CDC + MSC Throughput

A deliberately minimal CDC + MSC composite device for measuring pure USB bulk throughput — the ceiling set by the USB link and the TinyUSB driver, with no backing storage or per-byte work in the way.

How it measures the ceiling, not storage

MSC advertises a 1 GiB logical disk (2 Mi × 512-byte blocks) but has no real backing store. Writes are discarded; reads zero-fill only the low LBAs the host scans during enumeration (partition table / GPT header) and otherwise return whatever is already in the transfer buffer — so no memset/copy cost skews the result.
CDC drains its RX in tud_cdc_rx_cb and sources TX from a static zero filler, so dd can push data in either direction over /dev/ttyACMx.

The 1 GiB capacity lets dd run long enough for the rate to stabilise; high-speed peripherals show the most headroom.

USB Descriptors

Interface	Class driver
0–1	CDC (virtual serial)
2	MSC (mass storage)

Configuration

Notable tusb_config.h settings (tuned for throughput):

#define CFG_TUD_CDC              1
#define CFG_TUD_MSC              1
#define CFG_TUD_MSC_EP_BUFSIZE   (TUD_OPT_HIGH_SPEED  ? 4096 : 1024)  // large MSC bulk buffer
#define CFG_TUD_CDC_RX_EPSIZE    (TUD_OPT_HIGH_SPEED ? 2*512 : 2*64)
#define CFG_TUD_CDC_TX_EPSIZE    CFG_TUD_CDC_RX_EPSIZE
#define CFG_TUD_CDC_RX_BUFSIZE   (TUD_OPT_HIGH_SPEED ? 2*512 : 2*64)
#define CFG_TUD_CDC_TX_BUFSIZE   CFG_TUD_CDC_RX_BUFSIZE

Building

CMake:

mkdir build && cd build
cmake -DBOARD=raspberry_pi_pico ..
cmake --build .

Make:

make BOARD=raspberry_pi_pico all

Measuring throughput (Linux)

The MSC disk appears as a raw block device (e.g. /dev/sdX); the CDC port as /dev/ttyACMx.

# MSC read (device → host)
sudo dd if=/dev/sdX of=/dev/null bs=1M count=256 iflag=direct

# MSC write (host → device, discarded)
sudo dd if=/dev/zero of=/dev/sdX bs=1M count=256 oflag=direct

# CDC read (device → host)
dd if=/dev/ttyACM0 of=/dev/null bs=64k count=4096

Pick the right /dev/sdX carefully — writing to the wrong block device destroys data.