Re: [arm-platforms:timers/tval-ectomy 2/17] drivers/clocksource/arm_arch_timer.c:156:3: warning: variable 'val' is used uninitialized whenever switch default is taken

Marc Zyngier
2021-10-10T09:39:32Z

On 2021-10-09 22:12, kernel test robot wrote:
> tree:
> https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git
> timers/tval-ectomy
> head: c815d60a6292c90ea228c635b4150f022322908b
> commit: 9c6e4aa680ebd5ad816fc8f97858ec6057362c9e [2/17]
> clocksource/arm_arch_timer: Drop CNT*_TVAL read accessors
> config: arm64-randconfig-r026-20211010 (attached as .config)
> compiler: clang version 14.0.0 (https://github.com/llvm/llvm-project
> 30caca39f401ae17927439c0a0bd6d1b1916dd6a)
> reproduce (this is a W=1 build):
> wget
> https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross
> -O ~/bin/make.cross
> chmod +x ~/bin/make.cross
> # install arm64 cross compiling tool for clang build
> # apt-get install binutils-aarch64-linux-gnu
> #
> https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/commit/?id=9c6e4aa680ebd5ad816fc8f97858ec6057362c9e
> git remote add arm-platforms
> https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git
> git fetch --no-tags arm-platforms timers/tval-ectomy
> git checkout 9c6e4aa680ebd5ad816fc8f97858ec6057362c9e
> # save the attached .config to linux build tree
> COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1
> ARCH=arm64
>
> If you fix the issue, kindly add following tag as appropriate
> Reported-by: kernel test robot <lkp@intel.com>

This is *not* an issue, see below.

>
> All warnings (new ones prefixed by >>):
>
>>> drivers/clocksource/arm_arch_timer.c:156:3: warning: variable 'val'
>>> is used uninitialized whenever switch default is taken
>>> [-Wsometimes-uninitialized]
> default:
> ^~~~~~~
> drivers/clocksource/arm_arch_timer.c:163:9: note: uninitialized use
> occurs here
> return val;
> ^~~
> drivers/clocksource/arm_arch_timer.c:147:3: warning: variable 'val'
> is used uninitialized whenever switch default is taken
> [-Wsometimes-uninitialized]
> default:
> ^~~~~~~
> drivers/clocksource/arm_arch_timer.c:163:9: note: uninitialized use
> occurs here
> return val;
> ^~~
> drivers/clocksource/arm_arch_timer.c:139:9: note: initialize the
> variable 'val' to silence this warning
> u32 val;
> ^
> = 0
> 2 warnings generated.
>
>
> vim +/val +156 drivers/clocksource/arm_arch_timer.c
>
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 134
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 135 static __always_inline
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 136 u32
> arch_timer_reg_read(int access, enum arch_timer_reg reg,
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 137 struct
> clock_event_device *clk)
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 138 {
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 139 u32 val;
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 140
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 141 if (access ==
> ARCH_TIMER_MEM_PHYS_ACCESS) {
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 142 struct arch_timer
> *timer = to_arch_timer(clk);
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 143 switch (reg) {
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 144 case
> ARCH_TIMER_REG_CTRL:
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 145 val =
> readl_relaxed(timer->base + CNTP_CTL);
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 146 break;
> 0bc2c9327c236c Marc Zyngier 2021-09-20 147 default:
> 0bc2c9327c236c Marc Zyngier 2021-09-20 148 BUILD_BUG();
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 149 }
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 150 } else if (access ==
> ARCH_TIMER_MEM_VIRT_ACCESS) {
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 151 struct arch_timer
> *timer = to_arch_timer(clk);
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 152 switch (reg) {
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 153 case
> ARCH_TIMER_REG_CTRL:
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 154 val =
> readl_relaxed(timer->base + CNTV_CTL);
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 155 break;
> 0bc2c9327c236c Marc Zyngier 2021-09-20 @156 default:
> 0bc2c9327c236c Marc Zyngier 2021-09-20 157 BUILD_BUG();

If you hit this case, which results in a compilation failure...

> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 158 }
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 159 } else {
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 160 val =
> arch_timer_reg_read_cp15(access, reg);
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 161 }
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 162
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 163 return val;

... how can you return anything the first place?

> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 164 }
> f4e00a1a55c7f9 Marc Zyngier 2017-01-20 165

I think the problem squarely lies with the compiler (or the
way it interprets the BUILD_BUG helper). If it cannot see that
BUILD_BUG() trumps the control flow analysis, then this warning
is totally useless.

M.
--
Jazz is not dead. It just smells funny...