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

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Message-ID: <CAJZ5v0hxnYere19wXbua6zWEDRDgSPeJgSECugtwfgTP-UN8Bw@mail.gmail.com>
Date: Mon, 16 Dec 2024 18:28:55 +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 Mon, Dec 16, 2024 at 5:48 PM Manivannan Sadhasivam
<manivannan.sadhasivam@...aro.org> wrote:
>
> On Mon, Dec 16, 2024 at 05:42:30PM +0100, Rafael J. Wysocki wrote:
> > On Mon, Dec 16, 2024 at 5:23 PM Christoph Hellwig <hch@....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.
> >
>
> On suspend-to-idle, yes D3Cold doesn't make sense,

Why?

> but on suspend-to-ram it is pretty much required.

Well, I know for a fact that on x86 platforms ACPI S3 does not require
putting devices into D3cold in general.

Why is it required for NVMe?

> That applies to DT as well.

Again, why?