Message-ID: <pjns34mlopgv6wcfd55myh4qbbryfcokrvxdxclbmhkm4c4m5e@ysd4dnmbmsx5>
Date: Tue, 25 Nov 2025 19:12:15 +0530
From: Manivannan Sadhasivam <mani@...nel.org>
To: Chen-Yu Tsai <wenst@...omium.org>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@....qualcomm.com>, 
	Lorenzo Pieralisi <lpieralisi@...nel.org>, Krzysztof Wilczyński <kwilczynski@...nel.org>, 
	Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>, 
	Bartosz Golaszewski <brgl@...ev.pl>, linux-pci@...r.kernel.org, linux-arm-msm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Chen-Yu Tsai <wens@...nel.org>, 
	Brian Norris <briannorris@...omium.org>, Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>, 
	Niklas Cassel <cassel@...nel.org>, Alex Elder <elder@...cstar.com>
Subject: Re: [PATCH 3/5] PCI/pwrctrl: Add APIs for explicitly creating and
 destroying pwrctrl devices

On Tue, Nov 25, 2025 at 04:18:49PM +0800, Chen-Yu Tsai wrote:
> On Tue, Nov 25, 2025 at 3:13 PM Manivannan Sadhasivam
> <manivannan.sadhasivam@....qualcomm.com> wrote:
> >
> > From: Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>
> >
> > Previously, the PCI core created pwrctrl devices during pci_scan_device()
> > on its own and then skipped enumeration of those devices, hoping the
> > pwrctrl driver would power them on and trigger a bus rescan.
> >
> > This approach works for endpoint devices directly connected to Root Ports,
> > but it fails for PCIe switches acting as bus extenders. When the switch
> > requires pwrctrl support, and the pwrctrl driver is not available during
> > the pwrctrl device creation, it's enumeration will be skipped during the
> > initial PCI bus scan.
> >
> > This premature scan leads the PCI core to allocate resources (bridge
> > windows, bus numbers) for the upstream bridge based on available downstream
> > buses at scan time. For non-hotplug capable bridges, PCI core typically
> > allocates resources based on the number of buses available during the
> > initial bus scan, which happens to be just one if the switch is not powered
> > on and enumerated at that time. When the switch gets enumerated later on,
> > it will fail due to the lack of upstream resources.
> >
> > As a result, a PCIe switch powered on by the pwrctrl driver cannot be
> > reliably enumerated currently. Either the switch has to be enabled in the
> > bootloader or the switch pwrctrl driver has to be loaded during the pwrctrl
> > device creation time to workaround these issues.
> >
> > This commit introduces new APIs to explicitly create and destroy pwrctrl
> > devices from controller drivers by recursively scanning the PCI child nodes
> > of the controller. These APIs allow creating pwrctrl devices based on the
> > original criteria and are intended to be called during controller probe and
> > removal.
> >
> > These APIs, together with the upcoming APIs for power on/off will allow the
> > controller drivers to power on all the devices before starting the initial
> > bus scan, thereby solving the resource allocation issue.
> >
> > Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>
> > [mani: splitted the patch, cleaned up the code, and rewrote description]
> > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@....qualcomm.com>
> > ---
> >  drivers/pci/pwrctrl/core.c  | 112 ++++++++++++++++++++++++++++++++++++++++++++
> >  include/linux/pci-pwrctrl.h |   8 +++-
> >  2 files changed, 119 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/pci/pwrctrl/core.c b/drivers/pci/pwrctrl/core.c
> > index 6bdbfed584d6..6eca54e0d540 100644
> > --- a/drivers/pci/pwrctrl/core.c
> > +++ b/drivers/pci/pwrctrl/core.c
> > @@ -3,14 +3,21 @@
> >   * Copyright (C) 2024 Linaro Ltd.
> >   */
> >
> > +#define dev_fmt(fmt) "Pwrctrl: " fmt
> > +
> >  #include <linux/device.h>
> >  #include <linux/export.h>
> >  #include <linux/kernel.h>
> > +#include <linux/of.h>
> > +#include <linux/of_platform.h>
> >  #include <linux/pci.h>
> >  #include <linux/pci-pwrctrl.h>
> > +#include <linux/platform_device.h>
> >  #include <linux/property.h>
> >  #include <linux/slab.h>
> >
> > +#include "../pci.h"
> > +
> >  static int pci_pwrctrl_notify(struct notifier_block *nb, unsigned long action,
> >                               void *data)
> >  {
> > @@ -145,6 +152,111 @@ int devm_pci_pwrctrl_device_set_ready(struct device *dev,
> >  }
> >  EXPORT_SYMBOL_GPL(devm_pci_pwrctrl_device_set_ready);
> >
> > +static int pci_pwrctrl_create_device(struct device_node *np, struct device *parent)
> > +{
> > +       struct platform_device *pdev;
> > +       int ret;
> > +
> > +       for_each_available_child_of_node_scoped(np, child) {
> > +               ret = pci_pwrctrl_create_device(child, parent);
> > +               if (ret)
> > +                       return ret;
> > +       }
> > +
> > +       /* Bail out if the platform device is already available for the node */
> > +       pdev = of_find_device_by_node(np);
> > +       if (pdev) {
> > +               put_device(&pdev->dev);
> > +               return 0;
> > +       }
> > +
> > +       /*
> > +        * Sanity check to make sure that the node has the compatible property
> > +        * to allow driver binding.
> > +        */
> > +       if (!of_property_present(np, "compatible"))
> > +               return 0;
> > +
> > +       /*
> > +        * Check whether the pwrctrl device really needs to be created or not.
> > +        * This is decided based on at least one of the power supplies being
> > +        * defined in the devicetree node of the device.
> > +        */
> > +       if (!of_pci_supply_present(np)) {
> 
> This symbol is not exported for modules to use, and will cause the build
> to fail if PCI_PWRCTRL* is m.
> 

Ok. I'll export this symbol in next version. Thanks for spotting it!

- Mani

-- 
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