From de9e33bdfa22e607a88494ff21e9196d00bf4550 Mon Sep 17 00:00:00 2001 From: Ed Blake Date: Tue, 26 Sep 2017 11:40:03 +0100 Subject: [PATCH] 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 Signed-off-by: Greg Kroah-Hartman --- drivers/tty/serial/8250/8250_dw.c | 30 ++++++++++++++++++------------ 1 file changed, 18 insertions(+), 12 deletions(-) diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c index 7e638997bfc2..10b0aca8ae19 100644 --- a/drivers/tty/serial/8250/8250_dw.c +++ b/drivers/tty/serial/8250/8250_dw.c @@ -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;