Re: [PATCH] clk: qcom: rcg2: Cache rate changes for parked RCGs

[Bjorn Andersson <bjorn.andersson@linaro.org>]

On Wed 15 Dec 17:51 PST 2021, Stephen Boyd wrote:

> Quoting Bjorn Andersson (2021-12-02 19:56:01)
> > As GDSCs are turned on and off some associated clocks are momentarily
> > enabled for house keeping purposes. Failure to enable these clocks seems
> > to have been silently ignored in the past, but starting in SM8350 this
> > failure will prevent the GDSC to turn on.
> > 
> > At least on SM8350 this operation will enable the RCG per the
> > configuration in CFG_REG. This means that the current model where the
> > current configuration is written back to CF_REG immediately after
> > parking the RCG doesn't work.
> 
> Just to clarify, is the RCG off and "parked" at XO with the config
> register dirty and set to the desired frequency and then the RCG is
> turned on by the GDSC?
> 

Correct, that's exactly what I'm observing.

> > 
> > Instead, keep track of the currently requested rate of the clock and
> > upon enabling the clock reapply the configuration per the saved rate.
> 
> We already keep track of the requested rate and reapply it on enable,
> just we're lazy and stash that information in the hardware and not the
> software. I didn't think the gdsc would be turned on and ruin that all,
> but it's fair.
> 

Up until SM8350 I see no evidence that this has been a problem, but now
it is. So there's likely some changes in the hardware there...

> > 
> > Fixes: 7ef6f11887bd ("clk: qcom: Configure the RCGs to a safe source as needed")
> > Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
> > ---
> >  drivers/clk/qcom/clk-rcg.h  |  2 ++
> >  drivers/clk/qcom/clk-rcg2.c | 32 +++++++++++++++++---------------
> >  2 files changed, 19 insertions(+), 15 deletions(-)
> > 
> > diff --git a/drivers/clk/qcom/clk-rcg.h b/drivers/clk/qcom/clk-rcg.h
> > index 99efcc7f8d88..6939f4e62768 100644
> > --- a/drivers/clk/qcom/clk-rcg.h
> > +++ b/drivers/clk/qcom/clk-rcg.h
> > @@ -139,6 +139,7 @@ extern const struct clk_ops clk_dyn_rcg_ops;
> >   * @freq_tbl: frequency table
> >   * @clkr: regmap clock handle
> >   * @cfg_off: defines the cfg register offset from the CMD_RCGR + CFG_REG
> > + * @current_rate: cached rate for parked RCGs
> >   */
> >  struct clk_rcg2 {
> >         u32                     cmd_rcgr;
> > @@ -149,6 +150,7 @@ struct clk_rcg2 {
> >         const struct freq_tbl   *freq_tbl;
> >         struct clk_regmap       clkr;
> >         u8                      cfg_off;
> > +       unsigned long           current_rate;
> >  };
> >  
> >  #define to_clk_rcg2(_hw) container_of(to_clk_regmap(_hw), struct clk_rcg2, clkr)
> > diff --git a/drivers/clk/qcom/clk-rcg2.c b/drivers/clk/qcom/clk-rcg2.c
> > index e1b1b426fae4..b574b38dcbd5 100644
> > --- a/drivers/clk/qcom/clk-rcg2.c
> > +++ b/drivers/clk/qcom/clk-rcg2.c
> > @@ -167,6 +167,7 @@ clk_rcg2_recalc_rate(struct clk_hw *hw, unsigned long parent_rate)
> >  {
> >         struct clk_rcg2 *rcg = to_clk_rcg2(hw);
> >         u32 cfg, hid_div, m = 0, n = 0, mode = 0, mask;
> > +       unsigned long rate;
> >  
> >         regmap_read(rcg->clkr.regmap, RCG_CFG_OFFSET(rcg), &cfg);
> >  
> > @@ -186,7 +187,11 @@ clk_rcg2_recalc_rate(struct clk_hw *hw, unsigned long parent_rate)
> >         hid_div = cfg >> CFG_SRC_DIV_SHIFT;
> >         hid_div &= mask;
> >  
> > -       return calc_rate(parent_rate, m, n, mode, hid_div);
> > +       rate = calc_rate(parent_rate, m, n, mode, hid_div);
> > +       if (!rcg->current_rate)
> > +               rcg->current_rate = rate;
> 
> Instead of doing this in recalc_rate, all the time, why not make an init
> clk op that does it once during registration? The other problem I see is
> that the rate we calculate may be wrong if the parent is registered
> after this clk. I think this came up originally when the patch this is
> fixing was discussed.
> 

I would need to go back and reproduce the issue I saw, but I had to add
this because I ended up in clk_rcg2_shared_enable() with current_rate =
0, which I think would be equally bad to just committing the dirty
configuration.

> So instead of saving the current_rate can we save the cfg register value
> (or however many registers we need) to put back the frequency of the clk
> to what we want on enable? The other thing is that we made recalc_rate()
> work "seamlessly" here by stashing the frequency into the register but
> leaving it uncommitted until enable. We may need to now look at the
> software copy of the registers in the shared rcg recalc rate operation
> to figure out what the frequency is.
> 

I made an attempt at this, the problem I had was to come up within
something sane for how to deal with set_rate on parked clocks; because
we need to re-generate the register contents, without writing out the
value - and that got messy.

So stashing the frequency turned out to be much cleaner. I believe that
this is also what they do downstream...

Regards,
Bjorn