Hi Alexander,

On Mon, Feb 10, 2025 at 05:00:11PM +0100, Alexander Stein wrote:

The fracn gppll PLL so far only supports rates from a rate table passed
during initialization. Calculating PLL settings dynamically helps audio
applications to get their desired rates, so support for this is added
in this patch.

The strategy to get to the PLL setting for a rate is:

- The rate table is searched for suitable rates, so for standard rates the
  same settings are used as without this patch
- Then try to only adjust mfn, on fractional PLLs only, which specifies
  the fractional part of the PLL. This setting can be changed without
  glitches on the output and is therefore preferred

I wonder if this part is worth it. There might be cases in which a
glitch free switch is required, but without being able to enforce
a glitch free switch we can't rely on it.

Also this makes the result depend on the current PLL settings, so the
result is no longer reproducible. I.e. switching from a fari away
frequency to the desired frequency might yield in different settings
than switching from a nearby frequency to the desired frequency.

Finally I think the glitch free switch doesn't work currently, because
the PLL is fully disabled and re-enabled unconditionally in
clk_fracn_gppll_set_rate().

That said, glitch free switching would be great to have sometimes.

- As a last resort the best settings are calculated dynamically

Implementation is inspired by commit b09c68dc57c9d ("clk: imx: pll14xx:
Support dynamic rates")

Signed-off-by: Alexander Stein <redacted>
---

...

+			if (pll->flags & CLK_FRACN_GPPLL_FRACN) {
+				if (!dist) {
+					/* Disable fractional part upon exact match */
+					mfd = 1;
+					mfn = 0;
+				} else {
+					mfd = 100;
+					mfd = clamp(mfd, PLL_MFD_MIN, PLL_MFN_MAX);

With mfd = 100 this clamp looks like a no-op. Do we need this?

Sascha

-- 
Pengutronix e.K.                           |                             |
Steuerwalder Str. 21                       | http://www.pengutronix.de/  |
31137 Hildesheim, Germany                  | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |