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drivers: serial_sifive: Fix baud rate calculation

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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>
This commit is contained in:
Atish Patra 2019-02-25 08:15:02 +00:00 committed by Andes
parent b630d57d0a
commit a3682008a0
1 changed files with 26 additions and 2 deletions
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28
drivers/serial/serial_sifive.c
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@ -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)