linux-kernel - Re: randconfig build error with next-20141204, in drivers/pwm

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Message-ID: <20141219085015.GA7352@ulmo>
Date:	Fri, 19 Dec 2014 09:50:17 +0100
From:	Thierry Reding <thierry.reding@...il.com>
To:	Boris Brezillon <boris.brezillon@...e-electrons.com>
Cc:	Jim Davis <jim.epost@...il.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	linux-next <linux-next@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-pwm@...r.kernel.org
Subject: Re: randconfig build error with next-20141204, in drivers/pwm

On Thu, Dec 18, 2014 at 08:51:32PM +0100, Boris Brezillon wrote:
> Hi Thierry,
> 
> On Thu, 18 Dec 2014 10:44:44 +0100
> Thierry Reding <thierry.reding@...il.com> wrote:
> 
> > On Thu, Dec 04, 2014 at 09:10:55AM -0700, Jim Davis wrote:
> > > Building with the attached random configuration file,
> > > 
> > > ERROR: "____ilog2_NaN" [drivers/pwm/pwm-atmel-hlcdc.ko] undefined!  
> > 
> > This took a while to figure out. The attached patch fixes this build
> > failure, though the driver should probably be fixed to avoid division by
> > zero, just in case. Adding Boris for visibility.
> 
> Thanks for fixing this build issue. I'll propose a patch to prevent
> this div by 0 from happening.
> 
> > 
> > Thierry
> > 
> > From 7933af1d2e5f3941d934eec88f32f5547ee218c3 Mon Sep 17 00:00:00 2001
> > From: Thierry Reding <thierry.reding@...il.com>
> > Date: Thu, 18 Dec 2014 10:09:42 +0100
> > Subject: [PATCH] pwm: atmel-hlcdc: Depend on HAVE_CLK
> > 
> > The include/linux/clk.h header defines dummy implementations for the
> > various clk_*() functions if HAVE_CLK is not selected to improve build
> > coverage in randconfig builds.
> > 
> > The dummy implementation of clk_get_rate() returns 0, which causes the
> > Atmel HLCDC PWM driver's atmel_hlcdc_pwm_config() implementation to end
> > up calling:
> > 
> > 	do_div(clk_period_ns, 0)
> > 
> > On x86, do_div(n, base) will end up evaluating to this:
> > 
> > 	n >>= ilog2(base)
> > 
> > with base = 0, the implementation of ilog2() will call ____ilog2_NaN(),
> > which is purposely undefined and results in a linker failure:
> > 
> > 	ERROR: "____ilog2_NaN" [drivers/pwm/pwm-atmel-hlcdc.ko] undefined!
> > 
> > The implementation of do_div() checks that base is a power of 2 before
> > calling ilog2(). The compiler doesn't optimize this away, presumably
> > because is_power_of_2() is an inline function and the compiler doesn't
> > or can't inspect it closely enough. ilog2() being a macro it still ends
> > up generating the ____ilog2_NaN() because of the constant 0.
> > 
> > The root of the problem is that the driver really should be checking
> > before possibly dividing by zero. That should eventually be fixed, but
> > for now just assume that the clock runs at a sensible frequency when
> > available.
> > 
> > Reported-by: Jim Davis <jim.epost@...il.com>
> > Signed-off-by: Thierry Reding <thierry.reding@...il.com>
> 
> Acked-by: Boris Brezillon <boris.brezillon@...e-electrons.com>

Applied, thanks.

Thierry

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