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

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Message-ID: <aHGueAD70abjw8D_@google.com>
Date: Fri, 11 Jul 2025 17:38:16 -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

Sorry for so many individual reviews, but I've passed over this a few
times and had new questions/comments several times:

On Mon, Jul 07, 2025 at 11:48:39PM +0530, Manivannan Sadhasivam wrote:
> PERST# is an (optional) auxiliary signal provided by the PCIe host to
> components for signalling 'Fundamental Reset' as per the PCIe spec r6.0,
> sec 6.6.1.

>  void pci_pwrctrl_init(struct pci_pwrctrl *pwrctrl, struct device *dev)
>  {
> +	struct pci_host_bridge *host_bridge = to_pci_host_bridge(dev->parent);
> +	int devfn;
> +
>  	pwrctrl->dev = dev;
>  	INIT_WORK(&pwrctrl->work, rescan_work_func);
> +
> +	if (!host_bridge->perst)
> +		return;
> +
> +	devfn = of_pci_get_devfn(dev_of_node(dev));
> +	if (devfn >= 0 && host_bridge->perst[PCI_SLOT(devfn)])

This seems to imply a 1:1 correlation between slots and pwrctrl devices,
almost as if you expect everyone is using drivers/pci/pwrctrl/slot.c.
But there is also endpoint-specific pwrctrl support, and there's quite
a bit of flexibility around what these hierarchies can look like.

How do you account for that?

For example, couldn't you have both a "port" and an "endpoint" pwrctrl? Would
they both grab the same PERST# GPIO here? And might that incur excessive
resets, possibly even clobbering each other?

Or what if multiple slots are governed by a single GPIO? Do you expect
the bridge perst[] array to contain redundant GPIOs?

Brian

> +		pwrctrl->perst = host_bridge->perst[PCI_SLOT(devfn)];
>  }
>  EXPORT_SYMBOL_GPL(pci_pwrctrl_init);