linux-kernel - Re: [PATCH 0/7] arm/arm64: dts: Remove unused num-viewport from pcie node

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Date:   Fri, 7 Jan 2022 13:47:03 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
To:     Mark Kettenis <mark.kettenis@...all.nl>,
        Jisheng Zhang <jszhang@...nel.org>
Cc:     shawnguo@...nel.org, leoyang.li@....com, robh+dt@...nel.org,
        linux@...linux.org.uk, andrew@...n.ch,
        sebastian.hesselbarth@...il.com, thierry.reding@...il.com,
        jonathanh@...dia.com, hayashi.kunihiko@...ionext.com,
        mhiramat@...nel.org, nobuhiro1.iwamatsu@...hiba.co.jp,
        linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-samsung-soc@...r.kernel.org,
        linux-tegra@...r.kernel.org
Subject: Re: [PATCH 0/7] arm/arm64: dts: Remove unused num-viewport from pcie
 node

On 29/12/2021 17:50, Mark Kettenis wrote:
>> From: Jisheng Zhang <jszhang@...nel.org>
>> Date: Thu, 30 Dec 2021 00:02:38 +0800
>>
>> After commit 281f1f99cf3a("PCI: dwc: Detect number of iATU windows"),
>> the number of iATU windows is detected at runtime, what's more,
>> the 'num-viewport' property parsing has been removed, so remove the
>> unused num-viewport from pcie node(s).
>>
>> It's too late for linux-5.17-rc1, I will rebase and send out v2 if
>> necessary when 5.17-rc1 is released.
> 
> Please no.  This only makes the device trees unnecessarily
> incompatible with older kernels

Anyone who is running a new DTB with older kernel is doomed anyway, not
only because of this change but hundreds of other similar cleanups, e.g.
making DTS conforming to dtschema. Are you sure there are such use cases
of using new DTB with old kernel? I cannot imagine making a stable
product with such scenario...

> and other OSes that do rely on the
> "num-viewport" property.

Right. We should have move the DTS out of the kernel when it was still
small. :)

> It really doesn't hurt to keep this property
> even if future Linux kernels no longer look at it.

For Exynos PCIe, the property is still required by bindings, so
definitely it cannot be removed from DTS. I did not check the other
bindings.

Best regards,
Krzysztof