linux-kernel - Re: [PATCH v2 3/3] mmc: sdhci-msm: drop redundant of_device

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Message-ID: <CAD=FV=WdH0FLfQ-MMwkrHfo3xTjg8M2vxV5iO-CEpCDWG8NKKw@mail.gmail.com>
Date:   Wed, 13 Jul 2022 09:10:46 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
Cc:     Ulf Hansson <ulf.hansson@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Bhupesh Sharma <bhupesh.sharma@...aro.org>,
        Linux MMC List <linux-mmc@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] mmc: sdhci-msm: drop redundant of_device_id entries

Hi,

On Wed, Jul 13, 2022 at 9:07 AM Krzysztof Kozlowski
<krzysztof.kozlowski@...aro.org> wrote:
>
> On 13/07/2022 17:57, Doug Anderson wrote:
> > Hi,
> >
> > On Tue, Jul 12, 2022 at 8:02 AM Krzysztof Kozlowski
> > <krzysztof.kozlowski@...aro.org> wrote:
> >>
> >> This reverts three commits:
> >> 1. Revert "mmc: sdhci-msm: Add compatible string check for sdx65"
> >>    This reverts commit 953706844f0f2fd4dc6984cc010fe6cf51c041f2.
> >>
> >> 2. Revert "mmc: sdhci-msm: Add compatible string check for sm8150"
> >>    This reverts commit 5acd6adb65802cc6f9986be3750179a820580d37.
> >>
> >> 3. Revert "mmc: sdhci-msm: Add SoC specific compatibles"
> >>    This reverts commit 466614a9765c6fb67e1464d0a3f1261db903834b.
> >>
> >> The oldest commit 466614a9765c ("mmc: sdhci-msm: Add SoC specific
> >> compatibles") did not specify what benefits such multiple compatibles
> >> bring, therefore assume there is none.  On the other hand such approach
> >> brings a lot of churn to driver maintenance by expecting commit for
> >> every new compatible, even though it is already covered by the fallback.
> >>
> >> There is really no sense in duplicating of_device_id for each
> >> variant, which is already covered by generic compatible fallback
> >> qcom,sdhci-msm-v4 or qcom,sdhci-msm-v5.
> >>
> >> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
> >
> > Personally, I would have taken the extra step and added a comment in
> > the code to prevent someone from doing this again. Maybe like this:
> >
> > /*
> >  * In the device tree, all boards are required to have _two_ compatible
> >  * strings listed: a SoC-specific one followed by a more generic one.
> >  * Normally we can just rely on the generic string, but we always
> >  * include both so that if we ever find a bug on a specific SoC that
> >  * we need to workaround (like in sdm845/sc7180) that we can quickly
> >  * work around it without any changes to the dts.
> >  */
>
> This actually does not instruct the developer not to add new variants to
> the driver, so how about something like:
>
> /* Do not add new variants to the driver which are compatible with
> generic ones, unless they need customization. */
> ?

Sure, that would be fine.


> The problem is that this applies to several such drivers on several
> platforms (Qualcomm, NXP - these for sure use such pattern), so we would
> be documenting something obvious, IMO.

The problem is that the people adding to this file are probably not
device tree experts and may not know, so a short comment might be
worthwhile. If you don't think it's a good idea, though, I won't push.

-Doug