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

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Message-ID: <aOQLRhot8-MtXeE3@google.com>
Date: Mon, 6 Oct 2025 11:32:38 -0700
From: Brian Norris <briannorris@...omium.org>
To: Mika Westerberg <mika.westerberg@...ux.intel.com>
Cc: 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

Hi Mika,

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).

HTH,
Brian