SCF: FFT + TSQR Testcase

“SCF” is not a standalone accelerator — it is the name of the testcase (tools/generate/mosaic_scf.pl) that combines the FFT and TSQR accelerators in a single MoSAIC mesh. This page walks through that combined configuration.

Tile Layout

A 3x3 mesh, registering the FFT tile as acc1 and the TSQR tile as acc2:

$new_tile{'acc1'} = 'Tile_fft';
$new_tile{'acc2'} = 'Tile_tsqr';

$param{'r'} = 3;
$param{'c'} = 3;

@tile_array = (['pico', 'acc1', 'spad'],
               ['spad', 'spad', 'acc2'],
               ['spad', 'spad', 'spad']);

	Col 0	Col 1	Col 2
Row 0	pico (driver)	acc1 = FFT	spad
Row 1	spad	spad	acc2 = TSQR
Row 2	spad	spad	spad

@pico_program preloads two of the scratchpads with input data files (mosaic_scf_input_1.hex at (1,0), mosaic_scf_input_2.hex at (2,0)), rather than having the pico generate them.

Data Flow

graph LR
    A["spad (1,0)"] -->|mGet forwarded by pico| C["acc1: FFT (0,1)"]
    B["spad (2,0)"] -->|mGet forwarded by pico| C
    C -->|FFT output| D["spad (0,2)"]
    D -->|autonomous request| E["acc2: TSQR (1,2)"]
    E -->|R matrix result| F["spad (1,1)"]
    P["pico (0,0)"] -.orchestrates steps 1-2.-> A
    P -.-> B

1. The pico at (0,0) uses mGet’s dual-address (source + forward-destination) form to pull data out of the two preloaded scratchpads and route it directly into the FFT tile — the pico’s own registers never see the data.
2. The FFT tile computes the transform and forwards its output to the scratchpad at (0,2).
3. The TSQR tile at (1,2) autonomously requests data from that scratchpad (its own RequestData FSM states, no software involvement) and ingests it.
4. TSQR computes the QR decomposition and streams the R-matrix result to the scratchpad at (1,1).

Software: send_msg.c (tools/picorv_c/c_fft_tsqr/)

#define N 1024

int inp_spad_1 = 1 << 12;   // scratchpad tile (1,0)
int inp_spad_2 = 2 << 12;   // scratchpad tile (2,0)

qPut(8,  0x80000000);   // enable/kick the first scratchpad's target
qPut(17, 0x80000000);   // enable/kick the second

int inp_offst = 0;
for (int j = 0; j < 512; j++) {
    mGetH(inp_spad_1 + inp_offst, 3);   // request 8-word burst from scratchpad 1
    mGetD(8 << 12, 0);                  // ...forward result directly to the FFT tile
    inp_offst += 8;
}
inp_offst = 0;
for (int j = 0; j < 512; j++) {
    mGetH(inp_spad_2 + inp_offst, 3);   // request 8-word burst from scratchpad 2
    mGetD(8 << 12, 0);                  // ...forward result directly to the FFT tile
    inp_offst += 8;
}

The mGetH/mGetD pair issues a remote read whose result is forwarded to a second destination address (here, the FFT tile at 8 << 12) rather than returned to the requesting pico — a remote-DMA-style transfer. Across both loops, 8192 words (4096 per scratchpad) are moved from the two preloaded scratchpads into the FFT tile without the pico ever touching the data directly.

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