From 753d2a34667b41df648d9bc50b4d2db68523cdb5 Mon Sep 17 00:00:00 2001 From: raiden00pl Date: Tue, 21 Jul 2026 15:07:56 +0200 Subject: [PATCH] drivers/serial: fetch uart_writev data from the iovec segment 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 Assisted-by: Claude Code --- drivers/serial/serial.c | 24 ++++++++++++++++++++++-- 1 file changed, 22 insertions(+), 2 deletions(-) diff --git a/drivers/serial/serial.c b/drivers/serial/serial.c index c2c76b5249d..8c4704cd3c7 100644 --- a/drivers/serial/serial.c +++ b/drivers/serial/serial.c @@ -1480,6 +1480,9 @@ static ssize_t uart_writev(FAR struct file *filep, FAR struct uio *uio) { FAR struct inode *inode = filep->f_inode; FAR uart_dev_t *dev = inode->i_private; + FAR const char *segbuf = NULL; + size_t seglen = 0; + size_t nseg = 0; ssize_t nwritten; ssize_t buflen; bool oktoblock; @@ -1554,9 +1557,22 @@ static ssize_t uart_writev(FAR struct file *filep, FAR struct uio *uio) */ uart_disabletxint(dev); - for (; buflen; uio_advance(uio, 1), buflen--) + for (; buflen; buflen--, nseg++) { - uio_copyto(uio, 0, &ch, 1); + if (nseg >= seglen) + { + /* Consume the current segment and take a pointer to the next. + * uio_advance() steps over any zero-length iovec. + */ + + uio_advance(uio, nseg); + segbuf = (FAR const char *)uio->uio_iov->iov_base + + uio->uio_offset_in_iov; + seglen = uio->uio_iov->iov_len - uio->uio_offset_in_iov; + nseg = 0; + } + + ch = segbuf[nseg]; ret = OK; /* Do output post-processing */ @@ -1631,6 +1647,10 @@ static ssize_t uart_writev(FAR struct file *filep, FAR struct uio *uio) } } + /* Consume the bytes that were successfully queued */ + + uio_advance(uio, nseg); + if (dev->xmit.head != dev->xmit.tail) { #ifdef CONFIG_SERIAL_TXDMA