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

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Message-ID: <20240227225442.GA249898@bhelgaas>
Date: Tue, 27 Feb 2024 16:54:42 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Cc: Bjorn Helgaas <bhelgaas@...gle.com>,
	Bjorn Andersson <andersson@...nel.org>,
	Konrad Dybcio <konrad.dybcio@...aro.org>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Rob Herring <robh@...nel.org>, Lukas Wunner <lukas@...ner.de>,
	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 Wed, Feb 28, 2024 at 12:10:15AM +0530, Manivannan Sadhasivam wrote:
> On Tue, Feb 27, 2024 at 11:37:05AM -0600, Bjorn Helgaas wrote:
> > On Tue, Feb 27, 2024 at 10:38:40PM +0530, Manivannan Sadhasivam wrote:
> > > On Tue, Feb 27, 2024 at 10:25:35AM -0600, Bjorn Helgaas wrote:
> > > 
> > > [...]
> > > 
> > > > > Ok, I got the issue. TBH, I added the device tree property based on
> > > > > the existing quirks for the ACPI devices. But none of the DT based
> > > > > platforms I'm aware of (even the legacy Qcom MSM8996 chipset
> > > > > released in early 2016) doesn't have any issue with D3hot. But I'm
> > > > > just nervous to assume it is the case for all the DT based platforms
> > > > > in the wild.
> > > > > 
> > > > > But to proceed further, what is your preference? Should we ammend
> > > > > the DT property to make it explicit that the propery only focuses on
> > > > > the D3hot capability of the bridge and it works as per the spec
> > > > > (PMCSR) or bite the bullet and enable D3hot for all the non-ACPI
> > > > > platforms?
> > > > > 
> > > > > We can add quirks for the bridges later on if we happen to receive
> > > > > any bug report.
> > > > 
> > > > I would assume all devices support D3hot via PMCSR per spec.  We can
> > > > add quirks if we discover something that doesn't.
> > > 
> > > When you say "all devices", are you referring to bridges in DT
> > > platforms or the bridges across all platforms?
> > 
> > This patch is only concerned with DT, so that's what I'm commenting on
> > here.  I don't know how to untangle the question of ACPI systems.
> 
> Ok, I just wanted to confirm.
> 
> > This patch affects platform_pci_bridge_d3(), so just based on the
> > "platform" in the function name, I would expect it to be concerned
> > with the D3cold case and whether the platform supports controlling
> > main power.
> > 
> > It looks like this patch says "we can put devices in D3cold if DT has
> > 'supports-d3'".  But I don't know how to make sense of that because
> > that requires (a) platform hardware to control main power and (b)
> > software that knows how to use that hardware.  Wouldn't this require a
> > little more DT description, like "regulator X controls main power for
> > this bridge"?  And then an OS would only be able to actually use
> > D3cold if it knows how to *operate* the regulator, and it doesn't seem
> > like DT could answer that.
> 
> Fair point. And for most of the DT based platforms, there is no
> dedicated power supply for the bridge described in DT. So
> transitioning the bridge to D3cold is not entirely possible in the
> OS.
> 
> Since we concluded that enabling D3hot for all bridges in DT
> platforms is the way to go, I'll drop supporting the DT property in
> next version.
> 
> I'll also remove it from the binding.

Sounds good, thanks!