Date:   Tue, 22 May 2018 16:04:39 +0200
From:   Philipp Zabel <p.zabel@...gutronix.de>
To:     Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     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 Martin,

On Mon, 2018-05-21 at 12:40 +0200, Martin Blumenstingl wrote:
> Hello,
> 
> On Mon, May 21, 2018 at 3:27 AM, Masahiro Yamada
> <yamada.masahiro@...ionext.com> wrote:
> > 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.
> 
> OK, let's also hear Philipp's (reset framework maintainer) opinion on this

I'd like to know if there are other SoC families besides Amlogic Meson
that potentially could have this problem and about how many of the
resets that are documented in include/dt-bindings/reset/amlogic,meson*
we are actually talking. Are all of those initially deasserted and none
of the connected peripherals have power-on reset mechanisms?

> > > 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.
> 
> indeed, so let's skip this idea

Maybe we should at first add initial reset assertion to the Meson driver
on a case by case bases?
We can't add required reset hog DT bindings to the Meson reset
controller anyway without breaking DT backwards compatibility.

> > > - 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.
> 
> $ grep -R "of_reset_n_cells = [^1]" drivers/reset/
> drivers/reset/reset-berlin.c:   priv->rcdev.of_reset_n_cells = 2;
> drivers/reset/hisilicon/reset-hi3660.c: rc->rst.of_reset_n_cells = 2;
> drivers/reset/reset-ti-sci.c:   data->rcdev.of_reset_n_cells = 2;
> drivers/reset/reset-lantiq.c:   priv->rcdev.of_reset_n_cells = 2;
> 
> everything else uses only one reset cell
> from the lantiq reset dt-binding documentation: "The first cell takes
> the reset set bit and the second cell takes the status bit."
> 
> I'm not sure what to do with drivers that specify != 1 reset-cell
> though if we use a simple "init state pair"
> maybe Philipp can share his opinion on this one as well

See above, so far I am not convinced (either way) whether this should be
described in the DT at all.

> > > 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)
> 
> I believe we need to keep this if there's hardware out there:
> - where the reset controller only supports reset pulses
> - at least one reset line is shared between multiple devices
> 
> I didn't have a closer look at the Amlogic Meson6 SoC yet, but I think
> it matches above criteria. as far as I know:
> - the USB situation there is similar to Meson8b (USB controllers and
> PHYs share a reset line)
> - it uses an older reset controller IP block which does not support
> level resets (only reset pulses)

See my answer to Masahiro's first mail. I think somebody suggested in
the past to add a fallback from the deassert to the reset op. I think
this is something that should work in this case.

regards
Philipp

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