Patch "serial: 8250_dw: Revert "Improve clock rate setting"" has been added to the 4.15-stable tree

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 the 4.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
serial-8250_dw-revert-improve-clock-rate-setting.patch
and it can be found in the queue-4.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

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

From: Andy Shevchenko <[email protected]>

commit c14b65feac9ebed649d6fe79c6b6d64d21d0287d upstream.

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(-)

@@ -252,31 +252,25 @@ static void dw8250_set_termios(struct ua
 			       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;

Patches currently in stable-queue which might be from [email protected] are

queue-4.15/serial-8250_dw-revert-improve-clock-rate-setting.patch