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

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Message-ID: <20251020182327.0dd8958a.michal.pecio@gmail.com>
Date: Mon, 20 Oct 2025 18:23:27 +0200
From: Michal Pecio <michal.pecio@...il.com>
To: Alan Stern <stern@...land.harvard.edu>
Cc: yicongsrfy@....com, andrew+netdev@...n.ch, davem@...emloft.net,
 edumazet@...gle.com, kuba@...nel.org, linux-usb@...r.kernel.org,
 netdev@...r.kernel.org, oliver@...kum.org, pabeni@...hat.com
Subject: Re: [PATCH net v5 2/3] net: usb: ax88179_178a: add USB device
 driver for config selection

On Mon, 20 Oct 2025 11:56:50 -0400, Alan Stern wrote:
> On Sat, Oct 18, 2025 at 05:56:18PM +0200, Michal Pecio wrote:
> > On Sat, 18 Oct 2025 11:36:11 -0400, Alan Stern wrote:  
> > > How are prefer_vendor() and usb_driver_preferred() supposed to know 
> > > which configuration is being considered?  
> > 
> > Currently they don't need to know, but this could be added by passing
> > a temporary struct with more stuff in place of udev.
> > 
> > Really, this whole usb_drv->preferred business could be a simple
> > boolean flag, if not for r8152 needing to issue control transfers to
> > the chip to find whether it supports at all.
> > 
> > It seems that ax88179_preferred() could simply always return true.  
> 
> Instead of all this preferred() stuff, why not have the ax88179 driver's 
> probe routine check for a different configuration with a vendor-specific 
> interface?  If that other config is present and the chip is the right 
> type then you can call usb_driver_set_configuration() -- this is exactly 
> what it's meant for.

That could be doable and some code could be shared I guess, but how to
get the probe() routine to run in the first place?

The chip may be in other configuration, without this vendor interface.
If we remove _AND_INTERFACE_INFO, it's still a problem that cdc_ether
may already be bound to the CDC interface in CDC config.

Registering a *device* driver plows through such obstacles, because
core allows device drivers to immediately displace existing drivers.

It seems that this could work, if cdc_ether blacklisting and revert
of _AND_INTERFACE_INFO are applied as suggested in this series.
(But as part of the main commit, to avoid transient regressions).

I wonder if blacklisting is considered necessary evil? Without it, it's
possible that cdc_ether binds for a moment before it's kicked out by
the vendor driver. Looks weird in dmesg, at the very least.

FWIW, my RTL8153 is blacklisted in cdc_ether too. So much for the
promise that cfgselectors will allow users to choose drivers ;)

Regards,
Michal