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

From: Lukas Wunner <lukas@wunner.de>
To: Bjorn Helgaas <helgaas@kernel.org>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Bjorn Andersson <andersson@kernel.org>,
	Konrad Dybcio <konrad.dybcio@linaro.org>,
	Lorenzo Pieralisi <lpieralisi@kernel.org>,
	Krzysztof Wilczy??ski <kw@linux.com>,
	Rob Herring <robh@kernel.org>,
	Mika Westerberg <mika.westerberg@linux.intel.com>,
	quic_krichai@quicinc.com, linux-pci@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org
Subject: Re: [PATCH v3] PCI: Add D3 support for PCI bridges in DT based platforms
Date: Thu, 22 Feb 2024 10:40:52 +0100	[thread overview]
Message-ID: <20240222094052.GA25101@wunner.de> (raw)
In-Reply-To: <20240221182000.GA1533634@bhelgaas>

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.

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