The base baud value reported is supposed to be the highest baud rate
that can be set for a serial port. The SiFive FU740-C000 SOC's on-chip
UART supports baud rates of up to 1/16 of the input clock rate, which is
the bus clock `tlclk'[1], often at 130MHz in the case of the HiFive
Unmatched board.
However the sifive UART driver reports a fixed value of 115200 instead:
10010000.serial: ttySIF0 at MMIO 0x10010000 (irq = 1, base_baud = 115200) is a SiFive UART v0
10011000.serial: ttySIF1 at MMIO 0x10011000 (irq = 2, base_baud = 115200) is a SiFive UART v0
even though we already support setting higher baud rates, e.g.:
$ tty
/dev/ttySIF1
$ stty speed
230400
The baud base value is computed by the serial core by dividing the UART
clock recorded in `struct uart_port' by 16, which is also the minimum
value of the clock divider supported, so correct the baud base value
reported by setting the UART clock recorded to the input clock rate
rather than 115200:
10010000.serial: ttySIF0 at MMIO 0x10010000 (irq = 1, base_baud = 8125000) is a SiFive UART v0
10011000.serial: ttySIF1 at MMIO 0x10011000 (irq = 2, base_baud = 8125000) is a SiFive UART v0
References:
[1] "SiFive FU740-C000 Manual", v1p3, SiFive, Inc., August 13, 2021,
Section 16.9 "Baud Rate Divisor Register (div)", pp.143-144
Signed-off-by: Maciej W. Rozycki <macro@xxxxxxxxxxx>
Fixes: 1f1496a923b6 ("riscv: Fix sifive serial driver")
---
drivers/tty/serial/sifive.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
linux-serial-sifive-base-baud.diff
Index: linux-macro/drivers/tty/serial/sifive.c
===================================================================
--- linux-macro.orig/drivers/tty/serial/sifive.c
+++ linux-macro/drivers/tty/serial/sifive.c
@@ -998,7 +998,7 @@ static int sifive_serial_probe(struct pl
/* Set up clock divider */
ssp->clkin_rate = clk_get_rate(ssp->clk);
ssp->baud_rate = SIFIVE_DEFAULT_BAUD_RATE;
- ssp->port.uartclk = ssp->baud_rate * 16;
+ ssp->port.uartclk = ssp->clkin_rate;
__ssp_update_div(ssp);
platform_set_drvdata(pdev, ssp);
