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

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Date:   Tue, 8 Nov 2022 12:30:39 +0100
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>,
        "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

On 07/11/2022 21:16, Yu, Richard wrote:
> 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.

Wrap your lines properly, it's impossible to reply in-line to such messages.

Then how do you specify two devices? You allow here only one, right?

Which bit in which register? Your devices have separate address space,
so why they cannot poke the same register, right? Then just always set
it to 0...

I might miss here something but so far it looks to me like some hacky
description matching the driver, not hardware, not existing bindings.

> 
>>> +
>>> +  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.

Nope. So you create here some weird IDs to poke into syscon register.
First, syscon has offset if you need. You could treat it maybe as bits?
I don't know... but even then your design is poor - two devices changing
the same register. Even though it is sunchronized by regmap, it is
conflicting, obfuscated access.

Best regards,
Krzysztof