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

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Date:   Thu, 8 Sep 2016 17:39:15 -0700
From:   John Stultz <john.stultz@...aro.org>
To:     Rob Herring <robh+dt@...nel.org>
Cc:     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>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Grant Likely <grant.likely@...retlab.ca>
Subject: Re: [PATCH] of: Overlay manager

On Thu, Sep 8, 2016 at 5:33 PM, Rob Herring <robh+dt@...nel.org> wrote:
> On Thu, Sep 8, 2016 at 4:15 PM, Dmitry Shmidt <dimitrysh@...gle.com> wrote:
>> On Thu, Sep 8, 2016 at 2:08 PM, Frank Rowand <frowand.list@...il.com> wrote:
>>> What problem(s) are you trying to solve with this?
>>
>> Currently the Linux kernel doesn't provide a way for boot time
>> selection of different possible board configurations, when the board
>> peripherals are specified via Device Tree.
>
> Well, the kernel does provide a way. For different h/w you give it a
> different h/w description (i.e. dtb). The issue is having multiple
> dtbs for mostly similar h/w is a PIA.

Agreed.

> Also, it can be argued this should remain a bootloader problem. For
> the LCD case, the bootloader is going to need that information to
> support a splash screen. libfdt is learning how to apply overlays on
> flat trees.

I can imagine in many cases like devboards with various attached
shields/mezzanines the bootloader doesn't (and likely can't) have this
ability.


>>> What is the use case?
>>
>> It is usually useful for development board when you can use
>> different peripherals. For example you want to use special
>> LCD panel. Using it will go on account of other stuff like GPIOs,
>> so you don't want it all the time. And it is not convenient to
>> apply new patches and to recompile the kernel. And this code
>> allows to choose LCD panel configuration from command line.
>
> 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.

thanks
-john