linux-kernel - Re: [Patch v3 3/9] dt-bindings: arm: tegra: Add NVIDIA Tegra194 axi2apb binding

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Date:   Wed, 16 Mar 2022 13:15:35 +0530
From:   Sumit Gupta <sumitg@...dia.com>
To:     Thierry Reding <thierry.reding@...il.com>,
        Rob Herring <robh@...nel.org>
CC:     linux-tegra <linux-tegra@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        <devicetree@...r.kernel.org>, Jon Hunter <jonathanh@...dia.com>,
        <kbuild-all@...ts.01.org>, <bbasu@...dia.com>, <vsethi@...dia.com>,
        <jsequeira@...dia.com>, Thierry Reding <treding@...dia.com>,
        Sumit Gupta <sumitg@...dia.com>
Subject: Re: [Patch v3 3/9] dt-bindings: arm: tegra: Add NVIDIA Tegra194
 axi2apb binding



>>>>> Add device-tree binding documentation to represent the axi2apb bridges
>>>>> used by Control Backbone (CBB) 1.0 in Tegra194 SOC. All errors for APB
>>>>> slaves are reported as slave error because APB bas single bit to report
>>>>> error. So, CBB driver needs to further check error status registers of
>>>>> all the axi2apb bridges to find error type.
>>>>>
>>>>> Signed-off-by: Sumit Gupta<sumitg@...dia.com>
>>>>> Signed-off-by: Thierry Reding<treding@...dia.com>
>>>>> ---
>>>>>    .../arm/tegra/nvidia,tegra194-axi2apb.yaml    | 40 +++++++++++++++++++
>>>>>    1 file changed, 40 insertions(+)
>>>>>    create mode 100644 Documentation/devicetree/bindings/arm/tegra/nvidia,tegra194-axi2apb.yaml
>>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/arm/tegra/nvidia,tegra194-axi2apb.yaml b/Documentation/devicetree/bindings/arm/tegra/nvidia,tegra194-axi2apb.yaml
>>>>> new file mode 100644
>>>>> index 000000000000..788a13f8aa93
>>>>> --- /dev/null
>>>>> +++ b/Documentation/devicetree/bindings/arm/tegra/nvidia,tegra194-axi2apb.yaml
>>>>> @@ -0,0 +1,40 @@
>>>>> +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
>>>>> +%YAML 1.2
>>>>> +---
>>>>> +$id:"http://devicetree.org/schemas/arm/tegra/nvidia,tegra194-axi2apb.yaml#"
>>>>> +$schema:"http://devicetree.org/meta-schemas/core.yaml#"
>>>>> +
>>>>> +title: NVIDIA Tegra194 AXI2APB bridge
>>>>> +
>>>>> +maintainers:
>>>>> +  - Sumit Gupta<sumitg@...dia.com>
>>>>> +
>>>>> +properties:
>>>>> +  $nodename:
>>>>> +    pattern: "^axi2apb@([0-9a-f]+)$"
>>>>> +
>>>>> +  compatible:
>>>>> +    enum:
>>>>> +      - nvidia,tegra194-axi2apb
>>>>> +
>>>>> +  reg:
>>>>> +    maxItems: 6
>>>>> +    description: Physical base address and length of registers for all bridges
>>>>> +
>>>>> +additionalProperties: false
>>>>> +
>>>>> +required:
>>>>> +  - compatible
>>>>> +  - reg
>>>>> +
>>>>> +examples:
>>>>> +  - |
>>>>> +    axi2apb: axi2apb@...0000 {
>>>> As axi2apb appears to be a bus, then all the child nodes (APB devices)
>>>> should be under this node.
>>> axi2apb is a bridge which coverts an AXI to APB interface and not a bus.
>> A bus and bridge node are pretty much one and the same in DT
>> representation. A PCI host bridge has a PCI bus beneath it for
>> example.
> Sorry for taking so long to reply, this fell through the cracks.
> 
> These aren't really bridges as such. CBB (which we call /bus@0 in DT) is
> a sort of large container for all IP. Within that there are various shim
> layers that connect these "legacy" interfaces to CBB. I suppose you
> could call them bridges, but it's a bit of a stretch. From a software
> point of view there is no observable translation happening. The only
> reason why we need this is for improved error reporting.
> 
> The TRM also doesn't make a distinction between the various bridges. The
> devices are all just mapped into a single address space via the CBB.
> 
> My understanding is that this is also gone in newer chips, so matters
> become a bit simpler there.
> 
> Reorganizing /bus@0 into multiple bridges and busses would be a lot of
> churn and likely confuse people that want to correlate what's in the TRM
> to what's in DT, so I don't think it's worth it.
> 
> For newer chips we may want to keep this in mind so we structure the DT
> more accurately from the beginning, though as I said, things have been
> simplified a bit, so this may not be an issue anymore.
> 
> Thierry

Hi Thierry,
Thank you for answering the concern.

Hi Rob,
Can you please ACK to help queue the patch series for next.

Regards,
Sumit