linux-kernel - Re: [PATCH] ath9k: fix build error with LEDS

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Date:   Mon, 25 Jan 2021 14:08:42 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Krzysztof Kozlowski <krzk@...nel.org>
Cc:     QCA ath9k Development <ath9k-devel@....qualcomm.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Johannes Berg <johannes@...solutions.net>,
        Arnd Bergmann <arnd@...db.de>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Flavio Suligoi <f.suligoi@...m.it>,
        linux-wireless <linux-wireless@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ath9k: fix build error with LEDS_CLASS=m

On Mon, Jan 25, 2021 at 12:40 PM Krzysztof Kozlowski <krzk@...nel.org> wrote:
> On Mon, 25 Jan 2021 at 12:36, Arnd Bergmann <arnd@...nel.org> wrote:
> >
> > From: Arnd Bergmann <arnd@...db.de>
> >
> > When CONFIG_ATH9K is built-in but LED support is in a loadable
> > module, both ath9k drivers fails to link:
> >
> > x86_64-linux-ld: drivers/net/wireless/ath/ath9k/gpio.o: in function `ath_deinit_leds':
> > gpio.c:(.text+0x36): undefined reference to `led_classdev_unregister'
> > x86_64-linux-ld: drivers/net/wireless/ath/ath9k/gpio.o: in function `ath_init_leds':
> > gpio.c:(.text+0x179): undefined reference to `led_classdev_register_ext'
> >
> > The problem is that the 'imply' keyword does not enforce any dependency
> > but is only a weak hint to Kconfig to enable another symbol from a
> > defconfig file.
> >
> > Change imply to a 'depends on LEDS_CLASS' that prevents the incorrect
> > configuration but still allows building the driver without LED support.
> >
> > The 'select MAC80211_LEDS' is now ensures that the LED support is
> > actually used if it is present, and the added Kconfig dependency
> > on MAC80211_LEDS ensures that it cannot be enabled manually when it
> > has no effect.
>
> But we do not want to have this dependency (selecting MAC80211_LEDS).
> I fixed this problem here:
> https://lore.kernel.org/lkml/20201227143034.1134829-1-krzk@kernel.org/
> Maybe let's take this approach?

Generally speaking, I don't like to have a device driver specific Kconfig
setting 'select' a subsystem', for two reasons:

- you suddenly get asked for tons of new LED specific options when
  enabling seemingly benign options

- Mixing 'depends on' and 'select' leads to bugs with circular
  dependencies that usually require turning some other 'select'
  into 'depends on'.

The problem with LEDS_CLASS in particular is that there is a mix of drivers
using one vs the other roughly 50:50.

      Arnd