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Date: Thu, 5 Dec 2024 19:49:34 -0600
From: Bjorn Helgaas <helgaas@kernel.org>
To: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Cc: kbusch@kernel.org, axboe@kernel.dk, hch@lst.de, sagi@grimberg.me,
	linux-nvme@lists.infradead.org, linux-kernel@vger.kernel.org,
	linux-pci@vger.kernel.org, andersson@kernel.org,
	konradybcio@kernel.org, "Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Ulf Hansson <ulf.hansson@linaro.org>
Subject: Re: [PATCH] nvme-pci: Shutdown the device if D3Cold is allowed by
 the user
Message-ID: <20241206014934.GA3081609@bhelgaas>
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In-Reply-To: <20241205232900.GA3072557@bhelgaas>

On Thu, Dec 05, 2024 at 05:29:00PM -0600, Bjorn Helgaas wrote:
> On Sat, Nov 23, 2024 at 02:31:13PM +0530, Manivannan Sadhasivam wrote:
> > On Fri, Nov 22, 2024 at 04:20:50PM -0600, Bjorn Helgaas 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.
> ...

> > We did attempt to solve this problem in multiple ways, but the
> > lesson learned was, kernel cannot decide the power mode without help
> > from userspace. That's the reason I wanted to make use of this
> > 'd3cold_allowed' sysfs attribute to allow userspace to override the
> > D3Cold if it wants based on platform requirement.
> 
> It seems sub-optimal that this only works how you want if the user
> intervenes.

Oops, I think I got this part backwards.  The patch uses PCI PM if
d3cold_allowed is set, and it's set by default, so it does what you
need for the Qualcomm platform *without* user intervention.

But I guess using the flag allows users in other situations to force
use of NVMe power management by clearing d3cold_allowed via sysfs.
Does that mean some unspecified other platforms might only work
correctly with that user intervention?