[PATCH 1/4] serial: amba-pl011: Introduce skip_ibrd_fbrd vendor flag

Kartik Rajput
2026-02-25T06:59:46Z

The NVIDIA Tegra264 UART has a broken fractional baud rate
divisor register. Using IBRD and FBRD may cause the baud rate
to fall outside the required tolerance.

Introduce the skip_ibrd_fbrd vendor flag to skip IBRD/FBRD
programming. When set, the baud rate is derived directly from the
UART clock rate using a fixed divisor.

Signed-off-by: Kartik Rajput <kkartik@nvidia.com>
---
drivers/tty/serial/amba-pl011.c | 53 +++++++++++++++++++--------------
1 file changed, 31 insertions(+), 22 deletions(-)

diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index 7f17d288c807..79e1c937a600 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -114,6 +114,7 @@ struct vendor_data {
bool cts_event_workaround;
bool always_enabled;
bool fixed_options;
+ bool skip_ibrd_fbrd;

unsigned int (*get_fifosize)(struct amba_device *dev);
};
@@ -2115,11 +2116,6 @@ pl011_set_termios(struct uart_port *port, struct ktermios *termios,
uap->dmarx.poll_rate = DIV_ROUND_UP(10000000, baud);
#endif

- if (baud > port->uartclk / 16)
- quot = DIV_ROUND_CLOSEST(port->uartclk * 8, baud);
- else
- quot = DIV_ROUND_CLOSEST(port->uartclk * 4, baud);
-
switch (termios->c_cflag & CSIZE) {
case CS5:
lcr_h = UART01x_LCRH_WLEN_5;
@@ -2190,21 +2186,28 @@ pl011_set_termios(struct uart_port *port, struct ktermios *termios,
old_cr &= ~ST_UART011_CR_OVSFACT;
}

- /*
- * Workaround for the ST Micro oversampling variants to
- * increase the bitrate slightly, by lowering the divisor,
- * to avoid delayed sampling of start bit at high speeds,
- * else we see data corruption.
- */
- if (uap->vendor->oversampling) {
- if (baud >= 3000000 && baud < 3250000 && quot > 1)
- quot -= 1;
- else if (baud > 3250000 && quot > 2)
- quot -= 2;
+ if (!uap->vendor->skip_ibrd_fbrd) {
+ if (baud > port->uartclk / 16)
+ quot = DIV_ROUND_CLOSEST(port->uartclk * 8, baud);
+ else
+ quot = DIV_ROUND_CLOSEST(port->uartclk * 4, baud);
+
+ /*
+ * Workaround for the ST Micro oversampling variants to
+ * increase the bitrate slightly, by lowering the divisor,
+ * to avoid delayed sampling of start bit at high speeds,
+ * else we see data corruption.
+ */
+ if (uap->vendor->oversampling) {
+ if (baud >= 3000000 && baud < 3250000 && quot > 1)
+ quot -= 1;
+ else if (baud > 3250000 && quot > 2)
+ quot -= 2;
+ }
+ /* Set baud rate */
+ pl011_write(quot & 0x3f, uap, REG_FBRD);
+ pl011_write(quot >> 6, uap, REG_IBRD);
}
- /* Set baud rate */
- pl011_write(quot & 0x3f, uap, REG_FBRD);
- pl011_write(quot >> 6, uap, REG_IBRD);

/*
* ----------v----------v----------v----------v-----
@@ -2374,6 +2377,7 @@ static void pl011_console_get_options(struct uart_amba_port *uap, int *baud,
int *parity, int *bits)
{
unsigned int lcr_h, ibrd, fbrd;
+ unsigned int clkdiv;

if (!(pl011_read(uap, REG_CR) & UART01x_CR_UARTEN))
return;
@@ -2393,10 +2397,15 @@ static void pl011_console_get_options(struct uart_amba_port *uap, int *baud,
else
*bits = 8;

- ibrd = pl011_read(uap, REG_IBRD);
- fbrd = pl011_read(uap, REG_FBRD);
+ if (uap->vendor->skip_ibrd_fbrd) {
+ clkdiv = 64;
+ } else {
+ ibrd = pl011_read(uap, REG_IBRD);
+ fbrd = pl011_read(uap, REG_FBRD);
+ clkdiv = 64 * ibrd + fbrd;
+ }

- *baud = uap->port.uartclk * 4 / (64 * ibrd + fbrd);
+ *baud = uap->port.uartclk * 4 / clkdiv;

if (uap->vendor->oversampling &&
(pl011_read(uap, REG_CR) & ST_UART011_CR_OVSFACT))
--
2.43.0