Re: [PATCH] psci: add CPU_IDLE dependency

From: Kevin Brodsky
Date: Mon Jul 31 2017 - 12:22:24 EST


On 31/07/17 15:14, Jean Delvare wrote:
Hi Kevin,

On Mon, 31 Jul 2017 11:19:39 +0100, Kevin Brodsky wrote:
On 31/07/17 09:55, Arnd Bergmann wrote:
I ran into a build error for the psci_checker:

drivers/firmware/psci_checker.o: In function `psci_checker':
psci_checker.c:(.init.text+0x528): undefined reference to `cpuidle_devices'

As far as I can tell, this is simply a very rare combination of options,
but the problem has existed since the code was initially added.
Adding a Kconfig dependency makes it build properly.
Good catch! For some reason I missed this config option when figuring out the
dependencies... I wonder though, shouldn't cpuidle.h declare cpuidle_devices
conditionally on CONFIG_CPU_IDLE?
Such conditional declarations only make sense if there is a legitimate
use of the disabled case and if they make the disabled case fully
transparent to the users. This is typically done by replacing function
declarations by inline stubs doing nothing in the right way when the
feature is disabled. It avoids having to put the condition checks on the
side of all users.

In this case however, you can't stub out cpuidle_devices alone. If you
omit the declaration when CONFIG_CPU_IDLE isn't set, all you'll get is a
failure at compilation time, instead of at linkage time. This barely
helps. For it to be useful, you would additionally have to provide
wrappers around
this_cpu_read(cpuidle_devices)
and
per_cpu(cpuidle_devices, cpu)
and stub out these wrappers when CONFIG_CPU_IDLE is disabled (so you
don't refer to cpuidle_devices at all when it isn't available.)

But then again this would only make sense if the psci_checker still
serves a purpose when CONFIG_CPU_IDLE isn't set. Not my area, but after
a quick look at the code I strongly suspect this is not the case.

I see your point: unlike functions, you can't provide empty stubs for variables, and therefore you can't make their absence transparent for the users. That wasn't my intention. My point was simply that a variable that's not defined should not be declared either in the header (if possible), and I do think that a compilation error is preferable to a linkage error. As far as I can tell, other headers also apply this principle to per-cpu variables: bpf_prog_active is conditional on CONFIG_BPF_SYSCALL in linux/bpf.h, vm_event_states is conditional on CONFIG_VM_EVENT_COUNTERS in linux/vmstat.h, etc.

Kevin


Fixes: ea8b1c4a6019 ("drivers: psci: PSCI checker module")
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
Acked-by: Kevin Brodsky <kevin.brodsky@xxxxxxx>
Reviewed-by: Jean Delvare <jdelvare@xxxxxxx>