linux-kernel - Re: [PATCH 1/3] clk: meson: mpll: properly handle spread spectrum

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Date:   Mon, 01 Apr 2019 10:40:20 +0200
From:   Jerome Brunet <jbrunet@...libre.com>
To:     Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc:     Neil Armstrong <narmstrong@...libre.com>,
        linux-amlogic@...ts.infradead.org, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] clk: meson: mpll: properly handle spread spectrum

On Sat, 2019-03-30 at 16:56 +0100, Martin Blumenstingl wrote:
> Hi Jerome,
> 
> On Sat, Mar 30, 2019 at 12:07 AM Jerome Brunet <jbrunet@...libre.com> wrote:
> > On Fri, 2019-03-29 at 20:39 +0100, Martin Blumenstingl wrote:
> > > Hi Jerome,
> > > 
> > > On Fri, Mar 29, 2019 at 4:34 PM Jerome Brunet <jbrunet@...libre.com> wrote:
> > > > The bit 'SSEN' available on some MPLL DSS outputs is not related to the
> > > > fractional part of the divider but to the function called
> > > > 'Spread Spectrum'.
> > > > 
> > > > This function might be used to solve EM issues by adding a jitter on
> > > > clock signal. This widens the signal spectrum and weakens the peaks in it.
> > > > 
> > > > While spread spectrum might be useful for some application, it is
> > > > problematic for others, such as audio.
> > > > 
> > > > This patch introduce a new flag to the MPLL driver to enable (or not) the
> > > > spread spectrum function.
> > > > 
> > > > Fixes: 1f737ffa13ef ("clk: meson: mpll: fix mpll0 fractional part ignored")
> > > > Signed-off-by: Jerome Brunet <jbrunet@...libre.com>
> > > > ---
> > > >  drivers/clk/meson/clk-mpll.c | 9 ++++++---
> > > >  drivers/clk/meson/clk-mpll.h | 1 +
> > > >  2 files changed, 7 insertions(+), 3 deletions(-)
> > > > 

[...]

> > 
> > Yes, none of our application require spread spectrum
> thank you for the explanation. can you please add it to the patch description?

I think I described spread spectrum already in the commit description.

> 
> > The fact is that only 2 MPLL had this bit, mpll0 on gx (without effect) and
> > mpll0 on axg: actually spread spectrum impacts mpll2, making it unusable, as
> > explained in the related patch
> > 
> > > based on the original commit 1f737ffa13ef ("clk: meson: mpll: fix
> > > mpll0 fractional part ignored") it seems that
> > > CLK_MESON_MPLL_SPREAD_SPECTRUM should be set for mpll0 (at least on
> > > GXBB and Meson8b)
> > > 
> > 
> > There a patch specifically targeting gxbb.
> I missed that, sorry for the noise
> 
> > I have checked on GXL and this bit had no effect (fractional part still on, no
> > spread spectrum)
> > 
> > So either we fixed something since then or I messed up when doing the patch
> > initially.
> > 
> > Feel free to cross check
> I tried it on Meson8b and it seems that there's no difference. I've
> done a bit of software archaeology:
> $ grep -R mpll_cntl uboot-2015-01-15-23a3562521/ | grep "=" | head -n3
> uboot-2015-01-15-23a3562521/board/amlogic/m8_k100_1G/firmware/timming.c:
>        .mpll_cntl = 0x600009A9,        //2.5G, fixed
> uboot-2015-01-15-23a3562521/board/amlogic/m8_k100_2G/firmware/timming.c:
>        .mpll_cntl = 0x600009A9,        //2.5G, fixed
> uboot-2015-01-15-23a3562521/board/amlogic/m8b_ft_v1/firmware/timming.c:
> .mpll_cntl = 0x600009A9,        //2.5G, fixed
> 
> Ethernet is still working on my Odroid-C1, so you can add my:
> Tested-by: Martin Blumenstingl<martin.blumenstingl@...glemail.com>
> 
> 
> Regards
> Martin