# Ch421 — complete request-packet boundary (prefit)

## Trigger

Ch420 successfully removed the direct request-pointer-to-RAM-address family.
Its fit exposed ten paths from the wide request RAM's registered data output,
through scene-bit decode, into separate fragment/marker valid registers.  The
worst was `-0.273 ns`; no address-stage path appeared in the violated top ten.

## Structural cut

`gs_lpddr_zc_emit` now captures all `PW` request bits, including the scene bit,
into one `req_stage_packet` register.  A single `req_stage_valid` occupancy bit
is driven only by the already-registered `req_read_pending` control.

Fragment versus marker selection is decoded after that complete packet
boundary:

```
req_stage_frag_valid   = req_stage_valid && !req_stage_packet.scene
req_stage_marker_valid = req_stage_valid &&  req_stage_packet.scene
```

Therefore no request RAM output bit feeds stage occupancy or two competing
valid-register D cones.  The packet register intentionally has no reset;
`req_stage_valid=0` makes it unobservable until a complete read is captured.
Pop, pending, consume, marker ordering, and external latency are unchanged.

## Prefit verification

- async FIFO randomized CDC test: 3,284 writes/reads exact;
- Z/color emit: 1,500 fragments exact across three epochs;
- concurrent scanout/reload/probe: 16,384 pixels exact;
- full 283-epoch production-control replay: PASS;
- 1,177,489 fragments fed, 984,845 passed Z, zero drops/errors;
- final Z mismatch `0 / 307,200`, color mismatch `0 / 235,963`;
- framebuffer SHA-256 remains exactly
  `a88f1f6c926dd0db2b72ae2873c399e7dc5666f8003190184c34d360b13a5f44`;
- relevant `git diff --check`: PASS.

## Owner fit gate

Ready for one owner-controlled Quartus 26.1 GUI compile.  Acceptance requires:

- no request RAM output path to stage occupancy/fragment/marker valid state;
- any RAM-output-to-`req_stage_packet` data path is timing-clean;
- setup, hold, recovery, removal, and minimum pulse all close;
- request FIFO RAM inference remains intact at the existing device budget.

Do not deploy the Ch420 RBF.  If Ch421 closes, deploy that RBF and execute the
already-verified 283-epoch board scene before accepting the chapter.

## Owner fit result — request path closed, design path exposed

The owner-controlled Quartus 26.1 GUI fit completed successfully.  Ch421 did
close the intended 310 MHz EMIF/request family:

- EMIF setup WNS `+0.155 ns`, TNS `0`;
- EMIF hold slack `0.000 ns` and all other timing classes clean;
- no request FIFO path appeared in the violated setup paths;
- 38,332 ALMs (82%), 55,712 registers, 336/358 RAM blocks (94%), 160 DSPs.

The fit is nevertheless **not deployable** because placement exposed a separate
25 MHz design-clock failure: setup WNS `-1.993 ns`, TNS `-14.856 ns`.  All ten
reported paths launch at `u_tex|g_bilinear.tap[0][8/9]` and terminate at
`raster_pixel_color_q[16/18]`.  The worst path contains 30 logic levels and
41.955 ns of data delay: both bilinear interpolation stages plus TEX0 vertex
modulation were being evaluated in one cycle.  That is an independent, real
combinational boundary defect, not a reason to undo the now-clean request
packet cut.  Ch422 addresses it structurally.