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

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Message-ID: <CAJZ5v0gFmRVNEG_QL2+F7Dmvey4WrcHwt7Rhq2hizcxzGF2eWA@mail.gmail.com>
Date: Tue, 13 Jan 2026 23:05:33 +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 11:00 PM Mario Limonciello
<mario.limonciello@....com> wrote:
> On 1/13/2026 3:57 PM, Rafael J. Wysocki wrote:
> > 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.
>
> Ah I see.
>
> >
> >>>
> >>> 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.
>
> Good point.
>
> >
> >> 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.
>
> I mean; yeah.  But it really is debugging and most distros have debugfs
> enabled by default these days don't they?

Some of them don't.

That's one of the reasons why tracing has its own pseudo-filesystem
now, so it is a real obstacle.

> >
> >> 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.
>
> I agree if debugfs is decided to be out sysfs and module parameter are
> equal footing and then this patch makes sense.

I think that debugfs cannot be relied on to be always present.