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

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Date:   Fri, 5 Oct 2018 17:48:44 +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 Tue, Oct 02, 2018 at 01:47:33PM -0300, Rodrigo Exterckötter Tjäder wrote:
> > > > > 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.
> > >
> > > So that would mean that it's up to every distro to support the boards
> > > instead of relying on mainline support?
> >
> > Distros would have to integrate it either way. One would need to
> > detect and / or ask for the board variant in order to load say the BT
> > stack, or to know if you want to boot from the eMMC or from the SD
> > card.
> 
> Yeah, but now if a bug is found in the device tree it has to be fixed
> once per distro instead of only on mainline.

Yeah, well, I never said it was perfect.

> > > > > 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.
> > >
> > > For the LIME2 the dts for the emmc variant was commited two years
> > > after the base LIME2 dts.
> > >
> > > What if instead of keeping the current dt for the least featureful
> > > model we keep it for the most featureful model and create new dts for
> > > the two less featureful models?
> >
> > This is a different story though. The LIME2 eMMC variant was created
> > way after the original LIME2, with a separate name.
> 
> What about the idea of keeping the current dt for the most featureful
> variant and creating new dts for the other two?
>
> That would make it so that no one's device stops working and would
> have mailine support for all three devices.

IIRC that has been the first introduced version, so that would make
sense. Chen-Yu, any opinion?

> Also, the current device tree doesn't represent any existing device:
> it has wifi on but no emmc. That variation does not exist.

Most of our device tree are far from complete, so you shouldn't treat
them as such.

Maxime

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

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