linux-kernel - Re: [RFC] PCI: pwrctrl and link-up dependencies

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Message-ID: <uivlbxghkynwpmzenyr2b3xk4uxeuqf6dow6ao4mptcnzygrw7@ylfqavr3ry44>
Date: Mon, 14 Apr 2025 16:37:23 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Brian Norris <briannorris@...omium.org>
Cc: Bartosz Golaszewski <brgl@...ev.pl>, 
	Bjorn Helgaas <bhelgaas@...gle.com>, Jingoo Han <jingoohan1@...il.com>, 
	Lorenzo Pieralisi <lpieralisi@...nel.org>, Krzysztof Wilczyński <kw@...ux.com>, 
	Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>, Rob Herring <robh@...nel.org>, linux-pci@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org, dmitry.baryshkov@...aro.org, 
	Tsai Sung-Fu <danielsftsai@...gle.com>, Jim Quinlan <jim2101024@...il.com>, 
	Nicolas Saenz Julienne <nsaenz@...nel.org>, Florian Fainelli <florian.fainelli@...adcom.com>, 
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>
Subject: Re: [RFC] PCI: pwrctrl and link-up dependencies

On Tue, Apr 08, 2025 at 02:26:24PM -0700, Brian Norris wrote:
> + adding pcie-brcmstb.c folks
> 
> On Tue, Apr 08, 2025 at 12:59:39PM -0700, Brian Norris wrote:
> > TL;DR: PCIe link-up may depend on pwrctrl; however, link-startup is
> > often run before pwrctrl gets involved. I'm exploring options to resolve
> > this.
> 
> Apologies if a quick self-reply is considered nosiy or rude, but I
> nearly forgot that I previously was looking at "pwrctrl"-like
> functionality and noticed that drivers/pci/controller/pcie-brcmstb.c has
> had a portion of the same "pwrctrl" functionality for some time (commit
> 93e41f3fca3d ("PCI: brcmstb: Add control of subdevice voltage
> regulators")).
> 

Yes, the goal of the pwrctrl driver is to get rid of this clutter from the
controller drivers.

> Notably, it performs its power sequencing before starting its link, for
> (I believe) exactly the same reasons as I mention below. While I'm sure
> it could theoretically be nice for them to be able to use
> drivers/pci/pwrctrl/, I expect they cannot today, for the same reasons.
> 

If you look into brcm_pcie_add_bus(), they are ignoring the return value of
brcm_pcie_start_link() precisely for the reason I explained in the previous
thread. However, they do check for it in brcm_pcie_resume_noirq() which looks
like a bug as the controller will fail to resume from system suspend if no
devices are connected.

- Mani

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