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

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Message-ID: <CANCKTBuEyOmojZ0yw30VKF2b5xOYkirx2gWEhkQ1a=-YHud5Cw@mail.gmail.com>
Date: Tue, 15 Apr 2025 14:12:39 -0400
From: Jim Quinlan <jim2101024@...il.com>
To: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Cc: Brian Norris <briannorris@...omium.org>, 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>, 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 Mon, Apr 14, 2025 at 7:07 AM Manivannan Sadhasivam
<manivannan.sadhasivam@...aro.org> wrote:
>
> 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.

The reason our brcm_pcie_add_bus() does not check/return an error is
that the caller will invoke a WARN() on our error, or at least that
was the case when the commit was submitted.   We want to be good
citizens.  Also, for our driver, if the pcie-link-up fails, the
probe() fails.  There is no subsequent suspend/resume possible.  We do
not support PCIe hotplug in HW or SW.

Regards,
Jim Quinlan
Broadcom STB

>
> - Mani
>
> --
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