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

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Message-ID: <47a3fe9f-8adb-8f47-e7c1-342f8fc318aa@st.com>
Date:   Mon, 28 Jan 2019 14:20:32 +0100
From:   Fabrice Gasnier <fabrice.gasnier@...com>
To:     Arnd Bergmann <arnd@...db.de>
CC:     Lee Jones <lee.jones@...aro.org>, Rob Herring <robh+dt@...nel.org>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        Gabriel Fernandez <gabriel.fernandez@...com>,
        DTML <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        <linux-stm32@...md-mailman.stormreply.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v2 2/3] mfd: syscon: Add optional clock support

On 1/16/19 4:11 PM, Arnd Bergmann wrote:
> On Wed, Jan 16, 2019 at 3:10 PM Fabrice Gasnier <fabrice.gasnier@...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@...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@...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?

Hi Arnd,

Sorry for the late reply.

When no other driver references them, nothing happens at probe time on
the clock: no calls to get/prepare... the clock.

=> The clock will remain unrequested & unused until another driver calls
one of "of_syscon_register()" variants:
- syscon_node_to_regmap
- syscon_regmap_lookup_by_compatible
- syscon_regmap_lookup_by_phandle

When another driver references them (e.g. one of the above calls), then
it will acquire the optional clock and use it, e.g.:
- clk_prepare() upon of_syscon_register() variants
- clk_enable & clk_disable when accessing the registers

I hope this clarifies.

Please advise,
Best Regards,
Fabrice

> 
>>> 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
>