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

[Jim Quinlan]

On Mon, Apr 14, 2025 at 7:07 AM Manivannan Sadhasivam
<manivannan.sadhasivam@xxxxxxxxxx> 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
>
> --
> மணிவண்ணன் சதாசிவம்