[PATCH] soc: samsung: exynos-pmu: Fix for CONFIG_DEBUG_PREEMPT

[Peter Griffin]

Hi Mostafa,

Thanks for your patch and bug report, it's great to see more folks
running upstream Pixel 6 :)

On Fri, 5 Sept 2025 at 17:25, Mostafa Saleh <smostafa at google.com> wrote:
>
> Booting the kernel on Pixel-6 with `CONFIG_DEBUG_PREEMPT` prints the
> following WARN:
>
> [    0.784187][    T1] BUG: using smp_processor_id() in preemptible [00000000] code: swapper/0/1
> [    0.784328][    T1] caller is debug_smp_processor_id+0x20/0x30
> [    0.784433][    T1] CPU: 6 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.17.0-rc4-gd69eb204c255 #1 PREEMPT
> [    0.784439][    T1] Hardware name: Oriole (DT)
> [    0.784441][    T1] Call trace:
> [    0.784443][    T1]  show_stack+0x34/0xa0 (C)
> [    0.784453][    T1]  dump_stack_lvl+0x7c/0xb0
> [    0.784460][    T1]  dump_stack+0x18/0x24
> [    0.784464][    T1]  check_preemption_disabled+0xf8/0x100
> [    0.784470][    T1]  debug_smp_processor_id+0x20/0x30
> [    0.784476][    T1]  gs101_cpuhp_pmu_online+0x40/0x108
> [    0.784483][    T1]  cpuhp_invoke_callback+0x188/0x2d8
> [    0.784490][    T1]  cpuhp_issue_call+0xec/0x240
> [    0.784494][    T1]  __cpuhp_setup_state_cpuslocked+0x140/0x2c0
> [    0.784499][    T1]  __cpuhp_setup_state+0x58/0x88
> [    0.784504][    T1]  exynos_pmu_probe+0x2a4/0x380
> [    0.784508][    T1]  platform_probe+0x64/0xd0
> [    0.784516][    T1]  really_probe+0xd0/0x3b0
> [    0.784520][    T1]  __driver_probe_device+0x8c/0x170
> [    0.784524][    T1]  driver_probe_device+0x44/0x140
> [    0.784528][    T1]  __device_attach_driver+0xd8/0x180
> [    0.784532][    T1]  bus_for_each_drv+0x90/0xf8
> [    0.784536][    T1]  __device_attach+0xa8/0x1d0
> [    0.784540][    T1]  device_initial_probe+0x1c/0x30
> [    0.784544][    T1]  bus_probe_device+0xb4/0xc0
> [    0.784547][    T1]  device_add+0x4d0/0x700
> [    0.784550][    T1]  of_device_add+0x4c/0x78
> [    0.784556][    T1]  of_platform_device_create_pdata+0x9c/0x148
> [    0.784560][    T1]  of_platform_bus_create+0x1d0/0x370
> [    0.784563][    T1]  of_platform_bus_create+0x234/0x370
> [    0.784567][    T1]  of_platform_populate+0x84/0x178
> [    0.784571][    T1]  of_platform_default_populate_init+0xf0/0x120
> [    0.784579][    T1]  do_one_initcall+0x68/0x2d0
> [    0.784585][    T1]  kernel_init_freeable+0x2d8/0x358
> [    0.784589][    T1]  kernel_init+0x28/0x168
> [    0.784595][    T1]  ret_from_fork+0x10/0x20
>
> As this value is only read once, it doesn't require to be stable, so
> just use "raw_smp_processor_id" instead.

Can I ask what baseline you are running when you see this warning?

As this code got refactored recently in commit 78b72897a5c8 ("soc:
samsung: exynos-pmu: Enable CPU Idle for gs101") which is present in
linux-next but hasn't made its way to a proper release yet. After this
patch smp_processor_id() is always called with a raw_spin_lock() held
(so this warning shouldn't fire).

I just built next-20250909 locally to confirm, and with the above
patch reverted I see the warning you mention. So in summary I think
the issue has already been fixed by the above commit.

Thanks,

Peter