linux-kernel - Re: [PATCH v3 1/2] dt-bindings: exynos-dw-mshc-common: add exynos78xx variants

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Date:   Sun, 12 Mar 2023 10:26:13 -0700 (PDT)
From:   Sergey Lisov <sleirsgoevy@...il.com>
To:     Ulf Hansson <ulf.hansson@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Alim Akhtar <alim.akhtar@...sung.com>,
        Jaehoon Chung <jh80.chung@...sung.com>
Cc:     linux-mmc@...r.kernel.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] dt-bindings: exynos-dw-mshc-common: add exynos78xx
 variants

> Thanks for letting me know.
> 
> https://elixir.bootlin.com/linux/v6.1-rc1/source/Documentation/devicetree/bindings/writing-bindings.rst#L42

OK, at least its codified somewhere. Still, this results in the opposite
effect: DTBs written for one SoC, using compatibles from other SoCs just
because "they are the same anyway". And doing this properly, well, results
in essentially duplicate compatibles.

And "fallback compatibles" won't solve this case anyway, as there is no
common compatible that denotes "Exynos7 DW-MMC that has the bug".

>>> That's non-bisectable change (also breaking other users of DTS), so you
>>> need to explain in commit msg rationale - devices were never compatible
>>> and using exynos7 does not work in certain cases.

Probably it makes sense to put this patch after the actual implementation,
so that git bisect always gives a working setup.

> BTW, this rationale was only example - you need to come with something real.

Pretty much the only thing that is broken are SDIO cards, because they run
very short transfers (below the DMA threshold) over the data lines. That's
exactly what I stated.