From: Atish Patra atish.patra@wdc.com
Compute the baud rate multipler with more precision.
Signed-off-by: Atish Patra atish.patra@wdc.com Signed-off-by: Anup Patel anup.patel@wdc.com Reviewed-by: Alexander Graf agraf@suse.de Reviewed-by: Lukas Auer lukas.auer@aisec.fraunhofer.de --- drivers/serial/serial_sifive.c | 28 ++++++++++++++++++++++++++-- 1 file changed, 26 insertions(+), 2 deletions(-)
diff --git a/drivers/serial/serial_sifive.c b/drivers/serial/serial_sifive.c index 341728a690..ea4d35d48c 100644 --- a/drivers/serial/serial_sifive.c +++ b/drivers/serial/serial_sifive.c @@ -33,16 +33,40 @@ struct uart_sifive { };
struct sifive_uart_platdata { - unsigned int clock; + unsigned long clock; int saved_input_char; struct uart_sifive *regs; };
+/** + * Find minimum divisor divides in_freq to max_target_hz; + * Based on uart driver n SiFive FSBL. + * + * f_baud = f_in / (div + 1) => div = (f_in / f_baud) - 1 + * The nearest integer solution requires rounding up as to not exceed + * max_target_hz. + * div = ceil(f_in / f_baud) - 1 + * = floor((f_in - 1 + f_baud) / f_baud) - 1 + * This should not overflow as long as (f_in - 1 + f_baud) does not exceed + * 2^32 - 1, which is unlikely since we represent frequencies in kHz. + */ +static inline unsigned int uart_min_clk_divisor(unsigned long in_freq, + unsigned long max_target_hz) +{ + unsigned long quotient = + (in_freq + max_target_hz - 1) / (max_target_hz); + /* Avoid underflow */ + if (quotient == 0) + return 0; + else + return quotient - 1; +} + /* Set up the baud rate in gd struct */ static void _sifive_serial_setbrg(struct uart_sifive *regs, unsigned long clock, unsigned long baud) { - writel((u32)((clock / baud) - 1), ®s->div); + writel((uart_min_clk_divisor(clock, baud)), ®s->div); }
static void _sifive_serial_init(struct uart_sifive *regs)