linux-kernel - RE: [PATCH v1 2/7] dt-bindings: usb: hpe,gxp-udc: Add binding for gxp gadget

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Date:   Mon, 7 Nov 2022 20:16:29 +0000
From:   "Yu, Richard" <richard.yu@....com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        "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>,
        "linux@...linux.org.uk" <linux@...linux.org.uk>,
        "balbi@...nel.org" <balbi@...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>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: RE: [PATCH v1 2/7] dt-bindings: usb: hpe,gxp-udc: Add binding for gxp
 gadget

Hi Mr. Kozlowski,

> > +
> > +  vdevnum:
> > +    description:
> > +      virtual device number.

> That's unusual property... Why numbering devices is part of DT (hardware description)?

In HPE GXP virtual EHCI controller chipset, it can support up to 8 virtual devices(gadgets). Each device/gadget will be represented by a bit in 8 bits register. For example, the interrupt register bit 0 indicates the interrupt from device 0, bit 1 for device 1 ... so on.  When a user defines a device/gadget, he/she can define the device number as between 0 and 7. Thus, the driver can look up to the bit position. That is why we have numbering devices as part of DT.

> > +
> > +  fepnum:
> > +    description:
> > +      number of the flexible end-points this device is needed.

> Similar question.

In HPE GXP virtual EHCI Controller chipset, there is a flexible End-Point(EP) pool. Each flexible EP has its own mapping register. The mapping register bit 0 to 3 is for device number (vdevnum) and bit 4 to 7 is for EP number inside the device. The device driver configures the mapping register to assign a flexible EP to a specific device.  Here, "fepnum" is the input letting the driver know how many EPs are needed for this device/gadget.

Thanks

Richard.