Re: [PATCH] nvme-pci: Shutdown the device if D3Cold is allowed by the user

Manivannan Sadhasivam
2024-12-21T03:38:51Z

On Fri, Dec 20, 2024 at 04:15:21PM +0100, Konrad Dybcio wrote:
> On 16.12.2024 5:42 PM, Rafael J. Wysocki wrote:
> > On Mon, Dec 16, 2024 at 5:23 PM Christoph Hellwig <hch@lst.de> wrote:
> >>
> >> On Sat, Dec 14, 2024 at 12:00:23PM +0530, Manivannan Sadhasivam wrote:
> >>> 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.
> >>
> >> Maybe I'm misunderstanding things, but I think the important part is
> >> to indicate when a suspend actually MUST put the device into D3. Because
> >> doing that should always be safe, but not always optimal.
> >
> > I'm not aware of any cases when a device must be put into D3cold
> > (which I think is what you mean) during system-wide suspend.
> >
> > Suspend-to-idle on x86 doesn't require this, at least not for
> > correctness. I don't think any platforms using DT require it either.
>
> That would be correct.
>
> The Qualcomm platform (or class of platforms) we're looking at with this
> specific issue requires PCIe (implying NVMe) shutdown for S2RAM.
>
> The S2RAM entry mechanism is unfortunately misrepresented as an S2Idle
> state by Linux as of today, and I'm trying really hard to convince some
> folks to let me describe it correctly, with little success so far..
>

Perhaps you should say 'S2RAM is misrepresented as S2Idle by the firmware as of
today'...

But I'll leave it up to the PSCI folks to decide whether it makes sense to
expose PSCI SYSTEM_SUSPEND through CPU_SUSPEND or not.

For the people in this thread, I'm leaving the link to the PSCI discussion here:
https://lore.kernel.org/all/20241028-topic-cpu_suspend_s2ram-v1-0-9fdd9a04b75c@oss.qualcomm.com/

> That is the real underlying issue and once/if it's solved, this patch
> will not be necessary.
>
> > In theory, ACPI S3 or hibernation may request that, but I've never
> > seen it happen in practice.
> >
> > Suspend-to-idle on x86 may want devices to end up in specific power
> > states in order to be able to switch the entire platform into a deep
> > energy-saving mode, but that's never been D3cold so far.
>
> In our case the plug is only pulled in S2RAM, otherwise the best we can
> do is just turn off the devices individually to decrease the overall
> power draw
>

I don't think this is accurate. Qcom FW (the one we are discussing in this
thread) doesn't pull the plug (except on platforms like x13s due to hw
limitation). On ACPI though, the FW *might* pull the plug, so that's why drivers
prepare the devices by powering down them (largely) if pm_suspend_via_firmware()
succeeds. On Qcom platforms, we are trying to allow the SoC to transition to low
power state and that requires relinquishing the resource votes by the drivers.

I still have doubt that pm_set_suspend_via_firmware() applies to Qcom FW or not.
Also the API description doesn't exactly match its usecase.

- Mani

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