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

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Message-ID: <01d49f11-a68e-1674-e6b7-d1b87aa973c9@st.com>
Date:   Mon, 11 Feb 2019 17:32:30 +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 1/28/19 2:20 PM, Fabrice Gasnier wrote:
> 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

Hi Arnd,

Gentlemen reminder for this. I would appreciate to have your
feedback.

Many thanks,
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
>>