Re: [PATCH v6 3/5] clk: imx: add SCCG PLL type

[Abel Vesa]

On Tue, Aug 28, 2018 at 12:11:13PM -0700, Andrey Smirnov wrote:
> On Tue, Aug 28, 2018 at 3:58 AM Abel Vesa <abel.vesa@xxxxxxx> wrote:
> >
> > On Fri, Aug 24, 2018 at 09:40:11AM +0200, Sascha Hauer wrote:
> > > +Cc Andrey Smirnov who made me aware of this issue.
> > >
> > > On Wed, Aug 22, 2018 at 04:48:21PM +0300, Abel Vesa wrote:
> > > > From: Lucas Stach <l.stach@xxxxxxxxxxxxxx>
> > > >
> > > > The SCCG is a new PLL type introduced on i.MX8. Add support for this.
> > > > The driver currently misses the PLL lock check, as the preliminary
> > > > documentation mentions lock configurations, but is quiet about where
> > > > to find the actual lock status signal.
> > > >
> > > > Signed-off-by: Lucas Stach <l.stach@xxxxxxxxxxxxxx>
> > > > Signed-off-by: Abel Vesa <abel.vesa@xxxxxxx>
> > > > ---
> > > > +static int clk_pll1_set_rate(struct clk_hw *hw, unsigned long rate,
> > > > +                       unsigned long parent_rate)
> > > > +{
> > > > +   struct clk_sccg_pll *pll = to_clk_sccg_pll(hw);
> > > > +   u32 val;
> > > > +   u32 divf;
> > > > +
> > > > +   divf = rate / (parent_rate * 2);
> > > > +
> > > > +   val = readl_relaxed(pll->base + PLL_CFG2);
> > > > +   val &= ~(PLL_DIVF_MASK << PLL_DIVF1_SHIFT);
> > > > +   val |= (divf - 1) << PLL_DIVF1_SHIFT;
> > > > +   writel_relaxed(val, pll->base + PLL_CFG2);
> > > > +
> > > > +   /* FIXME: PLL lock check */
> > >
> > > Shouldn't be too hard to add, no?
> >
> > Added to the next version which I intend to send today.
> >
> > >
> > > > +
> > > > +   return 0;
> > > > +}
> > > > +
> > > > +static int clk_pll1_prepare(struct clk_hw *hw)
> > > > +{
> > > > +   struct clk_sccg_pll *pll = to_clk_sccg_pll(hw);
> > > > +   u32 val;
> > > > +
> > > > +   val = readl_relaxed(pll->base);
> > > > +   val &= ~(1 << PLL_PD);
> > > > +   writel_relaxed(val, pll->base);
> > >
> > > pll->base + PLL_CFG0 please.
> >
> > Same as above.
> >
> > >
> > > > +static const struct clk_ops clk_sccg_pll1_ops = {
> > > > +   .is_prepared    = clk_pll1_is_prepared,
> > > > +   .recalc_rate    = clk_pll1_recalc_rate,
> > > > +   .round_rate     = clk_pll1_round_rate,
> > > > +   .set_rate       = clk_pll1_set_rate,
> > > > +};
> > > > +
> > > > +static const struct clk_ops clk_sccg_pll2_ops = {
> > > > +   .prepare        = clk_pll1_prepare,
> > > > +   .unprepare      = clk_pll1_unprepare,
> > > > +   .recalc_rate    = clk_pll2_recalc_rate,
> > > > +   .round_rate     = clk_pll2_round_rate,
> > > > +   .set_rate       = clk_pll2_set_rate,
> > > > +};
> > >
> > > So these are two PLLs that share the same enable register. Doing the
> > > prepare/unprepare for only one PLL can lead to all kinds of trouble.
> > > Finding a good abstraction the properly handles this case with the
> > > clock framework is probably also not easy.
> > >
> > > I could imagine we'll need to track the enable state on both PLLs and
> > > only if both are disabled we disable it in hardware.
> > >
> > > With the current code we disable the PLLs when all consumers are
> > > reparented to pll1, which probably has bad effects.
> > >
> >
> > So it took me a while to understand exactly why this needs to stay like it is.
> >
> 
> IMHO this means that, if nothing else, all of the below should be
> documented in code as a comment, otherwise simpletons like me are
> going to continue stumbling over it and wondering what's going on.

Will try to add proper comment to explain what's happening.

> 
> > The PLL1 is never used by any device, instead it is used as a source for PLL2.
> >
> > But because the interlink between the two of them is too complicated,
> > the PLLs 1 and 2 need to be separate clocks.
> >
> 
> Can you go a little bit more into detail as for why PLL1 needs to be
> exposed in the first place and can't just be dealt with behind the
> scenes as a part of PLL2 abstraction? Are there use-cases where the
> rates of the two are going to be adjusted individually in Linux?
> 

Here is the SCCG PLL Block Diagram:

https://www.nxp.com/docs/en/reference-manual/IMX8MDQLQRM.pdf#page=834

Now lets take the System PLL 2 for example. If you follow the hierarchy,
when you reach the sys2_pll1_out you can see that one of its selectors is
actually sys1_pll1_ref_sel. Same thing applies to sys3_pll2_out with
sys2_pll1_ref_sel. This means there is an input reference clock interlinking
between different System PLLs.

> Thanks,
> Andrey Smirnov

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