linux-kernel - Re: [PATCH v5 4/4] PCI: Allow PCI bridges to go to D3Hot on all Devicetree based platforms

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Message-ID: <ZrIe70Z7uFven8HH@wunner.de>
Date: Tue, 6 Aug 2024 15:02:39 +0200
From: Lukas Wunner <lukas@...ner.de>
To: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Len Brown <lenb@...nel.org>, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
	mika.westerberg@...ux.intel.com, Hsin-Yi Wang <hsinyi@...omium.org>
Subject: Re: [PATCH v5 4/4] PCI: Allow PCI bridges to go to D3Hot on all
 Devicetree based platforms

On Tue, Aug 06, 2024 at 06:11:07PM +0530, Manivannan Sadhasivam wrote:
> On Tue, Aug 06, 2024 at 08:53:01AM +0200, Lukas Wunner wrote:
> > AFAICS we always program the device to go to D3hot and the platform
> > then cuts power, thereby putting it into D3cold.  So D3hot is never
> > skipped.  See __pci_set_power_state():
> > 
> > 	if (state == PCI_D3cold) {
> > 		/*
> > 		 * To put the device in D3cold, put it into D3hot in the native
> > 		 * way, then put it into D3cold using platform ops.
> > 		 */
> > 		error = pci_set_low_power_state(dev, PCI_D3hot, locked);
> > 
> > 		if (pci_platform_power_transition(dev, PCI_D3cold))
> > 			return error;
> > 
> 
> This is applicable only to pci_set_power_state(), but AFAIK PCIe spec
> doesn't mandate switching to D3Hot for entering D3Cold.

Per PCI Bus Power Management Interface Specification r1.2 sec 5.5 fig 5-1,
the only supported state transition to D3cold is from D3hot.

Per PCIe r6.2 sec 5.2, "PM is compatible with the PCI Bus Power Management
Interface Specification".

Granted, PCI-PM is an ancient spec, so I think anyone can be forgiven
for not knowing its intricacies off-the-cuff. :)


> So the PCIe host controller drivers (especically non-ACPI platforms)
> may just send PME_Turn_Off followed by removing the slot power
> (which again is not controlled by pci_set_power_state())
> as there are no non-ACPI related hooks as of now.

Ideally, devicetree-based platforms should be brought into the
platform_pci_*() fold to align them with ACPI and get common
behavior across all platforms.

Thanks,

Lukas