linux-kernel - Re: [PATCH v3 06/19] dt-bindings: memory: tegra: Squash tegra20-gart into tegra20-mc

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Date:   Mon, 20 Aug 2018 22:27:49 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Rob Herring <robh@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>
Cc:     Joerg Roedel <joro@...tes.org>,
        Robin Murphy <robin.murphy@....com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        iommu@...ts.linux-foundation.org, devicetree@...r.kernel.org,
        linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 06/19] dt-bindings: memory: tegra: Squash tegra20-gart
 into tegra20-mc

On 20.08.2018 22:12, Rob Herring wrote:
> On Sat, Aug 18, 2018 at 06:54:17PM +0300, Dmitry Osipenko wrote:
>> Splitting GART and Memory Controller wasn't a good decision that was made
>> back in the day. Given that the GART driver hasn't ever been used by
>> anything in the kernel, we decided that it will be better to correct the
>> mistakes of the past and merge two bindings into a single one. In a result
> 
> As a result...
> 
>> there is a DT ABI change for the Memory Controller that allows not to
>> break newer kernels using older DT by introducing a new required property,
>> the memory clock. Adding the new clock property also puts the tegra20-mc
>> binding in line with the bindings of the later Tegra generations.
> 
> I don't understand this part. It looks to me like you are breaking 
> compatibility. The driver failing to probe with an old DT is okay?

Yes, DT compatibility is broken. New driver won't probe/load with the old DT,
that's what we want.

> OS's like OpenSUSE use new DTs with older kernel versions, so you should 
> consider how to not break them as well. I guess if all this is optional 
> or has been unused, then there shouldn't be a problem.

That's interesting.. Memory Controller isn't optional, I guess we could change
compatible to "nvidia,tegra20-mc-gart".

Thierry, do you have any other suggestions?