Subject: i.MX6 Solo: maxcpus=1 vs. nosmp: Warning from smp_twd.c?

From: [email-masked] (Dirk Behme)
Date: Thu, 14 Feb 2013 15:10:43 +0100

Hi,

working on a i.MX6 Solo SoC (one Cortex A9 core) we are usually booting 
with maxcpus=1 a kernel built with CONFIG_NR_CPUS=4. This is done to be 
able to boot the same kernel on the Solo and Quad version of that SoC.

Now, we tried to switch from 'maxcpus=1' to 'nosmp'. With 'nosmp' we get 
the warning [1] in the boot messages. This isn't there with 'maxcpus=1'.

Is it a good idea to use 'nosmp' instead of 'maxcpus=1' on a single 
Cortex A9 SoC?

Looking into the warning, it's the result of setup_max_cpus being 0 in 
case of 'nosmp' (and being 1 in case of 'maxcpus=1'). So with 'nosmp' 
the function local_timer_register()

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=blob;f=arch/arm/kernel/smp.c;h=84f4cbf652e58b3acfea846ad1cad9ddbee72d95;hb=refs/heads/master#l514

returns -ENXIO (== -6).

Is this the intended behaviour for 'nosmp' resulting in below boot warning?

The call stack is

    imx6q_timer_init()
-> twd_local_timer_of_register()()
-> twd_local_timer_common_register()
-> local_timer_register()  (returns -ENXIO due to setup_max_cpus == 0)

Best regards

Dirk

[1]
...
sched_clock: 32 bits at 49MHz, resolution 20ns, wraps every 86767ms
------------[ cut here ]------------
WARNING: at arch/arm/kernel/smp_twd.c:345 
twd_local_timer_of_register+0x7c/0x90()
twd_local_timer_of_register failed (-6)
Modules linked in:
Backtrace:
[<80011f14>] (dump_backtrace+0x0/0x10c) from [<8044dd30>] 
(dump_stack+0x18/0x1c)
  r7:805e9f58 r6:805ba84c r5:80539331 r4:00000159
[<8044dd18>] (dump_stack+0x0/0x1c) from [<80020fbc>] 
(warn_slowpath_common+0x54/0x6c)
[<80020f68>] (warn_slowpath_common+0x0/0x6c) from [<80021078>] 
(warn_slowpath_fmt+0x38/0x40)
  r9:412fc09a r8:8fffffff r7:ffffffff r6:00000001 r5:80633b8c
r4:80b32da8
[<80021040>] (warn_slowpath_fmt+0x0/0x40) from [<805ba84c>] 
(twd_local_timer_of_register+0x7c/0x90)
  r3:fffffffa r2:8053934b
[<805ba7d0>] (twd_local_timer_of_register+0x0/0x90) from [<805c0bec>] 
(imx6q_timer_init+0x18/0x4c)
  r5:80633800 r4:8053b701
[<805c0bd4>] (imx6q_timer_init+0x0/0x4c) from [<805ba4e8>] 
(time_init+0x28/0x38)
  r5:80633800 r4:805dc0f4
[<805ba4c0>] (time_init+0x0/0x38) from [<805b6854>] 
(start_kernel+0x1a0/0x310)
[<805b66b4>] (start_kernel+0x0/0x310) from [<10008044>] (0x10008044)
  r8:1000406a r7:805f3f8c r6:805dc0c4 r5:805f0518 r4:10c5387d
---[ end trace 1b75b31a2719ed1c ]---
CPU identified as i.MX6S/DL, silicon rev 1.0
...