Since 00010089b8 uart_writev() takes the data one byte at a time with
uio_copyto() plus uio_advance(). Both of them walk the iovec list and
redo the byte counters for every single byte, so most of the work is
bookkeeping rather than copying. On slow cores this is what limits how
fast the TX buffer can be filled.
Take a pointer to the current iovec segment and read the bytes straight
from it, and move the uio forward once per segment instead of once per
byte. nseg counts only the bytes that really went into the buffer: it
is increased at the end of a loop pass, and that step is skipped when
uart_putxmitchar() fails.
Measured on nRF52840 (Cortex-M4, 64 MHz), 8 MiB write() to a CDC/ACM
port: 223 KB/s to 481 KB/s.
Signed-off-by: raiden00pl <raiden00@railab.me>
Assisted-by: Claude Code
753d2a3466