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Date: Mon, 6 Oct 2025 14:33:33 -0500
From: Bjorn Helgaas <helgaas@kernel.org>
To: Brian Norris <briannorris@chromium.org>
Cc: Mika Westerberg <mika.westerberg@linux.intel.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	"Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
	linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org,
	stable@vger.kernel.org
Subject: Re: [PATCH] PCI/PM: Avoid redundant delays on D3hot->D3cold
Message-ID: <20251006193333.GA537409@bhelgaas>
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In-Reply-To: <aOQLRhot8-MtXeE3@google.com>

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@google.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.

I didn't look into any of them in detail, but I would jump at any
chance to remove PCI details from driver suspend paths.  There are
only ~20 calls from suspend functions, ~25 from shutdown, and a few
from poweroff.  The fact that there are so few makes me think they
might be leftovers that could be more fully converted to generic PM.