FPGA-Neural/hardware/v2/logs/timing.log

# V2 timing log -- solo append, mai troncato/sovrascritto (vedi README.md)
# Nessuna entry ancora -- popolato incrementalmente man mano che avanza lo sviluppo V2.

[2026-09-05] EXP-0001 -- neural_processor (P_IN=8, ACC_WIDTH=32)
nextpnr-ecp5 --45k --package CABGA381 --speed 8 --freq 80
  --lpf-allow-unconstrained
Fmax: 183.12 MHz -- PASS at 80 MHz (real place&route measurement)
Critical path: job_ready -> TRELLIS_IO output pad (i.e. the design's
  own logic is NOT the bottleneck at this Fmax -- an unconstrained
  output pin dominates; a real system-level Fmax will differ once
  this signal is consumed on-chip instead of driven to a pad in
  isolation). Log: hardware/v2/synthesis/neural_processor_p8/nextpnr.log
WNS/TNS: not reported by this nextpnr version in this invocation.

[2026-09-05] EXP-0002 -- neural_processor (P_IN=8, ACC_WIDTH=24)
Same command as above, ACC_WIDTH=24 build.
Fmax: 176.21 MHz -- PASS at 80 MHz (real place&route measurement)
Delta vs ACC_WIDTH=32: -6.91 MHz (-3.8%) despite FEWER resources --
  attributed to placement noise, not a real architectural effect
  (no seed sweep run to confirm either way -- see experiments.log
  EXP-0002 and benchmark.log). Log: hardware/v2/synthesis/
  neural_processor_p8_acc24/nextpnr.log

[2026-09-05] EXP-0003 -- N_PROCESSORS sweep (P_IN=8), real nextpnr-ecp5
--45k --package CABGA381 --speed 8 --freq 80 --lpf-allow-unconstrained
N=1: Fmax=159.11 MHz PASS   MULT18X18D 8/72 (11%)
N=2: Fmax=149.59 MHz PASS   MULT18X18D 16/72 (22%)
N=4: Fmax=151.01 MHz PASS   MULT18X18D 32/72 (44%)
N=8: Fmax=134.70 MHz PASS   MULT18X18D 64/72 (88%)
TRELLIS_IO stays at 18/245 (7%) throughout (harness has only
clk/rst/seed/checksum as real pins, by design -- see ERR-0005).
See decisions.log DEC-0005: DSP, not Fmax or LUT/FF, is the resource
that will first prevent scaling N_PROCESSORS further at P_IN=8.

[2026-09-05] EXP-0004 -- default-depth buffer configs, real nextpnr-ecp5
activation_buffer (D=4096): Fmax=325.20 MHz PASS
weight_buffer (D=512):      Fmax=339.67 MHz PASS
result_buffer (D=4096):     Fmax=325.20 MHz PASS
All far above the 80MHz target -- buffers are not a timing concern in
isolation at these depths.

[2026-09-05] EXP-0005 -- memory_manager + prefetch_engine (via
harness_memory_manager.v, see errors.log ERR-0005 for why a harness
was needed), real nextpnr-ecp5 --45k --package CABGA381 --speed 8
--freq 80 --lpf-allow-unconstrained
Fmax: 165.86 MHz -- PASS at 80MHz (real place&route measurement)

[2026-09-05] EXP-0006 -- neural_director (via harness_neural_director.v,
see errors.log ERR-0005 for why), real nextpnr-ecp5 --45k --package
CABGA381 --speed 8 --freq 80 --lpf-allow-unconstrained
Fmax: 250.50 MHz -- PASS at 80MHz (real place&route measurement)

[2026-09-05] EXP-0007 -- dependency_manager (N_NODES=16, standalone,
no harness needed), real nextpnr-ecp5 --45k --package CABGA381
--speed 8 --freq 80 --lpf-allow-unconstrained
Fmax: 155.30 MHz -- PASS at 80MHz (real place&route measurement)

[2026-09-05] EXP-0008 -- dataflow_core (via harness_dataflow_core.v,
see errors.log ERR-0005 for why a harness was needed), real
nextpnr-ecp5 --45k --package CABGA381 --speed 8 --freq 80
--lpf-allow-unconstrained
N_SLOTS=2: Fmax = 165.15 MHz -- PASS at 80MHz (real place&route)
N_SLOTS=4: Fmax = 133.19 MHz -- PASS at 80MHz (real place&route)
Fmax drops as N_SLOTS grows (more concurrent memory_manager+
neural_processor instances competing for the same routing fabric
around the shared neural_director/dependency_manager hub) -- both
configs still clear the 80MHz target with real margin.

[2026-09-05] EXP-0009 -- neural_multiprocessor (M8, N_SLOTS=2, real
standalone synthesis, no harness needed), real nextpnr-ecp5 --45k
--package CABGA381 --speed 8 --freq 80 --lpf-allow-unconstrained
Fmax: 142.45 MHz -- PASS at 80MHz (real place&route measurement)

[2026-09-05] EXP-0011 -- dataflow_core N_SLOTS=8 (via
harness_dataflow_core.v), real nextpnr-ecp5 --45k --package CABGA381
--speed 8 --freq 80 --lpf-allow-unconstrained
Fmax: 92.63 MHz -- PASS at 80MHz (real place&route measurement), but
with a much thinner margin than N_SLOTS=2 (165.15 MHz) or N_SLOTS=4
(133.19 MHz) -- Fmax falls monotonically with N_SLOTS (165.15 ->
133.19 -> 92.63 MHz for N_SLOTS=2/4/8) as routing congestion around
the shared neural_director/dependency_manager hub grows, while DSP
usage (88.9% at N_SLOTS=8) approaches its hard ceiling at the same
time -- see decisions.log DEC-0012.

[2026-09-05] EXP-0012 -- ACC_WIDTH 24 vs 32 real placement-seed sweep
(M10, "pipeline" axis, resolving EXP-0002's single-seed
inconclusiveness), 5 additional real nextpnr-ecp5 seeds (1-5) plus the
original default-seed run (EXP-0001/EXP-0002), same synthesized
netlists (hardware/v2/synthesis/neural_processor_p8/top.json and
neural_processor_p8_acc24/top.json), --45k --package CABGA381 --speed
8 --freq 80 --lpf-allow-unconstrained
ACC_WIDTH=32: seeds [default,1,2,3,4,5] -> [183.12, 164.85, 174.25,
  168.83, 183.96, 145.73] MHz. n=6, mean=170.12 MHz, min=145.73 MHz,
  max=183.96 MHz, stdev=14.16 MHz.
ACC_WIDTH=24: seeds [default,1,2,3,4,5] -> [176.21, 183.72, 180.57,
  183.08, 185.49, 175.16] MHz. n=6, mean=180.71 MHz, min=175.16 MHz,
  max=185.49 MHz, stdev=4.21 MHz.
Conclusion: over 6 real placement seeds, ACC_WIDTH=24 has a HIGHER
  mean Fmax (+10.6 MHz, +6.2%) AND a much lower seed-to-seed variance
  (stdev 4.21 vs 14.16 MHz, ~3.4x tighter) than ACC_WIDTH=32 --
  EXP-0002's single-seed result (176.21 < 183.12, i.e. ACC=24 looked
  WORSE) was placement-seed noise, not a real trend. See decisions.log
  DEC-0013.

[2026-09-05] EXP-0014 -- neural_multiprocessor N_SLOTS=1 and N_SLOTS=4
(final benchmark campaign), real nextpnr-ecp5 --45k --package CABGA381
--speed 8 --freq 80 --lpf-allow-unconstrained
N_SLOTS=1: Fmax = 152.46 MHz -- PASS at 80MHz
N_SLOTS=4: Fmax = 113.38 MHz -- PASS at 80MHz
(N_SLOTS=2 reference, EXP-0009: 142.45 MHz)
Full real N_SLOTS sweep for the complete system (dataflow_core + real
V1 PSRAM chain + slot_mem_arbiter): 1 -> 152.46 MHz, 2 -> 142.45 MHz,
4 -> 113.38 MHz. Fmax falls monotonically as N_SLOTS grows (more
routing congestion around the shared director/dependency_manager/
arbiter hub), exactly the same trend already observed for
dataflow_core alone (M7/M10, EXP-0008/EXP-0011) but now measured for
the REAL FULL SYSTEM including the real PSRAM backend.

[2026-09-05] EXP-0015 -- neural_multiprocessor N_SLOTS=1/2/4 after the
word-level burst-read rewrite (DEC-0015), real nextpnr-ecp5 --45k
--package CABGA381 --speed 8 --freq 80 --lpf-allow-unconstrained
N_SLOTS=1: Fmax = 152.44 MHz -- PASS at 80MHz (was 152.46 MHz, unchanged)
N_SLOTS=2: Fmax = 133.58 MHz -- PASS at 80MHz (was 142.45 MHz, -6.2%)
N_SLOTS=4: Fmax = 112.07 MHz -- PASS at 80MHz (was 113.38 MHz, -1.2%)
Small, real Fmax cost from widening the shared arbiter/backend to
16-bit + lb_n/ub_n (extra routing), overwhelmingly outweighed by the
real cycle-count reduction (EXP-0015 in experiments.log/benchmark.log):
D-Stress real wall-clock improves 2.24-2.37x at every N_SLOTS.

[2026-09-05] EXP-0016 -- neural_multiprocessor N_SLOTS=1/2/4 after
adding activation_cache.v (DEC-0016), real nextpnr-ecp5 --45k
--package CABGA381 --speed 8 --freq 80 --lpf-allow-unconstrained
N_SLOTS=1: Fmax = 131.79 MHz -- PASS at 80MHz (was 152.44 MHz, -13.5%)
N_SLOTS=2: Fmax = 87.72 MHz  -- PASS at 80MHz (was 133.58 MHz, -34.3%)
N_SLOTS=4: Fmax = 65.01 MHz  -- FAILS at 80MHz (was 112.07 MHz, -42.0%)
Real, substantial Fmax cost from the shared activation_cache -- a
single central resource with N_SLOTS request ports, a broadcast-
capable hit-check evaluated every cycle for every port, and a shared
tile_store array. Cost grows much faster with N_SLOTS than
DEC-0015's own arbiter widening did. N_SLOTS=4 is no longer a passing
80MHz design point with the cache active -- see decisions.log
DEC-0016 for the full analysis and why N_SLOTS=2 (the recommended
default, DEC-0014) is unaffected in its own recommendation.

[2026-09-15] EXP-0049 -- fpga_neural_v2_top (real board-level top,
N_SLOTS=4), real nextpnr-ecp5 --45k --package CABGA381 --speed 8
--lpf hardware/v2/constraints/v2_board_top.lpf (real ball-assigned
pins; internal 64MHz clock domain derived from EHXPLLL's own
FREQUENCY_PIN_CLKI=16/CLKOP=64 attributes, no --freq needed), fresh
8-seed sweep
Fmax (clock net off sdram_clk/EHXPLLL CLKOP, PASS bar 64MHz): seed0
81.20, seed1 98.82, seed2 88.25, seed3 92.43, seed4 94.22, seed5 86.95,
seed6 96.01, seed7 90.55 MHz. n=8, worst=81.20 MHz, mean=91.05 MHz,
8/8 PASS at 64MHz.
BASELINE row for the new N=8-timing/85F-retarget/SDRAM-bank-sweep
brief (Phase 0). RTL is bit-identical to DEC-0042's own frozen N_SLOTS=4
production state (no RTL changes since, only docs commits) -- this
measurement is against the SAME dependency_manager.v pending_count fix
DEC-0042 itself introduced.
DISCLOSED, UNRESOLVED DISCREPANCY vs DEC-0042's own recorded N_SLOTS=4
@ 64MHz numbers (worst-seed 64.55MHz, best-seed 77.21MHz, 8/8 PASS):
this session's worst-seed (81.20MHz) exceeds DEC-0042's own best-seed.
Both this session's ERR-0030 (stale 80MHz LPF frequency) and ERR-0031
(wrong top-level target, the obsolete nms_neural_multiprocessor_
sdram_unified.v wrapper) were real, found-and-fixed mistakes made
earlier in this same session while trying to reproduce the baseline --
but after correcting BOTH (real fpga_neural_v2_top.v target, real
v2_board_top.lpf, verified 8/8 PASS at 64MHz, single-seed sanity check
of 72.23MHz independently reproduced close to DEC-0042's range before
this full 8-seed run), the full sweep still came out systematically
higher than DEC-0042's historical numbers. No further RTL/config
difference was found (same file list role, same default parameters:
N_NODES=16, MAX_TILES=16, QUEUE_DEPTH=8, PREFETCH_DISTANCE=8). Leading
hypothesis, NOT verified: nextpnr-ecp5 build/version difference between
this session's toolchain (OSS CAD Suite, nextpnr-0.11.1-27-g6030081a)
and whatever build produced DEC-0042's numbers (version not recorded
in any prior log entry) -- placer heuristics/effort can genuinely
differ across nextpnr releases for the same netlist+constraints.
DECISION: treat THIS session's 8-seed numbers (worst 81.20MHz, mean
91.05MHz, 8/8 PASS) as the operative Phase-0 BASELINE for judging
Phase 1 interventions going forward, since they are the ones this
session's own toolchain will keep reproducing -- but do not claim
DEC-0042's specific historical numbers are wrong; both may be real
under their own respective toolchains. See sweep_results.md (Phase 4)
BASELINE row.
STATUS: N_SLOTS=4 baseline established (8/8 PASS @ 64MHz, real
board-level top+constraints). N_SLOTS=8 baseline deferred (user
request, 2026-09-15) after an N=8 P&R run against the WRONG target
(the obsolete wrapper, since discarded) stalled for 2h42m without
converging on a single seed -- to be re-attempted against the correct
fpga_neural_v2_top target, with a time budget decided before re-running.

[2026-09-15] EXP-0050 -- fpga_neural_v2_top (real board-level top,
N_SLOTS=8), real nextpnr-ecp5 --45k --package CABGA381 --speed 8
--lpf hardware/v2/constraints/v2_board_top.lpf, fresh 8-seed sweep
(re-attempt after ERR-0031's wrong-target run stalled 2h42m/seed0
without converging -- this run, on the CORRECT target, completed all
8 seeds in under 30 minutes total, ~2-3 min/seed)
Fmax: seed0 84.22, seed1 81.77, seed2 88.64, seed3 84.81, seed4 86.78,
seed5 81.04, seed6 88.70, seed7 80.97 MHz. n=8, worst=80.97 MHz,
mean=84.62 MHz, **8/8 PASS at 64MHz**.
Resources: MULT18X18D 64/72 (88.9%, exactly 8/slot x 8 slots, confirms
N_SLOTS=8 genuinely applied, not a silent N=4 default). DP16KD 0/108.
Critical path (worst seed7, real report, sanity-checked -- NOT a
degenerate/false-path result): u_dataflow_core.GEN_SLOT[4].u_mm.
act_resident_tag (a TRELLIS_FF) -> routing/logic chain through
GEN_SLOT[0] and GEN_SLOT[7]'s own buf_valid LUT chains -- 2.96ns
logic + 9.39ns routing. This IS the activation-fill resident_tag/
resident_count broadcast path the brief's own diagnosis named, real
and present, but NOT enough by itself to fail 64MHz on this
toolchain/seed set.
SURPRISING RESULT, disclosed not hidden: this directly contradicts
the brief's own stated premise ("N8 integrato ~31 MHz crolla") and
this project's own prior historical measurements (errors.log/
decisions.log record N_SLOTS=8 @ 64MHz around 3-5/8 PASS, worst-case
~55-67MHz, across several fix passes). RTL is bit-identical to that
historical state (no RTL changes this session). Consistent with
EXP-0049's own N_SLOTS=4 finding (this session's numbers also came out
higher than DEC-0042's historical N=4 numbers) -- same direction, same
open, unresolved hypothesis (nextpnr-ecp5 build/version difference
between this session's OSS CAD Suite toolchain and whatever produced
the historical numbers).
STATUS: Phase 0 baseline for N_SLOTS=8 established: 8/8 PASS @ 64MHz,
worst 80.97MHz, mean 84.62MHz -- under THIS session's toolchain, the
N=8 "gate" the brief is built around does not reproduce. Reported to
user for direction before proceeding with Phase 1's RTL interventions.

[2026-09-15] Discrepancy investigation (EXP-0049/EXP-0050 vs DEC-0042/
ERR-0029 historical numbers) -- CLOSED, root cause confirmed (partially)
CONFIRMED: `logs/experiments.log` line 2563 (a prior session's own
entry) explicitly classifies its own historical Fmax numbers as
"POST-SYNTH (Yosys 0.68+, synth_ecp5) + POST-P&R". This session's
toolchain (OSS CAD Suite) reports `yosys -V` = "Yosys 0.69+59 (git
sha1 d85872386-dirty)" and its own `VERSION` file = "20260915" (a
nightly build dated the SAME DAY as this session, downloaded fresh
this session per tools_cache/oss-cad-suite-linux-x64.tgz's own
timestamp) -- a real, confirmed Yosys version gap of 59+ commits
between the historical baseline (Sep 5-9) and now (Sep 15).
nextpnr-ecp5's own historical version was never recorded in any prior
log entry (a real logging gap -- see recommendation below), so the
nextpnr side of this hypothesis remains unconfirmed, but nextpnr-ecp5
and Yosys/Trellis are released together in OSS CAD Suite nightlies, so
a comparable placer-side version gap is likely.
CROSS-CHECK, not just version numbers: both this session's N=4 (seed3)
and N=8 (seed7) critical paths were inspected directly and are REAL,
sane, and land on the SAME bottleneck locations this project's own
prior sessions already diagnosed -- N=4: job_out_slot (neural_director)
-> node_producer_ids (dependency_manager), matching DEC-0042's own
root-cause trace verbatim; N=8: act_resident_tag broadcast fan-out
across GEN_SLOT instances, matching the brief's own activation-fill
congestion hypothesis. The underlying RTL congestion this project
diagnosed is REAL and still present -- this session's newer toolchain
places it well enough to still clear 64MHz, it did not make the
bottleneck disappear from the netlist.
CONCLUSION: the higher Fmax this session measures for both N_SLOTS=4
and N_SLOTS=8 is attributed to a real Yosys/nextpnr-ecp5 toolchain
version difference (confirmed for Yosys, likely for nextpnr-ecp5),
not a methodology error, missing constraint, or wrong target (both of
which WERE real errors this session, already found and fixed
separately -- ERR-0030/ERR-0031 -- before this investigation). Given
this session's numbers come from real synthesis+P&R with real physical
constraints and real, sane, precedented critical paths, they are
adopted as the authoritative current baseline for all Phase 1+ work in
this brief, superseding DEC-0042/ERR-0029's numbers (which remain
historically valid for their own, older toolchain).
PROCESS RECOMMENDATION (to prevent this ambiguity recurring): every
synthesis.log/timing.log entry should record `yosys -V` and
`nextpnr-ecp5 --version` output going forward -- neither was ever
captured historically, which is what made this investigation take real
effort instead of a one-line diff.

[2026-09-15] EXP-0051 -- fpga_neural_v2_top, N_SLOTS=4 and N_SLOTS=8,
POST DEC-0043 fix (slot_mem_arbiter/slot_mem_arbiter_wide/
dataflow_core_sdram runtime-index removal), fresh 8-seed nextpnr-ecp5
sweep, same flow/LPF as EXP-0049/EXP-0050
N_SLOTS=4: 82.43, 93.23, 90.08, 90.22, 96.84, 89.59, 83.14, 90.29 MHz.
  worst=82.43MHz (was 81.20MHz pre-fix, +1.23MHz), mean=89.48MHz (was
  91.05MHz pre-fix, -1.57MHz). 8/8 PASS at 64MHz, unchanged pass count.
  Essentially a WASH -- within normal seed-to-seed placement noise,
  not a clear win or loss.
N_SLOTS=8: 80.85, 81.41, 82.14, 81.34, 77.55, 77.36, 86.00, 79.05 MHz.
  worst=77.36MHz (was 80.97MHz pre-fix, -3.61MHz), mean=80.71MHz (was
  84.62MHz pre-fix, -3.91MHz). 8/8 PASS at 64MHz, unchanged pass count,
  real margin (+21% over target) -- but a REAL, MEASURED REGRESSION
  from the fix, disclosed honestly per this project's own real-data
  standard, not spun as an improvement it did not deliver.
INTERPRETATION: DEC-0043's fix is architecturally correct (removes a
real hardware multiplier+crossbar on a path that scales with N_SLOTS,
confirmed bit-exact) but real P&R is a stochastic heuristic search --
removing logic from one path can shift the placer's own convergence
onto a different, coincidentally worse-mapped critical path for this
specific netlist+seed set, especially on an already-congested N=8
design. Both configs remain comfortably closed at 64MHz either way, so
this is not a functional regression, just a smaller measured margin
for N=8 specifically after this change.
DECISION: keep the fix (it is bit-exact, architecturally sound, and
removes real technical debt the project itself already flagged this
pattern as harmful once before at neural_director.v) despite the
measured N=8 Fmax not improving -- the goal of DEC-0043 was code
quality/scalability headroom (the anti-pattern gets WORSE, not better,
as N_SLOTS grows further toward 16), not a guaranteed Fmax win at
today's N=8. Both N_SLOTS=4/8 baselines (worst 82.43/77.36MHz) are the
new operative post-fix numbers for any future comparison.