patch "serial: 8250_dw: Revert "Improve clock rate setting"" added to tty-testing

This is a note to let you know that I've just added the patch titled

serial: 8250_dw: Revert "Improve clock rate setting"

to my tty git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git
in the tty-testing branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will be merged to the tty-next branch sometime soon, after it passes testing, and the merge window is open.

If you have any questions about this process, please let me know.

From: Andy Shevchenko <[email protected]>
Date: Fri, 19 Jan 2018 18:02:05 +0200
Subject: serial: 8250_dw: Revert "Improve clock rate setting"

The commit

de9e33bdfa22 ("serial: 8250_dw: Improve clock rate setting")

obviously tries to cure symptoms, and not a root cause.

The root cause is the non-flexible rate calculation inside the corresponding clock driver. What we need is to provide maximum UART divisor value to the clock driver to allow it do the job transparently to the caller.

Since from the initial commit message I have got no clue which clock driver actually needs to be amended, I leave this exercise to the people who know better the case.

Moreover, it seems [1] the fix introduced a regression. And possible even one more [2].

Taking above, revert the commit de9e33bdfa22 for now.

[1]: https://www.spinics.net/lists/linux-serial/msg28872.html
[2]: https://github.com/Dunedan/mbp-2016-linux/issues/29#issuecomment-357583782

---
 drivers/tty/serial/8250/8250_dw.c | 30 ++++++++++++------------------
 1 file changed, 12 insertions(+), 18 deletions(-)

index bda75d317d24..cd1b94a0f451 100644
@@ -252,31 +252,25 @@ 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 i, ret;
+	int ret;
 
 	if (IS_ERR(d->clk) || !old)
 		goto out;
 
-	/* 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);
-
-	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);
+	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);
-		if (!ret)
-			p->uartclk = rate;
-	}
+	clk_prepare_enable(d->clk);
+
+	if (!ret)
+		p->uartclk = rate;
 
 out:
 	p->status &= ~UPSTAT_AUTOCTS;

2.16.0