RE: [PATCH 0/3] On AMD platforms only offer s2idle w/ proper FW

["Deucher, Alexander" <Alexander.Deucher@amd.com>]

[AMD Official Use Only]

> -----Original Message-----
> From: Rafael J. Wysocki <rafael@kernel.org>
> Sent: Tuesday, January 11, 2022 12:06 PM
> To: Limonciello, Mario <Mario.Limonciello@amd.com>
> Cc: Rafael J. Wysocki <rafael@kernel.org>; Rafael J . Wysocki
> <rjw@rjwysocki.net>; ACPI Devel Maling List <linux-acpi@vger.kernel.org>;
> S-k, Shyam-sundar <Shyam-sundar.S-k@amd.com>; Natikar, Basavaraj
> <Basavaraj.Natikar@amd.com>; Deucher, Alexander
> <Alexander.Deucher@amd.com>; bjoern.daase@gmail.com
> Subject: Re: [PATCH 0/3] On AMD platforms only offer s2idle w/ proper FW
> 
> On Tue, Jan 11, 2022 at 5:23 PM Limonciello, Mario
> <mario.limonciello@amd.com> wrote:
> >
> > +Alex
> >
> > On 1/11/2022 09:52, Rafael J. Wysocki wrote:
> > > On Wed, Jan 5, 2022 at 8:39 PM Mario Limonciello
> > > <mario.limonciello@amd.com> wrote:
> > >>
> > >> Currently the Linux kernel will offer s2idle regardless of whether
> > >> the FADT indicates the system should use or on X86 if the LPS0 ACPI
> > >> device has been activated.
> > >>
> > >> On some non-AMD platforms s2idle can be offered even without
> proper
> > >> firmware support.  The power consumption may be higher in these
> > >> instances but the system otherwise properly suspends and resumes.
> > >
> > > Well, the idea is that s2idle should not require FW support at all.
> > > >
> >
> > May I ask - why?  It's an intentional design decision?
> 
> Yes, it is.
> 
> > > It may not be possible to reach the minimum power level of the
> > > platform without FW support, but that should not prevent s2idle from
> > > being used.
> > >
> > >> On AMD platforms however when the FW has been configured not to
> > >> offer s2idle some different hardware initialization has occurred
> > >> such that the system won't properly resume.
> > >
> > > That's rather unfortunate.
> > >
> > > Can you please share some details on what's going on in those cases?
> > >
> > > Technically, without FW support there should be no difference
> > > between the platform state reachable via s2idle and the platform
> > > state reachable via runtime idle.
> >
> > During resume there is a number of page faults that occur and during
> > initialization the ring tests fail.  The graphics is unusable at this
> > time as a result.
> >
> > The amdgpu code actually *does* distinguish between the 3 different
> > cases of S3, S0ix, and runtime suspend.
> 
> But s2idle doesn't guarantee S0ix in any case.
> 
> > The function "amdgpu_acpi_is_s0ix_active" causes different codepaths
> > to be used during the suspend routine.
> 
> Well, as I said, s2idle need not mean S0ix.
> 
> > In this particular case that FADT doesn't set the low power idle bit
> > and that function returns false meaning the s3 codepath is taken but
> > the hardware didn't go through a reset.
> 
> If there is a separate S3 code path, taking it when pm_suspend_target_state
> == PM_SUSPEND_TO_IDLE is incorrect.
> 
> > It *might* also be possible to solve this by mandating an ASIC reset
> > in such a case (we didn't try).
> 
> I'd rather do a PM-runtime path equivalent if the target sleep state is
> PM_SUSPEND_TO_IDLE and there is no FW support for S0ix.
> 
> > However it comes back to my first upleveveled question - is this a
> > case we really want to support and encourage?  This type of bug and
> > combination of codepaths is not a case that is going to be well tested.
> > This patch series will align the kernel behavior to only what AMD validates.
> 
> But this does not follow the definition of s2idle and its documentation.

At least for devices integrated into the SoC, the power rails are controlled by the firmware in the SoC.  For S3, the power rails are cut by the FW when the platform enters S3.  For S0ix, the power rails are cut when all of the devices on the rail suspended and various conditions are met.  Also, in the case of some devices, the device has to be in a very specific state for s0ix to work properly.  The GPU is the big one here.  For S3, the entire GPU has to be re-initialized at resume.  For S0ix, the GPU's state is largely handled by the firmware and attempting to re-initialize it won't work unless you reset it.  Integrated AMD graphics don't support runtime power management, only dGPUs do.  For integrated graphics the firmware dynamically controls the power at runtime so there is no need to do anything special for runtime pm.  For dGPUs we support d3cold either via ACPI on platforms like all-in-ones and laptops or via a driver initiated sequence for add-in-cards.

What does S2idle ultimately do when all devices have suspended?  Does it enter S0ix or S3 at the end when it want to ultimately suspend the platform, or is the assumption that if all devices have suspended, the that is equivalent to S0ix or S3?  For AMD platforms, either S3 or S0ix needs to be entered for the platform to actually power down most of the power rails.  It's not clear to me what we should do for s2idle.

Alex