Message-ID: <1494180042.13734.4.camel@paulk.fr>
Date:   Sun, 07 May 2017 20:00:42 +0200
From:   Paul Kocialkowski <contact@...lk.fr>
To:     Doug Anderson <dianders@...omium.org>,
        Heiko Stuebner <heiko@...ech.de>
Cc:     Brian Norris <briannorris@...omium.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Russell King <linux@...linux.org.uk>
Subject: Re: [PATCH 1/3] ARM: dts: rockchip: Move cros-ec-sbs to
 rk3288-veyron-chromebook-sbs

Hi,

Le lundi 01 mai 2017 à 08:49 -0700, Doug Anderson a écrit :
> On Mon, May 1, 2017 at 7:07 AM, Heiko Stuebner <heiko@...ech.de> wrote:
> > Am Sonntag, 30. April 2017, 22:56:52 CEST schrieb Paul Kocialkowski:
> > > Le dimanche 30 avril 2017 à 22:37 +0200, Heiko Stuebner a écrit :
> > > > Hi Paul,
> > > > 
> > > > Am Sonntag, 30. April 2017, 20:30:52 CEST schrieb Paul Kocialkowski:
> > > > > This moves the cros-ec-sbs dtsi to a new rk3288-veyron-chromebook-sbs
> > > > > dtsi since it only concerns rk3288 veyron Chromebooks.
> > > > > 
> > > > > Other Chromebooks (such as the tegra124 nyans) also have sbs batteries
> > > > > and don't use this dtsi, that only makes sense when used with
> > > > > rk3288-veyron-chromebook anyway.
> > > > 
> > > > That isn't true. The gru series (rk3399-based) also uses the
> > > > sbs-battery [0]. And while it is currently limited to Rockchip-based
> > > > Chromebooks it is nevertheless used on more than one platform, so
> > > > the probability is high that it will be used in future series as well.
> > > 
> > > That's good to know, but as pointed out, other cros devices are using a
> > > sbs
> > > battery without this header, so such a generic name isn't really a good
> > > fit.
> 
> It would be interesting to know if the "retry-count" ought to be the
> same across all Chromebooks.  I guess you could argue that maybe
> someone found it needed to be 10 in all "nyan" variants and needed to
> be 1 in all "veyron" variants, but it seems more likely that the
> difference is arbitrary, or that one of the two values would work for
> everyone.  It sure looks like we've just been copying values from
> device to device.  Given that all the "veyron" devices have vastly
> different batteries (and probably all the nyan ones too), it seems
> likely there ought to be one value.

Well, the retry-count is a maximum number of retries to detect a status change
on external power connection/disconnection. From my experience, it seems that
nyans do indeed more retries to detect the change than veyrons, on average.

I don't think setting this value to 1 is very reasonable (in the end, that's a
number of seconds), because power supply status changes tend to take a few
seconds to reflect on the battery status.

I think setting a high value (like 10) would always work and either way, the
status detection mechanism stops itself as soon as a change is detected (it
turns out this is not a good idea for bq27xxx batteries, because they go from
charging to full in the first seconds after AC connection instead of directly
reporting full, when full), but let's assume this is okay for sbs (and maybe
change it later).

> In terms of setting the "charger", that also could potentially be
> something that could be for all Chromebooks, or at least older ones
> that don't have their charger implemented by the type C driver.  ...or
> nyan devices could simply have a line in their dts like:
> 
> &battery {
>   power-supplies = <&charger>;
> };

That's true, but I think it makes as much sense to keep the whole binding.

In my opinion, the only reason to have a separate dtsi for this binding is that
veyrons have another dtsi for chromebooks where this binding should be. However,
it cannot be there because of minnie using another battery IC.

So my approach here would be to make it common for devices where other major
parts are also common, so we can avoid duplication when most of the device-tree
is already common. In cases where most of the device-tree is specific to a
device, I think the binding should be duplicated. This is done already for lots
of other components that could be made (somewhat) common anyway.

> 
> > > Note that &charger has to be defined (after my subsequent patches), which
> > > it is
> > > for devices that also include rk3288-veyron-chromebook, but not
> > > necessarily
> > > others.
> > > 
> > > Overall, I think having one -sbs dtsi file makes sense here because there
> > > is
> > > already a rk3288-veyron-chromebook dtsi that veyron chromebooks use. That
> > > file
> > > cannot contain the battery bindings because minnie has a different one and
> > > it
> > > would be a bit silly to copy it over all devices. That definitely makes
> > > sense.
> > > 
> > > As for other devices, I don't see why we should have a separate include
> > > file for
> > > the battery instead of having it in the device's dts. I think this should
> > > be the
> > > case on gru/kevin.
> > > 
> > > Also maybe not *all* gru-based devices will turn out to use a SBS battery,
> > > so it
> > > seems early to include this header in the gru dtsi.
> 
> For gru devices, we've moved to a "virtual sbs battery" provided by
> the EC.  I'm not 100% positive that everything will just magically
> work and be converted in the EC if we put a non-sbs battery on a board
> with this EC feature, but I would hope we'd convert everything
> properly.

Interesting and good to know!

> > > One last point, gru/kevin
> > > currently don't define a charger, which will break my subsequent patch
> > > (that is
> > > however needed for the veyrons that use this file).
> 
> Arguably this should be fixed.  On veyron-chromebook we just use
> "gpio-charger".  We didn't add a special charger driver w/ a property
> like "ti,external-control" since the only piece of information that
> Linux really needed from the charger was whether or not AC was
> connected.

Thanks for taking that choice, it indeed makes things easier on the kernel side
whith no drawbacks.

> 
> > > To me, it seems that there's little advantage and major drawbacks in
> > > keeping
> > > this file the way it is.
> > 
> > I don't have any set opinion right now but after looking through the
> > other uses of the sbs-battery the cros-ec-sbs.dtsi snippet really seems
> > somewhat veyron/gru-specific - especially wrt. the retry-count values.
> > 
> > What I'm not sure about is whether it is actually better to keep the include
> > around under a new name or just move the (rather tiny) sbs-battery node
> > into the relevant devicetrees directly, when there aren't that many users
> > anyway.
> 
> I'm fine with whatever you guys choose to do here.  It's nice not to
> have copied "code", but with device tree sometimes copies are cleaner
> than trying to share something.

I definitely agree. I think copies are a good fit here because overall, we have
enough disparity in the possible configurations among different SoC platforms to
justify having one per device. So I believe it would make sense to make that
binding common *among the same SoC family*.

Cheers,

-- 
Paul Kocialkowski, developer of free digital technology and hardware support

Website: https://www.paulk.fr/
Coding blog: https://code.paulk.fr/
Git repositories: https://git.paulk.fr/ https://git.code.paulk.fr/
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