linux-kernel - Re: [PATCH] PCI/PM: Avoid redundant delays on D3hot->D3cold

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Message-ID: <v7ynntv43urqjfdfzzbai2btsohaxpprni2pix2wnjfoazlfcl@xdbhvnpmoebt>
Date: Mon, 6 Oct 2025 16:13:26 -0700
From: Manivannan Sadhasivam <mani@...nel.org>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: Brian Norris <briannorris@...omium.org>, 
	Mika Westerberg <mika.westerberg@...ux.intel.com>, Bjorn Helgaas <bhelgaas@...gle.com>, 
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>, linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org, 
	stable@...r.kernel.org
Subject: Re: [PATCH] PCI/PM: Avoid redundant delays on D3hot->D3cold

On Mon, Oct 06, 2025 at 02:33:33PM -0500, Bjorn Helgaas wrote:
> On Mon, Oct 06, 2025 at 11:32:38AM -0700, Brian Norris wrote:
> > On Mon, Oct 06, 2025 at 03:52:22PM +0200, Mika Westerberg wrote:
> > > On Fri, Oct 03, 2025 at 03:40:09PM -0700, Brian Norris wrote:
> > > > From: Brian Norris <briannorris@...gle.com>
> > > > 
> > > > When transitioning to D3cold, __pci_set_power_state() will first
> > > > transition a device to D3hot. If the device was already in D3hot, this
> > > > will add excess work:
> > > > (a) read/modify/write PMCSR; and
> > > > (b) excess delay (pci_dev_d3_sleep()).
> > > 
> > > How come the device is already in D3hot when __pci_set_power_state() is
> > > called? IIRC PCI core will transition the device to low power state so that
> > > it passes there the deepest possible state, and at that point the device is
> > > still in D0. Then __pci_set_power_state() puts it into D3hot and then turns
> > > if the power resource -> D3cold.
> > > 
> > > What I'm missing here?
> > 
> > Some PCI drivers call pci_set_power_state(..., PCI_D3hot) on their own
> > when preparing for runtime or system suspend, so by the time they hit
> > pci_finish_runtime_suspend(), they're in D3hot. Then, pci_target_state()
> > may still pick a lower state (D3cold).
> 
> We might need this change, but maybe this is also an opportunity to
> remove some of those pci_set_power_state(..., PCI_D3hot) calls from
> drivers.
> 

Agree. The PCI client drivers should have no business in opting for D3Hot in the
suspend path. It should be the other way around, they should opt-out if they
want by calling pci_save_state(), but that is also subject to discussion.

- Mani

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