linux-kernel - Re: [PATCH 1/2] arm64: dts: allwinner: Olimex A64-OLinuXino: enable eMMC.

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Date:   Sat, 29 Sep 2018 17:47:54 +0200
From:   Maxime Ripard <maxime.ripard@...tlin.com>
To:     Rodrigo Exterckötter Tjäder 
        <rodrigo@...der.xyz>
Cc:     wens@...e.org, robh+dt@...nel.org, mark.rutland@....com,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] arm64: dts: allwinner: Olimex A64-OLinuXino: enable
 eMMC.

On Thu, Sep 27, 2018 at 11:49:20AM -0300, Rodrigo Exterckötter Tjäder wrote:
> On Thu, Sep 27, 2018 at 5:17 AM Maxime Ripard <maxime.ripard@...tlin.com> wrote:
> >
> > On Tue, Sep 25, 2018 at 02:47:59PM -0300, Rodrigo Exterckötter Tjäder wrote:
> > > On Tue, Sep 25, 2018 at 6:01 AM Maxime Ripard <maxime.ripard@...tlin.com> wrote:
> > > > We can't really do that, unfortunately. If the device tree name was to
> > > > change for a given board, we'd break all the build systems, boot
> > > > scripts and distros out there.
> > >
> > > What if we keep the device tree for the version without WiFi and eMMC
> > > with the current name and create new device trees for the other two
> > > versions?
> >
> > Wifi and Bluetooth should be dealt with with overlays in this case,
> > and since the eMMC is already enabled, then there's nothing to do, I
> > guess.
> 
> It's WiFi that is already enabled, not eMMC. Only one of the three
> variants has WiFi.

Ah, right, sorry. In the board that doesn't have an emmc, are the pins
usable for something else?

> We can't even remove a node from a device tree? Removing the WiFi node
> from the current tree would make it correspond to the variant with the
> least features.

Not really. How pissed would you be if you were a user, had wifi
running on your board, you upgrade your kernel, and then it's just
gone? :)

> About device tree overlays, I read overlay-notes.txt, but I went
> looking for an example with "git grep /plugin/ arch" and it came
> empty. Is this approach not used for other boards?

It is, it's simply not stored in the kernel, but through other third
party repos.

> Does the overlay approach make the device available at boot time? That
> is important for a storage device such as eMMC.
> 
> I went with the separate dts approach because that's what I saw was
> done for other similar cases, like Pine64 and Pine64+, OLinuXino-LIME2
> and its variant with eMMC, among others.

Yeah, but in all these cases, it was done from day one, not after the
facts.

Maxime

-- 
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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