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From:   Rob Herring <robh+dt@kernel.org>
Date:   Thu, 3 Feb 2022 08:44:47 -0600
Message-ID: <CAL_JsqLMit=e6vdum-xm1uxfCZcoJsTFe_S3k-QyVbvJPfNHew@mail.gmail.com>
Subject: Re: [PATCH] arm64: dts: qcom: add IPA qcom,qmp property
To:     Alex Elder <elder@linaro.org>
Cc:     Jakub Kicinski <kuba@kernel.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        "Gross, Andy" <agross@kernel.org>,
        David Miller <davem@davemloft.net>,
        Matthias Kaehlcke <mka@chromium.org>,
        Evan Green <evgreen@chromium.org>, cpratapa@codeaurora.org,
        avuyyuru@codeaurora.org, jponduru@codeaurora.org,
        subashab@codeaurora.org, Alex Elder <elder@kernel.org>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Thu, Feb 3, 2022 at 5:27 AM Alex Elder <elder@linaro.org> wrote:
>
> On 2/2/22 11:06 PM, Jakub Kicinski wrote:
> > On Tue,  1 Feb 2022 08:07:23 -0600 Alex Elder wrote:
> >> At least three platforms require the "qcom,qmp" property to be
> >> specified, so the IPA driver can request register retention across
> >> power collapse.  Update DTS files accordingly.
> >>
> >> Signed-off-by: Alex Elder <elder@linaro.org>
> >> ---
> >>
> >> Dave, Jakub, please let Bjorn take this through the Qualcomm tree.
> >
> > I don't know much about DT but the patch defining the property is
> > targeting net - will it not cause validation errors? Or Bjorn knows
> > to wait for the fixes to propagate? Or it doesn't matter? :)
>
> It might matter sometimes, but in this case it does not.
>
> If the DT property is present but never referenced by the
> code, it doesn't matter.
>
> The code in this patch looks up the DT property, and its
> behavior is affected by whether the property is there
> or not.  If it's not there, it's treated as an error
> that can be safely ignored.
>
> In the case this fix is actually needed, we'll need
> both the code present and DT property defined.  If
> the code is there but not the property, it's OK, but
> the bug won't be fixed quite yet.

If there's only one possible node that qcom,qmp points to, you can
just get the node by its compatible (of_find_compatible_node()). Then
you don't need a DT update to make things work. Of course, this
doesn't work too well if there are 10 possible compatibles without a
common fallback compatible.

Rob