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

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Date:   Thu, 5 May 2022 22:49:45 +0530
From:   Sumit Gupta <sumitg@...dia.com>
To:     Rob Herring <robh@...nel.org>
CC:     Thierry Reding <thierry.reding@...il.com>,
        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



On 05/05/22 19:34, Rob Herring wrote:
> External email: Use caution opening links or attachments
> 
> 
> On Thu, Apr 07, 2022 at 11:54:16AM +0530, Sumit Gupta wrote:
>>
>>>
>>>>>>>> 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.
> 
> No one is going to apply a 4 month old patch. For starters, the DT
> meta-schema evolves and this could now have errors. Please resend.
>
Sent v4 with rebased patches on linux-next.

Regards,
Sumit