Re: [PATCH 1/6] clk: samsung: Enable bus clock on init

[Sam Protsenko]

On Wed, 15 Sept 2021 at 15:51, Sylwester Nawrocki
<s.nawrocki@xxxxxxxxxxx> wrote:
>
> Hi,
>
> On 14.09.2021 17:56, Sam Protsenko wrote:
> > By default if bus clock has no users its "enable count" value is 0. It
> > might be actually running if it's already enabled in bootloader, but
> > then in some cases it can be disabled by mistake. For example, such case
> > was observed when dw_mci_probe() enabled bus clock, then failed to do
> > something and disabled that bus clock on error path. After that even
> > attempt to read the 'clk_summary' file in DebugFS freezed forever, as
> > CMU bus clock ended up being disabled and it wasn't possible to access
> > CMU registers anymore.
> >
> > To avoid such cases, CMU driver must increment the ref count for that
> > bus clock by running clk_prepare_enable(). There is already existing
> > '.clk_name' field in struct samsung_cmu_info, exactly for that reason.
> > It was added in commit 523d3de41f02 ("clk: samsung: exynos5433: Add
> > support for runtime PM"). But the clock is actually enabled only in
> > Exynos5433 clock driver. Let's mimic what is done there in generic
> > samsung_cmu_register_one() function, so other drivers can benefit from
> > that `.clk_name' field. As was described above, it might be helpful not
> > only for PM reasons, but also to prevent possible erroneous clock gating
> > on error paths.
> >
> > Another way to workaround that issue would be to use CLOCK_IS_CRITICAL
> > flag for corresponding gate clocks. But that might be not very good
> > design decision, as we might still want to disable that bus clock, e.g.
> > on PM suspend.
> >
> > Signed-off-by: Sam Protsenko <semen.protsenko@xxxxxxxxxx>
> > ---
> >  drivers/clk/samsung/clk.c | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> >
> > diff --git a/drivers/clk/samsung/clk.c b/drivers/clk/samsung/clk.c
> > index 1949ae7851b2..da65149fa502 100644
> > --- a/drivers/clk/samsung/clk.c
> > +++ b/drivers/clk/samsung/clk.c
> > @@ -357,6 +357,19 @@ struct samsung_clk_provider * __init samsung_cmu_register_one(
> >
> >       ctx = samsung_clk_init(np, reg_base, cmu->nr_clk_ids);
> >
> > +     /* Keep bus clock running, so it's possible to access CMU registers */
> > +     if (cmu->clk_name) {
> > +             struct clk *bus_clk;
> > +
> > +             bus_clk = __clk_lookup(cmu->clk_name);
> > +             if (bus_clk) {
> > +                     clk_prepare_enable(bus_clk);
> > +             } else {
> > +                     pr_err("%s: could not find bus clock %s\n", __func__,
> > +                            cmu->clk_name);
> > +             }
> > +     }
> > +
> >       if (cmu->pll_clks)
> >               samsung_clk_register_pll(ctx, cmu->pll_clks, cmu->nr_pll_clks,
> >                       reg_base);
>
> I would suggest to implement runtime PM ops in your driver instead, even though
> those would initially only contain single clk enable/disable. Things like
> the clk_summary will work then thanks to runtime PM support in the clk core
> (see clk_pm_runtime_* calls).

Can you please elaborate more? I don't see how adding PM ops would
solve the problem I'm trying to address, which is keeping core bus
clocks always running. For example, I'm looking at clk-exynos5433.c
implementation, which enables bus clock on resume path:

<<<<<<<<<<<<<<<< cut here >>>>>>>>>>>>>>>>
static int __maybe_unused exynos5433_cmu_resume(struct device *dev)
{
    ...
    clk_prepare_enable(data->clk);
    ...
}
<<<<<<<<<<<<<<<< cut here >>>>>>>>>>>>>>>>

But that resume operation won't be called on driver init, because it
configures runtime PM like this:

<<<<<<<<<<<<<<<< cut here >>>>>>>>>>>>>>>>
static int __init exynos5433_cmu_probe(struct platform_device *pdev)
{
    ...
    /*
     * Enable runtime PM here to allow the clock core using runtime PM
     * for the registered clocks. Additionally, we increase the runtime
     * PM usage count before registering the clocks, to prevent the
     * clock core from runtime suspending the device.
     */
    pm_runtime_get_noresume(dev);
    pm_runtime_set_active(dev);
    pm_runtime_enable(dev);
    ...
    pm_runtime_put_sync(dev);
    ...
}
<<<<<<<<<<<<<<<< cut here >>>>>>>>>>>>>>>>

When I tried to implement the same in my driver, only suspend function
is called during kernel startup.

Anyway, even clk-exynos5433.c driver (which also implements PM ops)
does the same for core bus clocks:

<<<<<<<<<<<<<<<< cut here >>>>>>>>>>>>>>>>
static int __init exynos5433_cmu_probe(struct platform_device *pdev)
{
    ...
    if (info->clk_name)
        data->clk = clk_get(dev, info->clk_name);
    clk_prepare_enable(data->clk);
    ...
}
<<<<<<<<<<<<<<<< cut here >>>>>>>>>>>>>>>>

So it looks like separate feature to me. Not sure how that can be
implemented only by adding PM ops. Also, my board lacks PM support in
upstream kernel right now, so I probably won't be able to test PM ops
if I implement those, that's why I decided to skip it for now.

> We could also make common runtime PM suspend/resume helpers but I wouldn't focus
> on that too much now, it could well be done later.
> And please avoid introducing new __clk_lookup() calls.
>

The reason I used __clk_lookup() is that it's the only API that works
in that case. I tried to use clk_get(), but we lack 'struct dev'
pointer in samsung_cmu_register_one(), so when providing dev=NULL into
clk_get() it fails to get the clock. That's happening because
LIST_HEAD(clocks) is probably empty in clkdev.c. So this chain fails:

<<<<<<<<<<<<<<<< cut here >>>>>>>>>>>>>>>>
clk_get()    // dev = NULL
  v
__clk_get_sys()
  v
clk_find_hw()
  v
clk_find()   // returns 0, because LIST_HEAD(clocks) is empty
<<<<<<<<<<<<<<<< cut here >>>>>>>>>>>>>>>>

I saw your patches which get rid of __clk_lookup() usage by accessing
ctx->clk_data.hws[], but that requires using clock index, not name.
'struct samsung_cmu_info' only stores bus clock name (.clk_name),
which seems logical to me, so we can't get away from using
__clk_lookup() in that case without refactoring 'struct
samsung_cmu_info' first.

All that said, I suggest next: I'll pull the code from this patch into
clk-exynos850.c, adding platform_driver registration there, so I can
actually use clk_get() for getting bus clocks. As for PM ops, I'd like
to skip it for now, if you don't mind, as I can't fully test those.
Otherwise please elaborate more on how PM ops can solve this problem.

Thanks!

> --
> Regards,
> Sylwester