Message-ID: <1448290682.5792.6.camel@sipsolutions.net> (sfid-20151123_155831_172025_1E3614FA)
Subject: Re: [RFC v5 4/5] mac80211: use Kconfig counters to elide
 unnecessary code
From: Johannes Berg <johannes@sipsolutions.net>
To: Michal Marek <mmarek@suse.com>
Cc: linux-wireless@vger.kernel.org
Date: Mon, 23 Nov 2015 15:58:02 +0100
In-Reply-To: <5653283C.8050207@suse.com>
References: <1447365652-23716-1-git-send-email-johannes@sipsolutions.net>
	 <1447365652-23716-5-git-send-email-johannes@sipsolutions.net>
	 <5653283C.8050207@suse.com>
Content-Type: text/plain; charset="UTF-8"
Mime-Version: 1.0
Sender: linux-wireless-owner@vger.kernel.org

On Mon, 2015-11-23 at 15:52 +0100, Michal Marek wrote:
> 
> So one issue with this logic is that it is not safe to build and use an
> out-of-tree mac80211 driver after the kernel has been built. Also, any
> new driver needs the 'count MAC80211_NUM_DRIVERS' annotation, but there
> is the runtime check to catch omissions. 

Indeed, the runtime check will catch both of these.

> Since this is targeting users
> of very specific configs, how about an opt-in scheme à la
> 
> - User has to select CONFIG_MAC80211_SINGLE_DRIVER, the help text
>   explains the caveats and lists drivers known to work in such mode.
> - mac80211 uses the Kconfig-defined constants + dynamic bits iff
>   CONFIG_MAC80211_SINGLE_DRIVER=y, otherwise it behaves as before.
> - Some build- or compile-time check ensuring that we are not building /
>   loading multiple drivers with CONFIG_MAC80211_SINGLE_DRIVER=y.

That'd be possible - but I know of at least one potential use case that
would like to have two similar drivers, with similar flags: some
routers ship with ath9k hardware for the 2.4 GHz band and ath10k
hardware for the 5 GHz band. Doing "single-driver" and avoiding the
counters would not allow those to have any kind of optimisation, and
such low-power platforms are the ones who'd most likely benefit from
it...

That said, we really do need to figure out whether this makes sense at
all.

I'd actually Cc'ed you out of confusion, thinking you were maintaining
kconfig. I guess I really should've Cc'ed Yann instead.

johannes