netdev - Re: nt: usb: USB_RTL8153_ECM should not default to y

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Message-ID: <CANP3RGfPzXTMX+FAvd73EWjQnqUPyczuTD0dTQ79RMoVpjyQMg@mail.gmail.com>
Date:   Fri, 17 Sep 2021 21:05:55 +0200
From:   Maciej Żenczykowski <zenczykowski@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux NetDev <netdev@...r.kernel.org>,
        Hayes Wang <hayeswang@...ltek.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: nt: usb: USB_RTL8153_ECM should not default to y

On Fri, Sep 17, 2021 at 8:49 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Fri, 17 Sep 2021 19:59:15 +0200 Maciej Żenczykowski wrote:
> > I've been browsing some usb ethernet dongle related stuff in the
> > kernel (trying to figure out which options to enable in Android 13
> > 5.~15 kernels), and I've come across the following patch (see topic,
> > full patch quoted below).
> >
> > Doesn't it entirely defeat the purpose of the patch it claims to fix
> > (and the patch that fixed)?
> > Certainly the reasoning provided (in general device drivers should not
> > be enabled by default) doesn't jive with me.
> > The device driver is CDC_ETHER and AFAICT this is just a compatibility
> > option for it.
> >
> > Shouldn't it be reverted (ie. the 'default y' line be re-added) ?
> >
> > AFAICT the logic should be:
> >   if we have CDC ETHER (aka. ECM), but we don't have R8152 then we
> > need to have R8153_ECM.
> >
> > Alternatively, maybe there shouldn't be a config option for this at all?
> >
> > Instead r8153_ecm should simply be part of cdc_ether.ko iff r8152=n
> >
> > I'm not knowledgeable enough about Kconfig syntax to know how to
> > phrase the logic...
> > Maybe there shouldn't be a Kconfig option at all, and just some Makefile if'ery.
> >
> > Something like:
> >
> > obj-$(CONFIG_USB_RTL8152) += r8152.o
> > obj-$(CONFIG_USB_NET_CDCETHER) += cdc_ether.o obj-
> > ifndef CONFIG_USB_RTL8152
> > obj-$(CONFIG_USB_NET_CDCETHER) += r8153_ecm.o
> > endif
> >
> > Though it certainly would be nice to use 8153 devices with the
> > CDCETHER driver even with the r8152 driver enabled...
>
> Yeah.. more context here:
>
> https://lore.kernel.org/all/7fd014f2-c9a5-e7ec-f1c6-b3e4bb0f6eb6@samsung.com/
>
> default !USB_RTL8152 would be my favorite but that probably doesn't
> compute in kconfig land. Or perhaps bring back the 'y' but more clearly
> mark it as a sub-option of CDCETHER? It's hard to blame people for
> expecting drivers to default to n, we should make it clearer that this
> is more of a "make driver X support variation Y", 'cause now it sounds
> like a completely standalone driver from the Kconfig wording. At least
> to a lay person like myself.

I think:
        depends on USB_NET_CDCETHER && (USB_RTL8152 || USB_RTL8152=n)
        default y
accomplished exactly what was wanted.

USB_NET_CDCETHER is a dependency, hence:

USB_NET_CDCETHER=n forces it off - as it should - it's an addon to cdcether.

USB_NET_CDCETHER=m disallows 'y' - module implies addon must be module.

similarly USB_RTL8152 is a dependency, so it being a module disallows 'y'.
This is desired, because if CDCETHER is builtin, so this addon could
be builtin, then RTL8152 would fail to bind it by default.
ie. CDCETHER=y && RTL8152=m must force RTL8153_ECM != y  (this is the bugfix)

basically the funky 'USB_RTL8152 || USB_RTL8152=n' --> disallows 'y'
iff RTL8152=m

'default y' enables it by default as 'y' if possible, as 'm' if not,
and disables it if impossible.

So I believe this had the exact right default behaviour - and allowed
all the valid options.