linux-kernel - Re: [PATCH v1 1/3] dt-bindings: usb: Add HPE GXP UDCG Controller

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Date:   Sat, 5 Aug 2023 21:08:35 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     "Yu, Richard" <richard.yu@....com>,
        "Verdun, Jean-Marie" <verdun@....com>,
        "Hawkins, Nick" <nick.hawkins@....com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "robh+dt@...nel.org" <robh+dt@...nel.org>,
        "krzysztof.kozlowski+dt@...aro.org" 
        <krzysztof.kozlowski+dt@...aro.org>,
        "conor+dt@...nel.org" <conor+dt@...nel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 1/3] dt-bindings: usb: Add HPE GXP UDCG Controller

On 01/08/2023 20:07, Yu, Richard wrote:
> 
>>> +title: HPE GXP USB Virtual EHCI controller
> 
>> The word "virtual" in bindings pretty often raises questions, because we
>> describe usually real hardware, not virtual. Some explanation in
>> description would be useful.
> 
> Here we are working with virtual devices that are created and have no

Unfortunately I do not know what are virtual devices which do not exist
physically. I have serious doubts that they fit Devicetree purpose...

> physical presence. We have modeled our code off of ASPEED's VHUB
> implementation to comply with the implementation in OpenBMC.
> 
>>> + The HPE GXP USB Virtual EHCI Controller implements 1 set of USB EHCI
>>> + register and several sets of device and endpoint registers to support
>>> + the virtual EHCI's downstream USB devices.
>>> +
> 
> 
>> If this is EHCI controller, then I would expect here reference to usb-hcd.
> 
> We will remove references to EHCI in code and documentation. It has been
> modeled to following ASPEEDs approach as mentioned above.
> 
>>> + hpe,vehci-downstream-ports:
>>> + description: Number of downstream ports supported by the GXP
> 
> 
>> Why do you need this property in DT and what exactly does it represent?
>> You have one device - EHCI controller - and on some boards it is further
>> customized? Even though it is the same device?
> 
> That is correct. We can configure this VHUB Controller to have one to
> 8 virtual ports. This is similar to the aspeed virtual USB HUB
> "aspeed,vhub-downstream-ports" moving forward in the next patch
> we are going to use "hpe,vhub-downstream-ports"

Moving forward you need to address this lack of physical presence...
Aren't these different devices and you just forgot to customize the
compatible?

Best regards,
Krzysztof