Re: [PATCH 0/2] Enable D3 support for Qualcomm bridges

From: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
To: Lukas Wunner <lukas@wunner.de>
Cc: 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 0/2] Enable D3 support for Qualcomm bridges
Date: Sat, 3 Feb 2024 10:25:12 +0530	[thread overview]
Message-ID: <20240203045512.GA3038@thinkpad> (raw)
In-Reply-To: <20240202193326.GA29000@wunner.de>

On Fri, Feb 02, 2024 at 08:33:26PM +0100, Lukas Wunner wrote:
> On Fri, Feb 02, 2024 at 03:30:41PM +0530, Manivannan Sadhasivam wrote:
> > On Fri, Feb 02, 2024 at 10:00:33AM +0100, Lukas Wunner wrote:
> > > Please amend platform_pci_bridge_d3() to call a new of_pci_bridge_d3()
> > > function which determines whether D3 is supported by the platform.
> > > 
> > > E.g. of_pci_bridge_d3() could contain a whitelist of supported VID/DID
> > > tuples.  Or it could be defined as a __weak function which always
> > > returns false but can be overridden at link time by a function
> > > defined somewhere in arch/arm/, arch/arm64/ or in some driver
> > > whose Kconfig option is enabled in Qualcomm platforms.
> > 
> > Hmm. If we go with a DT based solution, then introducing a new property like
> > "d3-support" in the PCI bridge node would be the right approach. But then, it
> > also requires defining the PCI bridge node in all the DTs. But that should be
> > fine since it will help us to support WAKE# (per bridge) in the future.
> 
> I'm not sure whether a "d3-support" property would be acceptable.
> My understanding is that capabilities which can be auto-sensed by
> the driver (or the PCI core in this case), e.g. by looking at the
> PCI IDs or compatible string, should not be described in the DT.
> 

We cannot whitelist platforms in DT. DT should describe the hardware and its
capabilities. In this case, the "supports-d3" property as I proposed [1] tells
the OS that this bridge is capable of supporting D3.

Blacklisting/whitelisting belongs to the OS. We can however, whitelist the
bridges in PCI core. But that has the downside of not being useful to other OSes
supporting DT. Hence, a DT property that describes the hardware capability
makes sense to me.

- Mani

[1] https://github.com/devicetree-org/dt-schema/pull/127

-- 
மணிவண்ணன் சதாசிவம்