Re: [V4] tty: serial: qcom-geni-serial: Fix get_clk_div_rate() which otherwise could return a sub-optimal clock rate.
From: Vijaya Krishna Nivarthi
Date: Tue Jul 12 2022 - 08:52:14 EST
On 7/11/2022 11:35 PM, Doug Anderson wrote:
Hi,
On Mon, Jul 11, 2022 at 10:33 AM Vijaya Krishna Nivarthi
<quic_vnivarth@xxxxxxxxxxx> 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>
---
v4: replaced pr_dbg calls with dev_dbg
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 | 89 +++++++++++++++++++++--------------
1 file changed, 54 insertions(+), 35 deletions(-)
diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
index 2e23b65..f88b042 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, struct device *dev,
+ 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) {
+ dev_dbg(dev, "Invalid frequency\n");
+ return 0;
}
- *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)
+ dev_err(dev, "Couldn't find suitable clock rate for %d\n", desired_clk);
+ else
+ dev_dbg(dev, "desired_clk-%d, ser_clk-%d, clk_div-%d\n",
+ desired_clk, ser_clk, *clk_div);
Pretty sure the robot is going to yell at you again here. Here is my
analysis in detail:
desired_clk
* type: unsigned long
* proper format code: %lu
* marginally acceptable code: %ld
* format code you used in v3: %lu
* did robot yell at you about this in v3: no
* format code you used in v4: %d
* will robot yell at you in v4: YES
ser_clk:
* type: unsigned long
* proper format code: %lu
* marginally acceptable code: %ld
* format code you used in v3: %lu
* did robot yell at you about this in v3: no
* format code you used in v4: %d
* will robot yell at you in v4: YES
*clk_div:
* type: unsigned int
* proper format code: %u
* marginally acceptable code: %d
* format code you used in v3: %lu
* did robot yell at you about this in v3: YES
* format code you used in v4: %d
* will robot yell at you in v4: no
* should you change it to %u: yes
-Doug
Thank you very much.
For some reason robot seems to be more forgiving towards dev_funcs than
pr_funcs, at least it did not yell at v4.
Also I found usage of %d for uls at many locations.
Made the changes and uploaded V5.
-Vijay/