A measuring instrument, and what it says about where the latency goes

"Too slow for interactive use" is a real complaint and not a number. This adds the
number, and the number says the wire is innocent.

## The instrument

`tools/perfproto.zig` is a small framed protocol - "P4", op, length, CRC-32 of the
payload, payload - shared VERBATIM by the host tool and `examples/uartperf.zig`, so
a frame one writes and the other parses cannot drift. It is imported as a module by
both, not copied.

The checksum is the whole point. RX overrun on this UART is undetected in hardware
and uncounted in the driver, so a byte that never arrived is indistinguishable from
a late one; a throughput figure that is not checksummed is a guess about how fast
data was corrupted. `sink` accumulates a CRC over every payload byte the board
received and `report` hands it back, so the host can prove that what arrived is
what it sent.

`tools/rtt.zig` is the two timing functions: `roundTrip` and `measure`. Round trip
is to the FIRST response byte, deliberately. A renderer that starts drawing in 8 ms
and finishes in 130 ms feels immediate; one that thinks for 130 ms and then draws in
8 ms feels broken; waiting for the wire to fall quiet cannot tell them apart. Time
to the last byte is recorded separately as `settle`. Microseconds, because at 115200
one byte is 87 us and a millisecond clock quantises the answer into buckets eleven
bytes wide.

`tools/bench_main.zig` is `p4-bench`: `--link` for the ceiling, `--editor` for how
much of it the editor uses, `--sweep` for one controlled variable at a time with
`--csv` raw per-trial output.

## What it measured

The link is essentially perfect: 11,496 B/s up and 11,413 B/s down, 99.8% of
capacity in both directions, CRC verified over 32,768 B each way, zero corruption.
Typing at 6 to 100 keys/s loses nothing and never uses more than 9% of the wire, so
H5 - "typing loses input" - is refuted.

Latency is compute per input event, not transmission. A 40-byte motion and a
206-byte insert-and-escape cost the SAME round trip to within 0.3 ms, across a
five-fold range of output. That is why raising the baud cannot fix typing: there is
almost no wire in it.

And an edit costs the whole document. Round trip against characters already in the
line is a straight line at 54.3 us per character per keystroke - 17.0 ms at an empty
line, 25.6 ms at 160. On a ~90 MHz core that is ~5,000 cycles per character, far
more than a copy alone, so the full-buffer copy the source does is accompanied by at
least one more full pass.

One controlled intervention: building the editor object ReleaseFast instead of
ReleaseSmall cuts the fixed cost 13% and the per-character cost 36%, for 35% more
flash (809,536 B of a 1,536,000 B partition). Its advantage grows with the document.
Nothing else measured comes close to that ratio.

## Three bugs found while building it

The responder printed garbage and looked dead: it read `.rodata` before evicting the
bootloader's stale cache lines. `flushFlashCache` moved from `src/pardes/app.zig` to
`soc.zig` with its measured evidence, since every application that touches `.rodata`
after hand-over needs it and exactly one file knew that.

Then it booted, printed its marker and went silent after ten seconds:
`rst:0x10 (CHIP_LP_WDT_RESET)`. The bootloader arms the RTC watchdog and expects the
application to take it over. Only the editor ever did.

`serial.Port.drain()` drains INPUT, not output - so timing a transfer to it reported
202% of the wire's capacity and ate the reply. Added `flushOutput` (tcdrain), named
so the two cannot be confused again.

Also: Zig 0.16 emits an explicit `+` for a non-negative SIGNED integer whenever a
width is given (std/Io/Writer.zig:1548-1559), which put a `+` in front of every
number in the first tables.

## The report

`experiments/report.typ` reads the raw CSVs and computes its own figures, so a
re-run changes the document instead of contradicting it. It states five hypotheses,
settles each against one experiment, and is explicit about the one that failed: the
geometry sweep is confounded, because characters accumulated across conditions and
the length experiment then proved that matters. It is reported as unsupported rather
than dressed up as a result.

1 files changed, 45 insertions, 0 deletions

diff --git a/src/soc.zig b/src/soc.zig
index 172966e..350948a 100644
--- a/src/soc.zig
+++ b/src/soc.zig
@@ -114,6 +114,51 @@ pub const rom = struct {
     }
 };
 
+/// Evict every flash-mapped cache line the bootloader left behind, by reading more flash than the
+/// caches can hold. Call it before the first byte of `.rodata` is touched.
+///
+/// This is a workaround for a real defect in the hand-over, not a tidiness measure. The second-stage
+/// bootloader leaves lines cached against a mapping it then replaces, so an application reads its
+/// own `.rodata` and gets its own `.text` back - **deterministically**, which is exactly what makes
+/// it look like anything other than a cache. Measured on this die: a load at `0x40035A1C` returned
+/// `93 85 85 0f` before eviction and `3c ee 08 40` after, and the second is what the image holds
+/// there. A string literal read before this runs is machine code, so a firmware whose first act is
+/// to print a marker prints garbage and looks like it never booted at all.
+///
+/// Everything cheaper was tried first and every one of them said the hardware was fine, which is
+/// why the list is here rather than being rediscovered:
+///
+///   * The MMU table is correct. Entries 0..9 read `0x1001`..`0x100a` - the valid bit plus physical
+///     page N+1 - which is exactly what the image builder's single flash-to-vaddr anchor requires,
+///     and 10..11 are unmapped as they should be. Read off the die through
+///     `SPI_MEM_C_MMU_ITEM_INDEX_REG`, not inferred.
+///   * The flash is correct. `zig build flash` verifies an MD5 of what the ROM stored, and the
+///     image matches the ELF byte for byte at the addresses that misread.
+///   * The page size is not in question: hardwired to 64 KiB on this chip
+///     (`hal/esp32p4/mmu_ll.h:126-130` returns `MMU_PAGE_64KB` and the setter asserts it).
+///   * Not fragmentation of the mapping either: the bad bytes arrive in one contiguous run of
+///     >= 192 B, not in 64-byte lines, and they are identical across three resets and two
+///     reflashes - determinism is what kept this looking like anything but a cache.
+///
+/// 512 KiB is four times the 128 KiB the L2 measured at (`examples/memprobe.zig` found real RAM
+/// stopping at `0x4FFA0000`, the cache taking the rest), with the L1s smaller still.
+///
+/// `rom.Cache_Invalidate_All` is the instrument that ought to do this and does not: called from an
+/// image the ROM did not launch, it faults inside the ROM with its argument stranded in `a2`.
+/// Capacity eviction needs no preconditions, which is the whole reason it is what ships. 512 KiB at
+/// a 64-byte stride is 8,192 loads, once per boot.
+pub fn flushFlashCache() void {
+    var sink: u32 = 0;
+    var p: u32 = 0x4000_0000;
+    while (p < 0x4008_0000) : (p += 64) {
+        sink +%= @as(*volatile u32, @ptrFromInt(p)).*;
+    }
+    // Consumed through a volatile store, or the optimiser drops the whole loop as dead.
+    @as(*volatile u32, &flush_sink).* = sink;
+}
+
+var flush_sink: u32 = 0;
+
 /// Busy-wait for a number of CPU cycles, using the cycle counter rather than the mask ROM. Useful
 /// when an image must not depend on ROM entry points at all, and for delays shorter than the ROM's
 /// microsecond granularity.