linux-kernel - Re: [PATCH v1] ACPI: PM: s2idle: Add module parameter for LPS0 constraints checking

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Message-ID: <CAJZ5v0gwAXU1MdiffgHz8bYJotrJEujwc14D9Dh1rAX0pE9Q_A@mail.gmail.com>
Date: Tue, 13 Jan 2026 22:57:34 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Mario Limonciello <mario.limonciello@....com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Linux ACPI <linux-acpi@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, Linux PM <linux-pm@...r.kernel.org>, 
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Subject: Re: [PATCH v1] ACPI: PM: s2idle: Add module parameter for LPS0
 constraints checking

On Tue, Jan 13, 2026 at 10:48 PM Mario Limonciello
<mario.limonciello@....com> wrote:
> On 1/13/2026 7:36 AM, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > Commit 32ece31db4df ("ACPI: PM: s2idle: Only retrieve constraints when
> > needed") attempted to avoid useless evaluation of LPS0 _DSM Function 1
> > in lps0_device_attach() because pm_debug_messages_on might never be set
> > (and that is the case on production systems most of the time), but it
> > turns out that LPS0 _DSM Function 1 is generally problematic on some
> > platforms and causes suspend issues to occur when pm_debug_messages_on
> > is set now.
>
> Any ideas why it's causing problems?  AML doing something it shouldn't?

It's not a clear AML bug AFAICS.  Rather, it seems to have
dependencies on something that is not ready when it is evaluated, so
an ordering issue or similar.

> >
> > In Linux, LPS0 _DSM Function 1 is only useful for diagnostics and only
> > in the cases when the system does not reach the deepest platform idle
> > state during suspend-to-idle for some reason.  If such diagnostics is
> > not necessary, evaluating it is a loss of time, so using it along with
> > the other pm_debug_messages_on diagnostics is questionable because the
> > latter is expected to be suitable for collecting debug information even
> > during production use of system suspend.
> >
> > For this reason, add a module parameter called check_lps0_constraints
> > to control whether or not the list of LPS0 constraints will be checked
> > in acpi_s2idle_prepare_late_lps0() and so whether or not to evaluate
> > LPS0 _DSM Function 1 (once) in acpi_s2idle_begin_lps0().
> >
> > Fixes: 32ece31db4df ("ACPI: PM: s2idle: Only retrieve constraints when needed")
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > ---
> >   drivers/acpi/x86/s2idle.c |    8 ++++++--
> >   1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > --- a/drivers/acpi/x86/s2idle.c
> > +++ b/drivers/acpi/x86/s2idle.c
> > @@ -28,6 +28,10 @@ static bool sleep_no_lps0 __read_mostly;
> >   module_param(sleep_no_lps0, bool, 0644);
> >   MODULE_PARM_DESC(sleep_no_lps0, "Do not use the special LPS0 device interface");
> >
> > +static bool check_lps0_constraints __read_mostly;
> > +module_param(check_lps0_constraints, bool, 0644);
> > +MODULE_PARM_DESC(check_lps0_constraints, "Check LPS0 device constraints");
>
> I'm personally not really a fan of another module parameter for
> debugging.  I know some other subsystem maintainers are very anti-module
> parameters too.
>
> I did like having /sys/power/pm_debug_messages able to be a one stop
> shop for debugging messages at runtime.

Well, this is not just debug messages, rather a whole debug facility
enabled via pm_debug_messages, which is kind of confusing.

> So I had another idea I wanted to throw around.  What if instead we had
> a file /sys/kernel/debug/x86/lps0_constraints?

Then you cannot use this without debugfs.

> If the file is never accessed never evaluate constraints.  If you read
> it once then you can get a dump of all the current constraints and any
> future suspends during that boot will also include constraints in the
> logs (IE call lpi_check_constraints()).

So if it is not in debugfs, it would need to be in sysfs and then I
don't see much difference between it and a module param, honestly.

I actually prefer the latter because it uses an existing infra.