On Thu, Dec 18, 2014 at 10:44:44AM +0100, Thierry Reding 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. > > 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. If I turn is_power_of_2() into a macro, then this build failure also goes away. I suppose the reason is that now the do_div() evaluates such that the branch that would reference the undefined symbol can be discarded. Still I think allowing that branch to remain will cause the linker failure on division by (constant) zero, which has some value, too. Thierry
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