linux-kernel - Re: [PATCH RFC 2/3] PCI/pwrctrl: Allow pwrctrl core to control PERST# GPIO if available

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Message-ID: <aHGjBxKYJCkhXcbo@google.com>
Date: Fri, 11 Jul 2025 16:49:27 -0700
From: Brian Norris <briannorris@...omium.org>
To: Manivannan Sadhasivam <mani@...nel.org>
Cc: Bartosz Golaszewski <brgl@...ev.pl>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Jingoo Han <jingoohan1@...il.com>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kwilczynski@...nel.org>,
	Rob Herring <robh@...nel.org>, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>
Subject: Re: [PATCH RFC 2/3] PCI/pwrctrl: Allow pwrctrl core to control
 PERST# GPIO if available

On Wed, Jul 09, 2025 at 01:35:08PM +0530, Manivannan Sadhasivam wrote:
> On Tue, Jul 08, 2025 at 08:15:39PM GMT, Brian Norris wrote:
> > On Mon, Jul 07, 2025 at 11:48:39PM +0530, Manivannan Sadhasivam wrote:
> > > --- a/drivers/pci/pwrctrl/core.c
> > > +++ b/drivers/pci/pwrctrl/core.c
> > > +static void pci_pwrctrl_perst_deassert(struct pci_pwrctrl *pwrctrl)
> > > +{
> > > +	/* Bail out early to avoid the delay if PERST# is not available */
> > > +	if (!pwrctrl->perst)
> > > +		return;
> > > +
> > > +	msleep(PCIE_T_PVPERL_MS);
> > > +	gpiod_set_value_cansleep(pwrctrl->perst, 0);
> > 
> > What if PERST# was already deasserted? On one hand, we're wasting time
> > here if so. On the other, you're not accomplishing your spec-compliance
> > goal if it was.
> > 
> 
> If controller drivers populate 'pci_host_bridge::perst', then they should not
> deassert PERST# as they don't control the supplies. I've mentioned it in the
> cover letter, but I will mention it in commit message also.

Sorry, I think I partially read that, but didn't really grasp how it fit
in here.

I commented on patch 3 where you try to implement this. IIUC, you're
making excessive assumptions about the use of pwrctrl.

Brian