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serial: 8250_dw: Improve clock rate setting 

Currently dw8250_set_termios sets the input clock to the nearest
achievable rate to baudx16.  If necessary, the input clock is then
divided down to baudx16 using an integer divider within the UART
device, with the divisor calculated in the 8250 core driver.

However, the clock rate set by dw8250_set_termios and subsequently
divided down could be considerably different to the target baudx16
rate, resulting in incorrect operation.  This patch fixes this by
iteratively searching for an input clock rate that is within +/-1.6%
of an integer multiple of the target baudx16 rate.

Signed-off-by: Ed Blake <ed.blake@sondrel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Ed Blake 2017-09-26 11:40:03 +01:00 committed by Greg Kroah-Hartman
parent 6263368c5b
commit de9e33bdfa
1 changed files with 18 additions and 12 deletions
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30
drivers/tty/serial/8250/8250_dw.c
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@ -256,25 +256,31 @@ static void dw8250_set_termios(struct uart_port *p, struct ktermios *termios,
struct ktermios *old)
{
unsigned int baud = tty_termios_baud_rate(termios);
unsigned int target_rate, min_rate, max_rate;
struct dw8250_data *d = p->private_data;
long rate;
int ret;
int i, ret;
if (IS_ERR(d->clk) || !old)
goto out;
clk_disable_unprepare(d->clk);
rate = clk_round_rate(d->clk, baud * 16);
if (rate < 0)
ret = rate;
else if (rate == 0)
ret = -ENOENT;
else
ret = clk_set_rate(d->clk, rate);
clk_prepare_enable(d->clk);
/* Find a clk rate within +/-1.6% of an integer multiple of baudx16 */
target_rate = baud * 16;
min_rate = target_rate - (target_rate >> 6);
max_rate = target_rate + (target_rate >> 6);
if (!ret)
p->uartclk = rate;
for (i = 1; i <= UART_DIV_MAX; i++) {
rate = clk_round_rate(d->clk, i * target_rate);
if (rate >= i * min_rate && rate <= i * max_rate)
break;
}
if (i <= UART_DIV_MAX) {
clk_disable_unprepare(d->clk);
ret = clk_set_rate(d->clk, rate);
clk_prepare_enable(d->clk);
if (!ret)
p->uartclk = rate;
}
out:
p->status &= ~UPSTAT_AUTOCTS;