operation = Protocol.Operation;
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/// The call floor, like every synchronous protocol. This module used to declare
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/// 64 — the *push* floor — which was simply wrong: nothing here is pushed, and a
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/// provider sizing its receive buffer to 64 refuses (`-E2BIG`) any caller that
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/// sends up to the floor it is entitled to.
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pub const message_maximum: usize = Protocol.message_maximum;
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