linux-kernel - Re: [PATCH v3] PCI: Add D3 support for PCI bridges in DT based platforms

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Message-ID: <20240305162537.GA8339@thinkpad>
Date: Tue, 5 Mar 2024 21:55:37 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Lukas Wunner <lukas@...ner.de>
Cc: Bjorn Helgaas <helgaas@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
	Bjorn Andersson <andersson@...nel.org>,
	Konrad Dybcio <konrad.dybcio@...aro.org>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczy??ski <kw@...ux.com>, Rob Herring <robh@...nel.org>,
	Mika Westerberg <mika.westerberg@...ux.intel.com>,
	quic_krichai@...cinc.com, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH v3] PCI: Add D3 support for PCI bridges in DT based
 platforms

On Thu, Feb 22, 2024 at 10:40:52AM +0100, Lukas Wunner wrote:
> On Wed, Feb 21, 2024 at 12:20:00PM -0600, Bjorn Helgaas wrote:
> >   1) D3hot doesn't work per spec.  This sounds like a hardware
> >      defect in the device that should be a quirk based on
> >      Vendor/Device ID, not something in DT.  I don't actually know if
> >      this is common, although there are several existing quirks that
> >      mention issues with D3.
> 
> My recollection is that putting Root Ports into D3hot on older x86
> systems would raise MCEs, which is why pci_bridge_d3_possible() only
> allows D3hot in cases which are known to work (e.g. Thunderbolt
> controllers, machines with a recent BIOS).  It was a conservative
> policy chosen to avoid regressions.
> 

So pci_bridge_d3_possible() is only checking for D3hot capability? If so, I'd
rename it to pci_bridge_d3hot_possible() and also 'bridge_d3' to 'bridge_d3hot'
to make it explicit.

Since the default value of 'd3cold_allowed' is true, I believe the code expects
all devices to support D0 and D3cold. Please correct me if I'm wrong.

- Mani

> I don't know if similar issues exist on non-ACPI systems.  If they
> don't exist, platform_pci_bridge_d3() could just return true for
> all devicetree-based systems.  Might be worth testing if any systems
> can be found which exhibit issues with such a policy.  That would
> obviate the need to specify "supports-d3" in the devicetree.
> Quite the opposite, ports which are known not to work could be
> blacklisted.  Of course if it turns out that's the majority then
> whitelisting via "supports-d3" is a better option.
> 
> 
> >   2) The platform doesn't support putting the bridge in D3cold and
> >      back to D0.  I don't understand this either because I assumed DT
> >      would describe *hardware*, and "supports-d3" might imply the
> >      presence of hardware power control, but doesn't tell us how to
> >      operate it, and it must be up to a native driver to know how to
> >      do it.
> 
> I think we're putting devices into D3hot first before cutting power
> (i.e. putting them into D3cold), so knowing that D3hot is safe is
> basically a prerequisite for D3cold.
> 
> Thanks,
> 
> Lukas

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