linux-kernel - Re: [PATCH] clk: meson: gxbb: Add the spread spectrum bit for MPLL0 on GXBB

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Message-ID: <1jfssumuxi.fsf@starbuckisacylon.baylibre.com>
Date:   Thu, 21 Oct 2021 14:18:01 +0200
From:   Jerome Brunet <jbrunet@...libre.com>
To:     Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc:     linux-amlogic@...ts.infradead.org,
        Neil Armstrong <narmstrong@...libre.com>,
        linux-clk@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        Christian Hewitt <christianshewitt@...il.com>
Subject: Re: [PATCH] clk: meson: gxbb: Add the spread spectrum bit for MPLL0
 on GXBB


On Wed 20 Oct 2021 at 20:16, Martin Blumenstingl <martin.blumenstingl@...glemail.com> wrote:

> Hi Jerome,
>
> On Mon, Oct 18, 2021 at 2:03 PM Jerome Brunet <jbrunet@...libre.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.

Fine by me if you are confident that HHI_MPLL_CNTL7[15] has indeed no
effect. Maybe add a comment about this oddity.

>
>
> Best regards,
> Martin