linux-kernel - Re: [PATCH v4 1/9] dt-bindings: imx6q-pcie: Add ref clock for i.MX95 PCIe RC

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Message-ID: <20241022163826.6w3it42o7kbp6ydp@thinkpad>
Date: Tue, 22 Oct 2024 22:08:26 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: Richard Zhu <hongxing.zhu@....com>, kw@...ux.com, bhelgaas@...gle.com,
	lpieralisi@...nel.org, frank.li@....com, l.stach@...gutronix.de,
	robh+dt@...nel.org, conor+dt@...nel.org, shawnguo@...nel.org,
	krzysztof.kozlowski+dt@...aro.org, festevam@...il.com,
	s.hauer@...gutronix.de, linux-pci@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org, kernel@...gutronix.de,
	imx@...ts.linux.dev
Subject: Re: [PATCH v4 1/9] dt-bindings: imx6q-pcie: Add ref clock for i.MX95
 PCIe RC

On Fri, Oct 18, 2024 at 06:13:05PM -0500, Bjorn Helgaas wrote:
> On Tue, Oct 15, 2024 at 04:33:25PM +0800, Richard Zhu wrote:
> > Previous reference clock of i.MX95 PCIe RC is on when system boot to
> > kernel. But boot firmware change the behavor, it is off when boot. So it
> > needs be turn on when it is used. Also it needs be turn off/on when suspend
> > and resume.
> 
> I think this background would make more sense in patch 2.  IIUC,
> that's where the driver behavior changes to do something with the
> "ref" clock.
> 
> I'm not sure how to interpret "Previous reference clock of i.MX95 PCIe
> RC is on when system boot to kernel. But boot firmware change the
> behavor, it is off when boot."
> 
> Does that mean a previous version of the boot firmware left the ref
> clock on at handoff to the OS, and newer firmware turns it off?  If
> so, I think it would be useful to include information about the
> relevant firmware versions.
> 

Most likely that the bootloader *used to* turn on the reference clock and now it
decides not to do so (for whatever reason). But ideally, the clock should be
voted/enabled by the kernel irrespective of that. So the reference of 'boot
firmware' is not relevant here.

- Mani

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