Message-ID: 
 <CAGwozwEDJbFoZJsqyOycHbv-LwGqOC-MG3K_duVEEqfKfXMUhA@mail.gmail.com>
Date: Wed, 3 Dec 2025 11:12:59 +0100
From: Antheas Kapenekakis <lkml@...heas.dev>
To: Dmitry Osipenko <dmitry.osipenko@...labora.com>
Cc: "Mario Limonciello (AMD) (kernel.org)" <superm1@...nel.org>,
 Lennart Poettering <lennart@...ttering.net>,
	Daniel Stone <daniels@...labora.com>,
 "Rafael J. Wysocki" <rafael@...nel.org>,
	Robert Beckett <bob.beckett@...labora.com>, linux-acpi@...r.kernel.org,
	kernel@...labora.com, linux-kernel@...r.kernel.org,
	Sebastian Reichel <sebastian.reichel@...labora.com>,
 Xaver Hugl <xaver.hugl@...il.com>,
	Richard Hughes <richard@...hsie.com>, William Jon McCann <mccann@....edu>,
	"Jaap A . Haitsma" <jaap@...tsma.org>, Benjamin Canou <bookeldor@...il.com>,
	Bastien Nocera <hadess@...ess.net>
Subject: Re: [RFC PATCH v1 1/1] ACPI: PM: s2idle: Add lps0_screen_off sysfs
 interface

On Wed, 3 Dec 2025 at 07:47, Dmitry Osipenko
<dmitry.osipenko@...labora.com> wrote:
>
> On 12/3/25 05:12, Mario Limonciello (AMD) (kernel.org) wrote:
> >
> >
> > On 12/2/2025 4:35 PM, Dmitry Osipenko wrote:
> >> On 12/3/25 00:25, Mario Limonciello (AMD) (kernel.org) wrote:
> >>>> An inhibitor process in logind can handle this gracefully very simply.
> >>>> Involving the DRM subsystem just adds a lot of complexity and it is
> >>>> not clear what the benefit would be. There are no known devices that
> >>>> hook DRM components into that DSM.
> >>>>
> >>>>> By doing it this way userspace still has control, but it's not
> >>>>> mandatory for userspace to be changed.
> >>>>
> >>>> On that note, the screen off calls/userspace implementations are
> >>>> optional under both patch series. If userspace is not aware of them,
> >>>> they are still called by the kernel when suspending.
> >>>
> >>> With the proposal I mentioned you can get the LPS0 _DSM called on a
> >>> handheld when the screen gets called without changing userspace.
> >>>
> >>>>
> >>>> Current userland also duplicates the functionality of the screen off
> >>>> call, which is primarily turning off the keyboard backlight. So along
> >>>> implementing this call, userspace software like powerdevil/upower
> >>>> needs to be tweaked to avoid doing that if the screen off state is
> >>>> available.
> >>>
> >>> Sure Any hooking for turning off LEDs manually based off the screen off
> >>> _DSM is totally feasible.
> >>
> >> It's not that trivial to add screen on/off hooks to DRM, there is no one
> >> central place for that from what I can tell. I'm seeing variant with DRM
> >> hooks as unnecessary complexity that doesn't solve any practical problem.
> >
> > Is it really that hard?  I figured that any time
> > connector->dpms != mode from drm_atomic_connector_commit_dpms() could
> > walk through all the connectors and make a judgement call whether to
> > notify the potentially exported symbol.
>
> - drm_atomic_connector_commit_dpms() is used only for atomic ioctl path
> - there is another legacy kms path
> - AFAICT, DRM takes a different path when display is enabled initially
> by kernel
>
> Here we have 3 places where to plug the hook. Gives a strong feeling of
> a red flag, IMO.
>
> >> A week ago in a private conversation, Daniel Stone gave an example of
> >> laptop's lid-close handling that is done purely in userspace.
> >> Technically, kernel could have DRM hooks for that too, but it doesn't.
> >
> > All the way into hardware sleep?  There are certain requirements needed
> > for hardware sleep that kernel drivers are normally used to put devices
> > into the right state.  I guess PCIe devices you can hack around with
> > userspace PCI config space writes but you're going to confuse the kernel
> > pretty badly.
>
> - Userspace gets notification for a changed lid state
> - Userspace takes action of turning display on/off
> - Kernel DRM doesn't know and doesn't care about lid state,
> force-disabling display on machine suspension
>
> Don't see how this is different for the case of the LPS0 notifications.
> Maybe I'm not getting your point well, in that case please clarify more.
>
> >> Userspace would need to be taught about new power modes in any case.
> >> Addition of DRM hooks should require a well-defined justification, which
> >> is currently absent.
> >>
> >
> > Why does userspace need to know about them?  Besides the inhibitor can't
> > this be invisible to userspace?  I thought this mostly is for the
> > firmware to flash some LEDs and maybe change some power limits.
>
> What I was saying is that LPS0 inhibitors would represent the power mode
> controls by themselves. Userspace would have to know how to drive them.
>
> Userspace power managers are already driving displays DPMS. Combining
> this with knowledge about the LPS0 inhibitors gives userspace ability to
> support the new device power states. Hence, there is no practical need
> to bother kernel DRM with the LPS0 burden.

_Technically_ DPMS is driven by the compositor, not by any power manager.

However, I think for Gnome and KDE they link to logind's inactivity
hook so practically you are correct for inactivity.

Moreover, traditionally, compositors do not fire DPMS for suspend, the
kernel does. I think KDE fixed that recently though. This is why the
display used to wake up twice during hibernation, and why you get a
frozen display when suspending/resuming. This also complicates
suspend-then-hibernate checks because the display wakes up. i.e., they
should fire DPMS for suspend but a lot for them don't

With compositors such as gamescope that do not have a dbus API it gets
more hairy. And it also means that the power manager/kernel cannot
control DPMS without the compositor's consent. If they do that, the
compositor will crash due to rejected commits. We have a suspend hook
for gamescope on Bazzite though, it improves suspend appearance.

Just throwing in this for context, although it builds more of a case
of not involving DRM.

Best,
Antheas

> --
> Best regards,
> Dmitry
>

On Wed, 3 Dec 2025 at 07:47, Dmitry Osipenko
<dmitry.osipenko@...labora.com> wrote:
>
> On 12/3/25 05:12, Mario Limonciello (AMD) (kernel.org) wrote:
> >
> >
> > On 12/2/2025 4:35 PM, Dmitry Osipenko wrote:
> >> On 12/3/25 00:25, Mario Limonciello (AMD) (kernel.org) wrote:
> >>>> An inhibitor process in logind can handle this gracefully very simply.
> >>>> Involving the DRM subsystem just adds a lot of complexity and it is
> >>>> not clear what the benefit would be. There are no known devices that
> >>>> hook DRM components into that DSM.
> >>>>
> >>>>> By doing it this way userspace still has control, but it's not
> >>>>> mandatory for userspace to be changed.
> >>>>
> >>>> On that note, the screen off calls/userspace implementations are
> >>>> optional under both patch series. If userspace is not aware of them,
> >>>> they are still called by the kernel when suspending.
> >>>
> >>> With the proposal I mentioned you can get the LPS0 _DSM called on a
> >>> handheld when the screen gets called without changing userspace.
> >>>
> >>>>
> >>>> Current userland also duplicates the functionality of the screen off
> >>>> call, which is primarily turning off the keyboard backlight. So along
> >>>> implementing this call, userspace software like powerdevil/upower
> >>>> needs to be tweaked to avoid doing that if the screen off state is
> >>>> available.
> >>>
> >>> Sure Any hooking for turning off LEDs manually based off the screen off
> >>> _DSM is totally feasible.
> >>
> >> It's not that trivial to add screen on/off hooks to DRM, there is no one
> >> central place for that from what I can tell. I'm seeing variant with DRM
> >> hooks as unnecessary complexity that doesn't solve any practical problem.
> >
> > Is it really that hard?  I figured that any time
> > connector->dpms != mode from drm_atomic_connector_commit_dpms() could
> > walk through all the connectors and make a judgement call whether to
> > notify the potentially exported symbol.
>
> - drm_atomic_connector_commit_dpms() is used only for atomic ioctl path
> - there is another legacy kms path
> - AFAICT, DRM takes a different path when display is enabled initially
> by kernel
>
> Here we have 3 places where to plug the hook. Gives a strong feeling of
> a red flag, IMO.
>
> >> A week ago in a private conversation, Daniel Stone gave an example of
> >> laptop's lid-close handling that is done purely in userspace.
> >> Technically, kernel could have DRM hooks for that too, but it doesn't.
> >
> > All the way into hardware sleep?  There are certain requirements needed
> > for hardware sleep that kernel drivers are normally used to put devices
> > into the right state.  I guess PCIe devices you can hack around with
> > userspace PCI config space writes but you're going to confuse the kernel
> > pretty badly.
>
> - Userspace gets notification for a changed lid state
> - Userspace takes action of turning display on/off
> - Kernel DRM doesn't know and doesn't care about lid state,
> force-disabling display on machine suspension
>
> Don't see how this is different for the case of the LPS0 notifications.
> Maybe I'm not getting your point well, in that case please clarify more.
>
> >> Userspace would need to be taught about new power modes in any case.
> >> Addition of DRM hooks should require a well-defined justification, which
> >> is currently absent.
> >>
> >
> > Why does userspace need to know about them?  Besides the inhibitor can't
> > this be invisible to userspace?  I thought this mostly is for the
> > firmware to flash some LEDs and maybe change some power limits.
>
> What I was saying is that LPS0 inhibitors would represent the power mode
> controls by themselves. Userspace would have to know how to drive them.
>
> Userspace power managers are already driving displays DPMS. Combining
> this with knowledge about the LPS0 inhibitors gives userspace ability to
> support the new device power states. Hence, there is no practical need
> to bother kernel DRM with the LPS0 burden.
>
> --
> Best regards,
> Dmitry
>

Powered by blists - more mailing lists