Re: [PATCH net v5 2/3] net: usb: ax88179_178a: add USB device driver for config selection

[Michal Pecio <michal.pecio@gmail.com>]

On Fri, 17 Oct 2025 09:10:22 -0400, Alan Stern wrote:
> On Fri, Oct 17, 2025 at 10:42:29AM +0800, yicongsrfy@163.com wrote:
> > > > +	/* The vendor mode is not always config #1, so to find it out. */
> > > > +	c = udev->config;
> > > > +	num_configs = udev->descriptor.bNumConfigurations;
> > > > +	for (i = 0; i < num_configs; (i++, c++)) {
> > > > +		struct usb_interface_descriptor	*desc = NULL;
> > > > +
> > > > +		if (!c->desc.bNumInterfaces)
> > > > +			continue;
> > > > +		desc = &c->intf_cache[0]->altsetting->desc;
> > > > +		if (desc->bInterfaceClass == USB_CLASS_VENDOR_SPEC)
> > > > +			break;
> > > > +	}
> > > > +
> > > > +	if (i == num_configs)
> > > > +		return -ENODEV;
> > > > +
> > > > +	return c->desc.bConfigurationValue;
> > > > +}  
> > >
> > > I wonder how many copies of this code would justify making it some
> > > sort of library in usbnet or usbcore?  
> > 
> > Yes, there are many similar code instances in the USB subsystem.
> > However, I'm primarily focused on the networking subsystem,
> > so my abstraction work here might not be thorough enough.
> > Hopefully, an experienced USB developer may can optimize this issue.  
> 
> Would it help to have a USB quirks flag that tells the core to prefer 
> configurations with a USB_CLASS_VENDOR_SPEC interface class when we 
> choose the device's configuration?  Or something similar to that (I'm 
> not sure exactly what you are looking for)?

Either that or just patch usb_choose_configuration() to prefer vendor
config *if* an interface driver is available. But not 100% sure if that
couldn't backfire, so maybe only if the driver asked for it, indeed.

I was looking into making a PoC of that (I have r8152 with two configs)
but there is a snag: r8152-cfgselector bails out if a vendor-specific
check indicates the chip isn't a supported HW revision.

So to to fully replicate r8152-cfgselector, core would neet to get new
"pre_probe" callback in the interface driver. It gets complicated.

A less complicated improvement could be moving the common part of all
cfgselectors into usbnet. Not sure if it's worth it yet for only two?

Regards,
Michal