linux-kernel - Re: [PATCH] of: Overlay manager

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Date:   Fri, 9 Sep 2016 22:26:25 +0300
From:   Pantelis Antoniou <pantelis.antoniou@...sulko.com>
To:     John Stultz <john.stultz@...aro.org>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Dmitry Shmidt <dimitrysh@...gle.com>,
        Frank Rowand <frowand.list@...il.com>,
        "<linux-kernel@...r.kernel.org>" <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Grant Likely <grant.likely@...retlab.ca>
Subject: Re: [PATCH] of: Overlay manager

Hi John,

> On Sep 9, 2016, at 06:06 , John Stultz <john.stultz@...aro.org> wrote:
> 
> On Thu, Sep 8, 2016 at 7:27 PM, Rob Herring <robh+dt@...nel.org> wrote:
>> On Thu, Sep 8, 2016 at 7:39 PM, John Stultz <john.stultz@...aro.org> wrote:
>>> On Thu, Sep 8, 2016 at 5:33 PM, Rob Herring <robh+dt@...nel.org> wrote:
>>>> If you can figure out how to change the command line, then you can
>>>> just change the dtb. At least for how Android boot works, those aren't
>>>> really changed separately.
>>> 
>>> Ehh.. that's not so simple. The dtb is often appended to the kernel on
>>> Android devices. Changing the boot arguments is much simpler to do.
>> 
>> How? You typically make a new bootimage assembling the kernel/dtb,
>> ramdisk and kernel command-line. If things were done differently such
>> that the dtb is part of the bootloader (how it is supposed to be
>> done), then I would buy the argument that we can't update the dtb and
>> need to either have a way to add and/or select overlays. But Android
>> folks like to update *everything*, so I don't buy that here.
> 
> So in many cases the dtb is appended when the kernel is built, not
> when the abootimg is assembled.
> 
> So its much easier to use abootimg -u to update a prebuilt boot.img in
> place and reflash. That way users don't need to regenerate the kernel
> w/ appended dtb.
> 

I understand what you’re trying to do, but it’s not going to work.

It will only work for a very small subset of overlays since you can’t
have more than a single phandle label.

For instance this will not work:

overlays {
	overlay_0 {
		opt: opt_0 { bar; };
	};
	overlay_1 {
		opt: opt_1 { baz; };
	};
};


frob_device {
	compatible = “frob”;
	use = <&opt>;
};

If your use case is simple enough you’ll never hit this, but it does happen in
more complex examples.

> thanks
> -john

Regards

— Pantelis