linux-kernel - Re: [PATCHv2 1/3] net: phy: prevent linking breakage

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Message-ID: <6466898.RnYOe3jpGG@wuerfel>
Date:	Tue, 04 Jun 2013 17:07:39 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	David Miller <davem@...emloft.net>
Cc:	alexandre.belloni@...e-electrons.com, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux@....linux.org.uk, shawn.guo@...aro.org, kernel@...gutronix.de
Subject: Re: [PATCHv2 1/3] net: phy: prevent linking breakage

On Thursday 30 May 2013 02:42:01 David Miller wrote:
> From: Alexandre Belloni <alexandre.belloni@...e-electrons.com>
> > On 28/05/2013 22:09, David Miller wrote:
> > But that is making it impossible to compile a kernel without any network
> > stack for those platforms or we are going back to either enclosing the
> > calls to phy_register_fixup{,_for_uid,_for_id} with #ifdef CONFIG_PHYLIB
> > or if(IS_BUILTIN(CONFIG_PHYLIB)). And as you can see, it is quite error
> > prone and is done only done for 2 platforms on a total of 6. I believe
> > fixing that in phy.h is more foolproof.
> 
> Or you properly segregate the networking bits of the platform code so
> that it can be built only when the necessary networking portions are
> enabled.
> 
> Sometimes having dummy stubs makes sense, but not in this situation.

Currently most users of this function are doing something like

static int foo_phy_fixup(struct phy_device *phydev)
{
	...
}

static int __init boo_board_init(void)
{
	 if (IS_BUILTIN(CONFIG_PHYLIB))
		phy_register_fixup_for_uid(phy_id, foo_phy_fixup);
}

which is practically the same as having a dummy stub. It leads to
the foo_phy_fixup() function always getting compiled and then discarded
by gcc when CONFIG_PHYLIB is disabled.

The method is currently broken when network drivers are enabled as
modules, because we are missing the fixup then.

I think we should use IS_ENABLED() here to force a build error
in that case, and have something like

config ARCH_FOO
	bool "support for the foo platform"
	select PHYLIB if NET

in the platform Kconfig file, to ensure PHYLIB is always built-in.

I still think the inline alternatives would be helpful, but using
if (IS_ENABLED(CONFIG_PHYLIB)) in the platform code would also
work.

	Arnd
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