Messages in this thread | | | Subject | Re: [PATCH 1/3] clk: meson: mpll: properly handle spread spectrum | From | Jerome Brunet <> | Date | Sat, 30 Mar 2019 00:07:53 +0100 |
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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@baylibre.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@baylibre.com> > > --- > > drivers/clk/meson/clk-mpll.c | 9 ++++++--- > > drivers/clk/meson/clk-mpll.h | 1 + > > 2 files changed, 7 insertions(+), 3 deletions(-) > > > > diff --git a/drivers/clk/meson/clk-mpll.c b/drivers/clk/meson/clk-mpll.c > > index 64d31c8ba3d0..2d39a8bc367c 100644 > > --- a/drivers/clk/meson/clk-mpll.c > > +++ b/drivers/clk/meson/clk-mpll.c > > @@ -141,9 +141,12 @@ static void mpll_init(struct clk_hw *hw) > > /* Enable the fractional part */ > > meson_parm_write(clk->map, &mpll->sdm_en, 1); > > > > - /* Set additional fractional part enable if required */ > > - if (MESON_PARM_APPLICABLE(&mpll->ssen)) > > - meson_parm_write(clk->map, &mpll->ssen, 1); > > + /* Set spread spectrum if possible */ > > + if (MESON_PARM_APPLICABLE(&mpll->ssen)) { > > + unsigned int ss = > > + mpll->flags & CLK_MESON_MPLL_SPREAD_SPECTRUM ? 1 : 0; > > + meson_parm_write(clk->map, &mpll->ssen, ss); > > + } > this changes the "ssen" flag on all supported clocks from 1 (before > this patch) to 0 (after this patch). > is this on purpose and how does it affect existing clocks?
Yes, none of our application require spread spectrum
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 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
> > Martin
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