[Bug 218865] NULL pointer reference if X86_AMD_PSTATE_DEFAULT_MODE=1 on a AMD CPU that supports CPPC

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=218865

--- Comment #4 from Perry Yuan(AMD) (Perry.Yuan@amd.com) ---
(In reply to Matthew Stapleton from comment #0)
> I recently deployed a custom kernel build with X86_AMD_PSTATE_DEFAULT_MODE
> set to 1 (Disabled) to a newer AMD system: AMD Ryzen 3 4100 that supports
> cppc and got a NULL pointer reference crash so I had to add
> amd_pstate=active on that system to get it to boot.  The same kernel boots
> fine on older AMD systems that don't support cppc.
> 
> The kernel is 6.6.22
> 
> This is what I think is happening when no amd_pstate parameter has been
> specified although I haven't tested a patch yet:
> in function: amd_pstate_init the cppc_state starts as AMD_PSTATE_UNDEFINED.
> 
> On older systems that don't support CPPC, !boot_cpu_has(X86_FEATURE_CPPC)
> tests true so "driver load is disabled, boot with specific mode to enable
> this" is printed and -ENODEV is returned.
> 
> On the newer systems that support CPPC , it goes on to "ret =
> amd_pstate_set_driver(CONFIG_X86_AMD_PSTATE_DEFAULT_MODE);" .  Then
> amd_pstate_set_driver sees AMD_PSTATE_DISABLE , prints "driver is explicitly
> disabled" , doesn't assign a driver to current_pstate_driver, but still
> returns 0.  So then the switch statement in amd_pstate_init breaks and then
> I suspect it crashes at "current_pstate_driver->adjust_perf =
> amd_pstate_adjust_perf;" just below the switch statement.
> 
> If this is the case, maybe adjusting amd_pstate_set_driver to return -ENODEV
> if cppc_state == AMD_PSTATE_DISABLE might fix the problem.
> 
> Here is some of the crash output (manually typed from a photo):
> amd_pstate: driver is explicitly disabled
> BUG: kernel NULL pointer deference, address: 0000000000000050
> ...
> ? __die+0x1a/0x60
> ? page_fault_oops+0x17c/0x4a0
> ? _prb_read_valid+0x263/0x2e0
> ? exc_page_fault+0x33f/0x610
> ? prb_read_valid+0x12/0x20
> ? asm_exc_page_fault+0x22/0x30
> ? amd_pstate_init+0x90/0x260
> ? amd_pstate_param+0xb0/0xb0
> do_one_initcall+0x82/0x2c0
> kernel_init_freeable+0x1af/0x260
> ? rest_init+0xb0/0xb0
> kernel_init+0x11/0x1b0
> ret_from_fork+0x2b/0x40
> ? rest_init+0xb0/0xb0
> ret_from_fork_asm+0x11/0x20

Could you try the patch lately posted while I am trying to find a system to
reproduce?
https://lore.kernel.org/lkml/9b31fbcdfd4e4f00c3302f45e655aa43589b224c.1715356532.git.perry.yuan@amd.com/

I have changed the mode selection code a bit, it will be great if you can help
to try this on your system,  if the issue still can be reproduced, I will need
to chang the patch accordingly.
thanks.

Perry.

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