Re: [PATCH v2 2/3] mfd: syscon: Add optional clock support

From: Arnd Bergmann <arnd@arndb.de>
To: Fabrice Gasnier <fabrice.gasnier@st.com>
Cc: Lee Jones <lee.jones@linaro.org>,
	Rob Herring <robh+dt@kernel.org>,
	Alexandre Torgue <alexandre.torgue@st.com>,
	Maxime Coquelin <mcoquelin.stm32@gmail.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Gabriel Fernandez <gabriel.fernandez@st.com>,
	DTML <devicetree@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-stm32@st-md-mailman.stormreply.com,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v2 2/3] mfd: syscon: Add optional clock support
Date: Wed, 16 Jan 2019 16:11:08 +0100	[thread overview]
Message-ID: <CAK8P3a0kV4-OF7JPwM9cEqxaPvNM9dFZdyZetc47t+D5izkFwQ@mail.gmail.com> (raw)
In-Reply-To: <9e0ee154-a399-9bff-3beb-adfd9d6c6fee@st.com>

On Wed, Jan 16, 2019 at 3:10 PM Fabrice Gasnier <fabrice.gasnier@st.com> wrote:
>
> On 1/16/19 1:14 PM, Arnd Bergmann wrote:
> > (sorry for the late reply, I just realized that I had never sent out the
> > mail after Lee asked me for a review last year and I had drafted
> > my reply).
>
> Hi Arnd,
>
> Many thanks for reviewing, no worries :-)
>
> >
> > On Wed, Dec 12, 2018 at 9:48 AM Fabrice Gasnier <fabrice.gasnier@st.com> wrote:
> >>
> >> Some system control registers need to be clocked, so the registers can
> >> be accessed. Add an optional clock and attach it to regmap.
> >>
> >> Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
> >
> > This looks ok to me in principle, but I have one question: When we
> > do a clk_get() and clk_prepare() as part of regmap_mmio_attach_clk(),
> > does that change the behavior of syscon nodes that are otherwise
> > unused?
>
> I'm not sure I correctly understand this question. I don't think it will
> change the behavior for "unused" nodes. These should remain unused with
> this patch.

What I mean is that nodes that listed as 'compatible="syscon"' get
probed by the syscon driver even when no other driver references
them, and that in turn would acquire the clock, right?

> > I think we have a bunch of devices that started out as a syscon but
> > then we added a proper driver for them, which would handle the
> > clocks explicitly. Is it guaranteed that this will keep working (including
> > shutting down the clocks when they are unused) if we have two drivers
> > that call clk_get() on the same device node?
>
> I'd expect nothing wrong happens when two drivers call clk_get() for the
> same clock.
> Are there some case where two drivers are bind (e.g. syscon driver +
> another driver) for the same piece of hardware ?

You won't actually have two drivers binding to the same device, but you
could have a driver and a syscon user that does relies on the
syscon_regmap_lookup_by_*() functions.

I think we've had a couple of cases where we started out representing
a device as syscon, and then later decided that a high-level abstraction
would be needed because syscon didn't quite support all the needed
features.

Since each syscon node should also have a more specific
compatible value, you can then have another driver that binds
to that compatible string.

      Arnd