netdev - Re: [PATCH] arm64: dts: qcom: add IPA qcom,qmp property

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Date:   Thu, 3 Feb 2022 14:11:48 -0600
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Rob Herring <robh+dt@...nel.org>
Cc:     Alex Elder <elder@...aro.org>, Jakub Kicinski <kuba@...nel.org>,
        "Gross, Andy" <agross@...nel.org>,
        David Miller <davem@...emloft.net>,
        Matthias Kaehlcke <mka@...omium.org>,
        Evan Green <evgreen@...omium.org>, cpratapa@...eaurora.org,
        avuyyuru@...eaurora.org, jponduru@...eaurora.org,
        subashab@...eaurora.org, Alex Elder <elder@...nel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm64: dts: qcom: add IPA qcom,qmp property

On Thu 03 Feb 08:44 CST 2022, Rob Herring wrote:

> On Thu, Feb 3, 2022 at 5:27 AM Alex Elder <elder@...aro.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@...aro.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.
> 

In one of the latest platforms there's two QMP instances, so we decided
to make the reference explicit to prepare for the introduction of that.

So I intend to pick this patch through the qcom tree.

And we don't need to synchronize it with the net-changes; the two
changes can go independently and only when they are both in place will
the change take effect and the issue resolved.

Regards,
Bjorn