linux-kernel - Re: [PATCH v2 10/12] watchdog: s3c2410: Support separate source clock

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Message-ID: <CAPLW+4mhD+k03S1cR-AdNk4etPiK=wsCoR4+o39gDe==XdoY8w@mail.gmail.com>
Date:   Sun, 7 Nov 2021 17:55:55 +0200
From:   Sam Protsenko <semen.protsenko@...aro.org>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
Cc:     Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Rob Herring <robh+dt@...nel.org>,
        linux-watchdog@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org
Subject: Re: [PATCH v2 10/12] watchdog: s3c2410: Support separate source clock

On Tue, 2 Nov 2021 at 12:15, Krzysztof Kozlowski
<krzysztof.kozlowski@...onical.com> wrote:
>
> On 31/10/2021 13:22, Sam Protsenko wrote:
> > Right now all devices supported in the driver have the single clock: it
> > acts simultaneously as a bus clock (providing register interface
> > clocking) and source clock (driving watchdog counter). Some newer Exynos
> > chips, like Exynos850, have two separate clocks for that. In that case
> > two clocks will be passed to the driver from the resource provider, e.g.
> > Device Tree. Provide necessary infrastructure to support that case:
> >   - use source clock's rate for all timer related calculations
> >   - use bus clock to gate/ungate the register interface
> >
> > All devices that use the single clock are kept intact: if only one clock
> > is passed from Device Tree, it will be used for both purposes as before.
> >
> > Signed-off-by: Sam Protsenko <semen.protsenko@...aro.org>
> > ---
> > Changes in v2:
> >   - Reworded commit message to be more formal
> >   - Used separate "has_src_clk" trait to tell if source clock is present
> >   - Renamed clock variables to match their purpose
> >   - Removed caching source clock rate, obtaining it in place each time instead
> >   - Renamed err labels for more consistency
> >
> >  drivers/watchdog/s3c2410_wdt.c | 52 +++++++++++++++++++++++++---------
> >  1 file changed, 39 insertions(+), 13 deletions(-)
> >
> > diff --git a/drivers/watchdog/s3c2410_wdt.c b/drivers/watchdog/s3c2410_wdt.c
> > index fdb1a1e9bd04..c733917c5470 100644
> > --- a/drivers/watchdog/s3c2410_wdt.c
> > +++ b/drivers/watchdog/s3c2410_wdt.c
> > @@ -118,7 +118,9 @@ struct s3c2410_wdt_variant {
> >
> >  struct s3c2410_wdt {
> >       struct device           *dev;
> > -     struct clk              *clock;
> > +     struct clk              *bus_clk; /* for register interface (PCLK) */
> > +     struct clk              *src_clk; /* for WDT counter */
> > +     bool                    has_src_clk;
>
> Why do you need the has_src_clk value? If clk_get() fails, just store
> there NULL and clk API will handle it.
>

I've added that 'has_src_clk' field for somewhat different reason.

1. As we discussed earlier, in case when src_clk is not present, I do
'src_clk = bus_clk' in v2. This way I don't have to check if src_clk
is NULL every time and use bus_clk to get rate. If I set src_clk =
NULL, I'll have to add selector code, which wouldn't be so elegant,
and contradicts what we've discussed.
2. On the other hand, I don't want to enable bus_clk twice in case
when src_clk is not present, that just doesn't feel right (user would
see incorrect refcount value in DebugFS, etc). And if "clk_src =
bus_clk', and I haven't enabled it second time, then I shouldn't try
to disable it second time, right?

The only way I can see to accomplish (1) and (2) together, is to use
that 'has_src_clk' flag. For me it still looks better than enabling
bus_clk twice, or checking if clk_src is NULL every time we need to
get clock rate. Please let me know if you still have a strong opinion
on this one.

> Best regards,
> Krzysztof