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

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Message-ID: <20241118145820.2ecu5hzb2r5ygjvi@thinkpad>
Date: Mon, 18 Nov 2024 20:28:20 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Christoph Hellwig <hch@....de>
Cc: 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, Bjorn Helgaas <bhelgaas@...gle.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: [PATCH] nvme-pci: Shutdown the device if D3Cold is allowed by
 the user

On Mon, Nov 18, 2024 at 01:58:17PM +0100, Christoph Hellwig wrote:
> On Mon, Nov 18, 2024 at 01:53:44PM +0530, Manivannan Sadhasivam wrote:
> > PCI core allows users to configure the D3Cold state for each PCI device
> > through the sysfs attribute '/sys/bus/pci/devices/.../d3cold_allowed'. This
> > attribute sets the 'pci_dev:d3cold_allowed' flag and could be used by users
> > to allow/disallow the PCI devices to enter D3Cold during system suspend.
> >
> > So make use of this flag in the NVMe driver to shutdown the NVMe device
> > during system suspend if the user has allowed D3Cold for the device.
> > Existing checks in the NVMe driver decide whether to shut down the device
> > (based on platform/device limitations), so use this flag as the last resort
> > to keep the existing behavior.
> 
> Umm, what?  The documentation of this attribute says:
> 
> "d3cold_allowed is bit to control whether the corresponding PCI
>  device can be put into D3Cold state.  If it is cleared, the
>  device will never be put into D3Cold state.  If it is set, the
>  device may be put into D3Cold state if other requirements are
>  satisfied too.  Reading this attribute will show the current
>  value of d3cold_allowed bit. Writing this attribute will
>  the value of d3cold_allowed bit."
> 
> Which honestly already sounds rather non-specific, but everything but
> a mandate for drivers to act on it.
> 
> The only place currently checking it is pci_dev_check_d3cold in the
> PCI core, which is used to set the bridge_d3 attibute.
> 

Yeah, it is pretty much used internally up until now. But the attribute looks
like a close match of what I could find for this usecase and that's why I used
it.

> So blindly using it in a driver to force a different PM strategy feels
> completely wrong.  Even if the attrite should have that effect it
> needs to happen through a well documented PCI or PM layer helper and
> open coded like this.
> 

Ok. I'd like to get some feedback from Bjorn H (PCI maintainer) about using this
attribute before moving forward with a helper.

Thanks!

- Mani

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