[PATCH] fix linux 5.16 freeze regression on 32-bit ThinkPad T40

[Woody Suwalski]

From: Woody Suwalski <wsuwalski@xxxxxxxxx>

Add and ACPI idle power level limit for 32-bit ThinkPad T40.

There is a regression on T40 introduced by commit d6b88ce2, starting 
with kernel 5.16:

commit d6b88ce2eb9d2698eb24451eb92c0a1649b17bb1
Author: Richard Gong <richard.gong@xxxxxxx>
Date:   Wed Sep 22 08:31:16 2021 -0500

    ACPI: processor idle: Allow playing dead in C3 state

The above patch is trying to enter C3 state during init, what is causing 
a T40 system freeze. I have not found a similar issue on any other of my 
32-bit machines.

The fix is to add another exception to the processor_power_dmi_table[] list.
As a result the dmesg shows as expected:

    2.155398] ACPI: IBM ThinkPad T40 detected - limiting to C2 
max_cstate. Override with "processor.max_cstate=9"
[    2.155404] ACPI: processor limited to max C-state 2

The fix is trivial and affects only vintage T40 systems.

Signed-off-by: Woody Suwalski <wsuwalski@xxxxxxxxx>
---
--- a/drivers/acpi/processor_idle.c    2022-02-04 09:09:54.515906362 -0500
+++ b/drivers/acpi/processor_idle.c    2022-02-04 14:21:08.264911271 -0500
@@ -96,6 +96,11 @@ static const struct dmi_system_id proces
       DMI_MATCH(DMI_SYS_VENDOR, "ASUSTeK Computer Inc."),
       DMI_MATCH(DMI_PRODUCT_NAME,"L8400B series Notebook PC")},
      (void *)1},
+    /* T40 can not handle C3 idle state */
+    { set_max_cstate, "IBM ThinkPad T40", {
+      DMI_MATCH(DMI_SYS_VENDOR, "IBM"),
+      DMI_MATCH(DMI_PRODUCT_NAME, "23737CU")},
+     (void *)2},
     {},
 };