Message-ID: <CAJZ5v0gLMx+tBo+MA3AQZ7qP28Z91d04oVBHVeTNcd-QD=kJZg@mail.gmail.com>
Date: Mon, 16 Dec 2024 17:24:40 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Christoph Hellwig <hch@....de>, Ulf Hansson <ulf.hansson@...aro.org>, 
	"Rafael J. Wysocki" <rjw@...ysocki.net>, Bjorn Helgaas <helgaas@...nel.org>, kbusch@...nel.org, 
	axboe@...nel.dk, sagi@...mberg.me, linux-nvme@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org, andersson@...nel.org, 
	konradybcio@...nel.org, Len Brown <len.brown@...el.com>, linux-pm@...r.kernel.org
Subject: Re: [PATCH] nvme-pci: Shutdown the device if D3Cold is allowed by the user

On Sat, Dec 14, 2024 at 7:30 AM Manivannan Sadhasivam
<manivannan.sadhasivam@...aro.org> wrote:
>
> On Fri, Dec 13, 2024 at 03:35:15PM +0100, Rafael J. Wysocki wrote:
> > On Thu, Dec 12, 2024 at 4:14 PM Christoph Hellwig <hch@....de> wrote:
> > >
> > > On Thu, Dec 12, 2024 at 01:49:15PM +0100, Ulf Hansson wrote:
> > > > Right. This seems to somewhat work for ACPI types of systems, because
> > > > ACPI is controlling the low power state for all the devices. Based on
> > > > the requested system wide low power state, ACPI can then decide to
> > > > call pm_set_suspend_via_firmware() or not.
> > > >
> > > > Still there is a problem with this for ACPI too.
> > > >
> > > > How does ACPI know whether it's actually a good idea to keep the NVMe
> > > > storage powered in s2idle (ACPI calls pm_set_suspend_via_firmware()
> > > > only for S2R and S2disk!?)? Especially when my laptop only supports
> > > > s2idle and that's what I will use when I close the lid. In this way,
> > > > the NMVe storage will certainly contribute to draining the battery,
> > > > especially when I won't be using my laptop for a couple of days.
> > > >
> > > > In my opinion, we need a better approach that is both flexible and
> > > > that dynamically adjusts based upon the use case.
> > >
> > > Agreed.  I'd be happy to work with the PM maintainers to do this,
> > > but I don't really know enough about the PM core to drive it
> > > (as the reply from Rafael to my mail makes pretty clear :))
> >
> > I'm here to help.
> >
> > Let me know what exactly you want to achieve and we'll see how to make it work.
>
> I'll try to summarize the requirement here since I started this thread:
>
> Problem statement
> =================
>
> We need a PM core API that tells the device drivers when it is safe to powerdown
> the devices. The usecase here is with PCIe based NVMe devices but the problem is
> applicable to other devices as well.
>
> Drivers are relying on couple of options now:
>
> 1. If pm_suspend_via_firmware() returns true, then drivers will shutdown the
> device assuming that the firmware is going to handle the suspend. But this API
> is currently used only by ACPI. Even there, ACPI relies on S2R being supported
> by the platform and it sets pm_set_suspend_via_firmware() only when the suspend
> is S2R. But if the platform doesn't support S2R (current case of most of the
> Qcom SoCs), then pm_suspend_via_firmware() will return false and NVMe won't be
> powered down draining the battery.

So my question here would be why is it not powered down always during
system-wide suspend?

Why exactly is it necessary to distinguish one case from the other
(assuming that we are talking about system-wide suspend only)?

There are drivers that use pm_suspend_via_firmware() to check whether
or not something special needs to be done to the device because if
"false" is returned, the platform firmware is not going to remove
power from it.

However, you seem to be talking about the opposite, so doing something
special to the device if "true" is returned.  I'm not sure why this is
necessary.

> If the platform is using DT, then there is no entity setting
> pm_set_suspend_via_firmware().

That's true and so the assumption is that in this case the handling of
all devices will always be the same regardless of which flavor of
system suspend is chosen by user space.

> So NVMe will be kept in low power state all the
> time (still draining the battery).

So what would be the problem with powering it down unconditionally?

> There were attempts to set this flag from
> PSCI [1], but there were objections on setting this flag when PSCI_SUSPEND is
> not supported by the platform (again, the case with Qcom SoCs). Even if this
> approach succeeds, then there are concerns that if the platform is used in an
> OS like Android where the S2Idle cycle is far more high, NVMe will wear out
> very quickly.

I see.

> So this is where the forthcoming API need to "dynamically adjusts
> based upon the use case" as quoted by Ulf in his previous reply. One way to
> achieve would be by giving the flexibility to the userspace to choose the
> suspend state (if platform has options to select). UFS does something similar
> with 'spm_lvl' [2] sysfs attribute that I believe Android userspace itself makes
> use of.

Before we're talking about APIs, let's talk about the desired behavior.

It looks like there are cases in which you'd want to turn the device
off completely (say put it into D3cold in the PCI terminology) and
there are cases in which you'd want it to stay in a somewhat-powered
low-power state.

It is unclear to me what they are at this point.

> 2. Making use of pm_suspend_target_state to differentiate between suspend states
> and powering down the devices only during PM_SUSPEND_MEM (S2R). But this also
> suffers from the same issue as mentioned above (when platform doesn't support
> S2R).
>
> TLDR: We need a PM core API that that sets a sane default suspend state for the
> platform and also allows dynamically changing/overriding the state (by taking
> inputs from userspace etc...). This API should also forbid override, if the
> platform has limitations (like if it requires powering down the devices all the
> time (x13s laptops)). Finally, this API would be used by the device drivers to
> decide when to safely power down the devices during suspend.
>
> @Ulf/others: Please chime in if you don't agree with anything I said above.

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