linux-kernel - Re: [PATCH 0/2] add ripple counter dt binding and driver

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Message-ID: <285d739a-b343-c411-5461-0fe1f44177a5@rasmusvillemoes.dk>
Date:   Mon, 1 Mar 2021 09:34:38 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Guenter Roeck <linux@...ck-us.net>, Arnd Bergmann <arnd@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        DTML <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        linux-clk <linux-clk@...r.kernel.org>,
        Wim Van Sebroeck <wim@...ux-watchdog.org>,
        "linux-watchdog@...r.kernel.org" <linux-watchdog@...r.kernel.org>
Subject: Re: [PATCH 0/2] add ripple counter dt binding and driver

On 26/02/2021 20.53, Guenter Roeck wrote:
> On 2/26/21 8:35 AM, Rasmus Villemoes wrote:
>> On 26/02/2021 15.35, Arnd Bergmann wrote:
>>> On Fri, Feb 26, 2021 at 3:14 PM Rasmus Villemoes
>>> <linux@...musvillemoes.dk> wrote:
>>>
>>>>
>>>> So I'm thinking that the proper way to handle this is to be able to
>>>> represent that ripple counter as a clock consumer in DT and have a
>>>> driver do the clk_prepare_enable(), even if that driver doesn't and
>>>> can't do anything else. But I'm certainly open to other suggestions.
>>>
>>> How about adding support for the optional clock to the gpio_wdt driver,
>>> would that work?
>>
>> I think it would _work_ (all I need is some piece of code doing the
>> clock_prepare_enable(), and until now we've just stashed that in some
>> otherwise unrelated out-of-tree driver, but we're trying to get rid of
>> that one), but the watchdog chip isn't really the consumer of the clock
>> signal, so in-so-far as DT is supposed to describe the hardware, I don't
>> think it's appropriate.
>>
>> OTOH, one could argue that the watchdog chip and the ripple counter
>> together constitute the watchdog circuit.
>>
>> Cc += watchdog maintainers. Context: I have a gpio-wdt which can
>> unfortunately effectively be disabled by disabling a clock output, and
>> that happens automatically unless the clock has a consumer in DT. But
>> the actual consumer is not the gpio-wdt.
>> Please see
>> https://lore.kernel.org/lkml/20210226141411.2517368-1-linux@rasmusvillemoes.dk/
>> for the original thread.
>>
> 
> Sorry, I am missing something. If the watchdog is controlled by the clock,
> it is a consumer of that clock.

But that's just it, the watchdog chip is _not_ a consumer of the clock -
I don't think I've ever seen a gpio_wdt that is not internally clocked,
but even if they exist, that's not the case for this board.

 What else does "consumer" mean ? And why
> not just add optional clock support to the gpio_wdt driver ?

Because, the consumer is a piece of electronics sitting _between_ the
watchdog chip's reset output and the SOCs reset pin, namely the ripple
counter that implements a 64 ms delay from the watchdog fires till the
actual reset. (The watchdog's reset is also routed directly to an
interrupt; so software gets a 64 ms warning that a hard reset is imminent).

Rasmus