Messages in this thread | | | From | Martin Blumenstingl <> | Date | Wed, 20 Oct 2021 20:16:29 +0200 | Subject | Re: [PATCH] clk: meson: gxbb: Add the spread spectrum bit for MPLL0 on GXBB |
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Hi Jerome,
On Mon, Oct 18, 2021 at 2:03 PM Jerome Brunet <jbrunet@baylibre.com> wrote: [...] > > The difference here is BIT(14). un-setting BIT(14) (documented as > > EN_DDS0) did not change anything according to Christian's test. > > That also means that SDM, SDM_EN and N2 have the expected values. > > I manually did the maths: > > (2000000000Hz * 16384) / ((16384 * 6) + 12808) = 294909640.7Hz > > which matches what clk_summary sees: > > 294909641Hz > > ... and (2000000000Hz * 16384) / ((16384 * 6) = 333MHz which is fairly close > to what you get w/o flipping the bit This is actually a great hint. So far MPLL clocks have "just worked" for me and I didn't have to work with this. With your explanation it makes sense that SDM_EN makes the hardware use or ignore the SDM value.
[...] > For example yes. I am asking check a bit more what this bit does and > what it does not: > - I need confirmation whether or not it does spread spectrum. Yes this > needs to be observed on a SoC pin, like MCLK with a fairly low divider > to the averaging effect which could partially mask spread spectrum. I did some more tests with Christian. It turns out that on GXBB HHI_MPLL_CNTL7[15] has no impact on the rate seen by meson-clk-msr. On the other hand, HHI_MPLL_CNTL[25] makes MPLL0 use or ignore the SDM value (again, verified through meson-clk-msr).
> - Get an idea what it actually does. The 2 calculations above are an > hint. (Spread spectrum does not change the rate mean value) Indeed! My conclusion is that on GXBB: 1) HHI_MPLL_CNTL[25] doesn't control the spread spectrum setting of MPLL0 - just like you thought 2) HHI_MPLL_CNTL[25] is actually SDM_EN (and HHI_MPLL_CNTL7[15] doesn't seem to have any impact on MPLL0's output rate)
Please let me know if there's anything else we can test. Else I'll send a patch for making HHI_MPLL_CNTL[25] the SDM_EN bit of MPLL0 on GXBB.
Best regards, Martin
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