linux-kernel - Re: [reset-control] How to initialize hardware state with the shared reset line?

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Date:   Mon, 21 May 2018 10:27:21 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc:     Philipp Zabel <p.zabel@...gutronix.de>,
        Rob Herring <robh+dt@...nel.org>,
        Lee Jones <lee.jones@...aro.org>,
        Hans de Goede <hdegoede@...hat.com>,
        Felipe Balbi <felipe.balbi@...ux.intel.com>,
        DTML <devicetree@...r.kernel.org>, Arnd Bergmann <arnd@...db.de>,
        Linus Walleij <linus.walleij@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [reset-control] How to initialize hardware state with the shared
 reset line?

Hi.


2018-05-20 19:57 GMT+09:00 Martin Blumenstingl
<martin.blumenstingl@...glemail.com>:
> Hi,
>
> On Thu, May 10, 2018 at 11:16 AM, Masahiro Yamada
> <yamada.masahiro@...ionext.com> wrote:
> [snip]
>> I may be missing something, but
>> one solution might be reset hogging on the
>> reset provider side.  This allows us to describe
>> the initial state of reset lines in the reset controller.
>>
>> The idea for "reset-hog" is similar to:
>>  - "gpio-hog" defined in
>>    Documentation/devicetree/bindings/gpio/gpio.txt
>>  - "assigned-clocks" defined in
>>    Documetation/devicetree/bindings/clock/clock-bindings.txt
>>
>>
>>
>> For example,
>>
>>    reset-controller {
>>             ....
>>
>>             line_a {
>>                   reset-hog;
>>                   resets = <1>;
>>                   reset-assert;
>>             };
>>    }
>>
>>
>> When the reset controller is registered,
>> the reset ID '1' is asserted.
>>
>>
>> So, all reset consumers that share the reset line '1'
>> will start from the asserted state
>> (i.e. defined state machine state).
> I wonder if a "reset hog" can be board specific:
> - GPIO hogs are definitely board specific (meson-gxbb-odroidc2.dts for
> example uses it to take the USB hub out of reset)
> - assigned-clock-parents (and the like) can also be board specific (I
> made up a use-case since I don't know of any actual examples: board A
> uses an external XTAL while board B uses some other internal
> clock-source because it doesn't have an external XTAL)
>
> however, can reset lines be board specific? or in other words: do we
> need to describe them in device-tree?

Indeed.

I did not come up with board-specific cases.

The problem we are discussing is SoC-specific,
and reset-controller drivers are definitely SoC-specific.

So, I think the initial state can be coded in drivers instead of DT.


> we could extend struct reset_controller_dev (= reset controller
> driver) if they are not board specific:
> - either assert all reset lines by default except if they are listed
> in a new field (may break backwards compatibility, requires testing of
> all reset controller drivers)

This is quite simple, but I am afraid there are some cases where the forcible
reset-assert is not preferred.

For example, the earlycon.  When we use earlycon, we would expect it has been
initialized by a boot-loader, or something.
If it is reset-asserted on the while, the console output
will not be good.



> - specify a list of reset lines and their desired state (or to keep it
> easy: specify a list of reset lines that should be asserted)
> (I must admit that this is basically your idea but the definition is
> moved from device-tree to the reset controller driver)

Yes, I think the list of "reset line ID" and "init state" pairs
would be nicer.


> any "chip" specific differences could be expressed by using a
> different of_device_id
>
> one the other hand: your "reset hog" solution looks fine to me if
> reset lines can be board specific
>
>> From the discussion with Martin Blumenstingl
>> (https://lkml.org/lkml/2018/4/28/115),
>> the problem for Amlogic is that
>> the reset line is "de-asserted" by default.
>> If so, the 'reset-hog' would fix the problem,
>> and DWC3 driver would be able to use
>> shared, level reset, I think.
> I think you are right: if we could control the initial state then we
> should be able to use level resets


Even further, can we drop the shared reset_control_reset() support, maybe?
(in other words, revert commit 7da33a37b48f11)


Thanks for your comment!


>
> Regards
> Martin
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-- 
Best Regards
Masahiro Yamada