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

[Thierry Reding]

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.

Thierry
From 7933af1d2e5f3941d934eec88f32f5547ee218c3 Mon Sep 17 00:00:00 2001
From: Thierry Reding <thierry.reding@xxxxxxxxx>
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@xxxxxxxxx>
Signed-off-by: Thierry Reding <thierry.reding@xxxxxxxxx>
---
 drivers/pwm/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/pwm/Kconfig b/drivers/pwm/Kconfig
index a3ecf5809634..468af1b4ca30 100644
--- a/drivers/pwm/Kconfig
+++ b/drivers/pwm/Kconfig
@@ -53,6 +53,7 @@ config PWM_ATMEL
 config PWM_ATMEL_HLCDC_PWM
 	tristate "Atmel HLCDC PWM support"
 	depends on MFD_ATMEL_HLCDC
+	depends on HAVE_CLK
 	help
 	  Generic PWM framework driver for the PWM output of the HLCDC
 	  (Atmel High-end LCD Controller). This PWM output is mainly used
-- 
2.1.3

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