Re: [V3] tty: serial: qcom-geni-serial: Fix get_clk_div_rate() which otherwise could return a sub-optimal clock rate.

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On 7/8/2022 6:41 PM, Greg KH wrote:
On Fri, Jul 08, 2022 at 12:47:37AM +0530, Vijaya Krishna Nivarthi wrote:
In the logic around call to clk_round_rate(), for some corner conditions,
get_clk_div_rate() could return an sub-optimal clock rate. Also, if an
exact clock rate was not found lowest clock was being returned.

Search for suitable clock rate in 2 steps
a) exact match or within 2% tolerance
b) within 5% tolerance
This also takes care of corner conditions.

Fixes: c2194bc999d4 ("tty: serial: qcom-geni-serial: Remove uart frequency table. Instead, find suitable frequency with call to clk_round_rate")
Signed-off-by: Vijaya Krishna Nivarthi <quic_vnivarth@xxxxxxxxxxx>
---
v3: simplified algorithm further, fixed robot compile warnings
v2: removed minor optimisations to make more readable
v1: intial patch contained slightly complicated logic
---
  drivers/tty/serial/qcom_geni_serial.c | 88 +++++++++++++++++++++--------------
  1 file changed, 53 insertions(+), 35 deletions(-)

diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
index 2e23b65..ac2df1c 100644
--- a/drivers/tty/serial/qcom_geni_serial.c
+++ b/drivers/tty/serial/qcom_geni_serial.c
@@ -943,52 +943,71 @@ static int qcom_geni_serial_startup(struct uart_port *uport)
  	return 0;
  }
-static unsigned long get_clk_div_rate(struct clk *clk, unsigned int baud,
-			unsigned int sampling_rate, unsigned int *clk_div)
+static unsigned long find_clk_rate_in_tol(struct clk *clk, unsigned int desired_clk,
+			unsigned int *clk_div, unsigned int percent_tol)
  {
-	unsigned long ser_clk;
-	unsigned long desired_clk;
-	unsigned long freq, prev;
+	unsigned long freq;
  	unsigned long div, maxdiv;
-	int64_t mult;
-
-	desired_clk = baud * sampling_rate;
-	if (!desired_clk) {
-		pr_err("%s: Invalid frequency\n", __func__);
-		return 0;
-	}
+	u64 mult;
+	unsigned long offset, abs_tol, achieved;
+ abs_tol = div_u64((u64)desired_clk * percent_tol, 100);
  	maxdiv = CLK_DIV_MSK >> CLK_DIV_SHFT;
-	prev = 0;
-
-	for (div = 1; div <= maxdiv; div++) {
-		mult = div * desired_clk;
-		if (mult > ULONG_MAX)
+	div = 1;
+	while (div <= maxdiv) {
+		mult = (u64)div * desired_clk;
+		if (mult != (unsigned long)mult)
  			break;
- freq = clk_round_rate(clk, (unsigned long)mult);
-		if (!(freq % desired_clk)) {
-			ser_clk = freq;
-			break;
-		}
+		offset = div * abs_tol;
+		freq = clk_round_rate(clk, mult - offset);
- if (!prev)
-			ser_clk = freq;
-		else if (prev == freq)
+		/* Can only get lower if we're done */
+		if (freq < mult - offset)
  			break;
- prev = freq;
+		/*
+		 * Re-calculate div in case rounding skipped rates but we
+		 * ended up at a good one, then check for a match.
+		 */
+		div = DIV_ROUND_CLOSEST(freq, desired_clk);
+		achieved = DIV_ROUND_CLOSEST(freq, div);
+		if (achieved <= desired_clk + abs_tol &&
+			achieved >= desired_clk - abs_tol) {
+			*clk_div = div;
+			return freq;
+		}
+
+		div = DIV_ROUND_UP(freq, desired_clk);
  	}
- if (!ser_clk) {
-		pr_err("%s: Can't find matching DFS entry for baud %d\n",
-								__func__, baud);
-		return ser_clk;
+	return 0;
+}
+
+static unsigned long get_clk_div_rate(struct clk *clk, unsigned int baud,
+			unsigned int sampling_rate, unsigned int *clk_div)
+{
+	unsigned long ser_clk;
+	unsigned long desired_clk;
+
+	desired_clk = baud * sampling_rate;
+	if (!desired_clk) {
+		pr_err("%s: Invalid frequency\n", __func__);
Note, this is a driver, ALWAYS use dev_err() and friends instead.

Also do not allow userspace to flood the kernel logs like this looks is
possible, this should just be dev_dbg().

And of course, never use __func__, it's not needed anymore for
dev_dbg().

Ok.


+		return 0;
Why if you have a error, are you returning 0?

Yes, and it has been so earlier too.

0 is an invalid clock rate and will be handled accordingly by caller.

  	}
- *clk_div = ser_clk / desired_clk;
-	if (!(*clk_div))
-		*clk_div = 1;
+	/*
+	 * try to find a clock rate within 2% tolerance, then within
+	 */
+	ser_clk = find_clk_rate_in_tol(clk, desired_clk, clk_div, 2);
+	if (!ser_clk)
+		ser_clk = find_clk_rate_in_tol(clk, desired_clk, clk_div, 5);
+
+	if (!ser_clk)
+		pr_err("Couldn't find suitable clock rate for %lu\n", desired_clk);
return an error?

dev_err().

As mentioned, we didn't (and don't) return error from here but 0.


+	else
+		pr_debug("desired_clk-%lu, ser_clk-%lu, clk_div-%lu\n",
+			desired_clk, ser_clk, *clk_div);
dev_dbg()?
Ok.

Also, as the kernel test robot says, this does not build cleanly :(

change to dev_dbg should take care of these.

Will do.

Thank you.

Vijay/



thanks,

greg k-h



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