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

From: "Yu, Richard" <richard.yu@hpe.com>
To: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>,
	"Verdun, Jean-Marie" <verdun@hpe.com>,
	"Hawkins, Nick" <nick.hawkins@hpe.com>,
	"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
	"robh+dt@kernel.org" <robh+dt@kernel.org>,
	"krzysztof.kozlowski+dt@linaro.org" 
	<krzysztof.kozlowski+dt@linaro.org>,
	"linux@armlinux.org.uk" <linux@armlinux.org.uk>,
	"balbi@kernel.org" <balbi@kernel.org>,
	"linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>
Subject: RE: [PATCH v1 2/7] dt-bindings: usb: hpe,gxp-udc: Add binding for gxp gadget
Date: Mon, 7 Nov 2022 20:16:29 +0000	[thread overview]
Message-ID: <SJ0PR84MB2085E6B922DAF1070DC802EF8D3C9@SJ0PR84MB2085.NAMPRD84.PROD.OUTLOOK.COM> (raw)
In-Reply-To: <b85230d4-8fce-ba49-0d6b-8c4d20132cda@linaro.org>

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.