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

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Date:   Mon, 1 Apr 2019 19:13:26 +0200
From:   Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:     Jerome Brunet <jbrunet@...libre.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

Hi Jerome,

On Mon, Apr 1, 2019 at 10:40 AM Jerome Brunet <jbrunet@...libre.com> wrote:
>
> 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.
I haven't been clear about this one -  let me do it better this time:
if you have to re-send the patches can you please add a note stating
that "none of our application require spread spectrum" to the
description so it's clear that you're disabling spread spectrum on
purpose.
your explanation about spread spectrum itself is perfectly fine


Regards
Martin