[U-Boot] [PATCH 08/11] drivers: serial_sifive: Fix baud rate calculation

Anup Patel
2019-01-17T11:03:53Z

From: Atish Patra <atish.patra@wdc.com>

Compute the baud rate multipler with more precision.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
---
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), &regs->div);
+ writel((uart_min_clk_divisor(clock, baud)), &regs->div);
}

static void _sifive_serial_init(struct uart_sifive *regs)
--
2.17.1