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

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Date:   Thu, 7 Apr 2022 11:54:16 +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

Ping.

Regards,
Sumit