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+# 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):
+
+```c
+#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:
+
+```bash
+mkdir build && cd build
+cmake -DBOARD=raspberry_pi_pico ..
+cmake --build .
+```
+
+Make:
+
+```bash
+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`.
+
+```bash
+# 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.