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serial: amba-pl011: Introduce skip_ibrd_fbrd vendor flag

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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>
Link: https://patch.msgid.link/20260225065915.341522-2-kkartik@nvidia.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Kartik Rajput 2026-02-25 12:29:11 +05:30 committed by Greg Kroah-Hartman
parent cc8de922bb
commit 42157639dd
1 changed files with 31 additions and 22 deletions
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53
drivers/tty/serial/amba-pl011.c
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@ -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))