linux-kernel - Re: [Patch v4 01/10] dt-bindings: memory: tegra: add bpmp ref in tegra234-mc node

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Message-ID: <ZCLiCWRYbO98qwCn@orome>
Date:   Tue, 28 Mar 2023 14:48:09 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Sumit Gupta <sumitg@...dia.com>
Cc:     treding@...dia.com, dmitry.osipenko@...labora.com,
        viresh.kumar@...aro.org, rafael@...nel.org, jonathanh@...dia.com,
        robh+dt@...nel.org, lpieralisi@...nel.org,
        linux-kernel@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-pm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-pci@...r.kernel.org, mmaddireddy@...dia.com, kw@...ux.com,
        bhelgaas@...gle.com, vidyas@...dia.com, sanjayc@...dia.com,
        ksitaraman@...dia.com, ishah@...dia.com, bbasu@...dia.com
Subject: Re: [Patch v4 01/10] dt-bindings: memory: tegra: add bpmp ref in
 tegra234-mc node

On Tue, Mar 28, 2023 at 01:22:26PM +0200, Krzysztof Kozlowski wrote:
> On 28/03/2023 12:48, Thierry Reding wrote:
> > On Tue, Mar 28, 2023 at 09:23:04AM +0200, Krzysztof Kozlowski wrote:
> >> On 27/03/2023 18:14, Sumit Gupta wrote:
> >>> For Tegra234, add the "nvidia,bpmp" property within the Memory
> >>> Controller (MC) node to reference BPMP node. This is needed in
> >>> the MC driver to pass the client info to the BPMP-FW when memory
> >>> interconnect support is available.
> >>>
> >>> Signed-off-by: Sumit Gupta <sumitg@...dia.com>
> >>> ---
> >>>  .../bindings/memory-controllers/nvidia,tegra186-mc.yaml    | 7 +++++++
> >>>  1 file changed, 7 insertions(+)
> >>>
> >>> diff --git a/Documentation/devicetree/bindings/memory-controllers/nvidia,tegra186-mc.yaml b/Documentation/devicetree/bindings/memory-controllers/nvidia,tegra186-mc.yaml
> >>> index 935d63d181d9..398d27bb2373 100644
> >>> --- a/Documentation/devicetree/bindings/memory-controllers/nvidia,tegra186-mc.yaml
> >>> +++ b/Documentation/devicetree/bindings/memory-controllers/nvidia,tegra186-mc.yaml
> >>> @@ -58,6 +58,10 @@ properties:
> >>>    "#interconnect-cells":
> >>>      const: 1
> >>>  
> >>> +  nvidia,bpmp:
> >>> +    $ref: /schemas/types.yaml#/definitions/phandle
> >>> +    description: phandle of the node representing the BPMP
> >>
> >> Why do you need this multiple times? Both in parent and all external-mc
> >> children?
> > 
> > We've had nvidia,bpmp in the external memory controller node since
> > basically the beginning because we've always needed it there. For newer
> > chips we now also need it for the memory controller.
> > 
> > Ideally I think we would only have this in the MC and have the EMC
> > driver reference it via the EMC's parent (i.e. MC), but that would break
> > backwards-compatibility. Reaching into the EMC's DT node from the MC was
> > another option that we discussed internally, but it didn't look right
> > given how this is also needed by the MC.
> > 
> > One thing we could potentially do is deprecate the nvidia,bpmp phandle
> > in the EMC and only keep it as a fallback in the drivers in case the
> > parent MC doesn't find it's own in the DT.
> 
> Yes, deprecation would answer to my question.

Okay, great. Sumit, you can resolve this by adding a "deprecated: true"
to the EMC's nvidia,bpmp property schema. In the driver we can then try
to look at the MC's ->bpmp and if it exists reuse that. If it doesn't
exist, we can keep the existing lookup as a fallback for device trees
that haven't been updated yet.

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