linux-kernel - Re: [RFC PATCH 3/3] arm64: dts: bcmbca: update bcm4808 board dts file

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Date:   Wed, 13 Jul 2022 14:00:06 +0200
From:   Rafał Miłecki <rafal@...ecki.pl>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc:     Florian Fainelli <f.fainelli@...il.com>,
        William Zhang <william.zhang@...adcom.com>,
        Linux ARM List <linux-arm-kernel@...ts.infradead.org>,
        kursad.oney@...adcom.com, anand.gore@...adcom.com,
        dan.beygelman@...adcom.com,
        Broadcom Kernel List <bcm-kernel-feedback-list@...adcom.com>,
        joel.peshkin@...adcom.com,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Rob Herring <robh+dt@...nel.org>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 3/3] arm64: dts: bcmbca: update bcm4808 board dts file

On 2022-07-13 13:09, Krzysztof Kozlowski wrote:
> On 13/07/2022 12:55, Rafał Miłecki wrote:
>> On 2022-07-12 17:36, Florian Fainelli wrote:
>>> On 7/12/22 00:47, Krzysztof Kozlowski wrote:
>>>> On 12/07/2022 04:11, William Zhang wrote:
>>>>> Update compatible string based on the new bcmbca binding rule
>>>>> for BCM4908 famliy based boards
>>>> 
>>>> Typo - family
>>>> 
>>>> Please explain why breaking the ABI (and users of these DTS_ is
>>>> acceptable.
>>> 
>>> This will be largely targeted towards Rafal who supports these kinds
>>> of devices with an upstream kernel. My understanding is that this is
>>> OK because we will always ship a DTB matching the Linux kernel, and I
>>> believe this is true for both the way that William and his group
>>> support these devices, as well as how OpenWrt, buildroot or other
>>> build systems envision to support these devices.
>>> 
>>> Rafal, does that sound about right?
>> 
>> Right - in all cases I'm aware of - Linux gets shipped with DTB files.
>> So such change won't actually break anything in real world.
> 
> We don't really talk here about Linux, but other projects, like
> bootloaders or *BSD...

Right, let me more specific.

BCM4908 uses pkgtb firmware images. Those images contain:
1. bootfs (atf, u-boot, kernel, DTB files)
2. rootfs (filesystem)

So when you flash BCM4908 firmware it always contains:
1. U-Boot and DTB for it
2. Kernel and DTB for it
(+ more stuff)

There isn't any on-flash DTB file that doesn't get updated when flashing
a new image.