linux-kernel - Re: [PATCH v2 2/3] PCI: qcom: Check for the presence of a device instead of Link up during suspend

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Message-ID: <20251113172250.GA2291436@bhelgaas>
Date: Thu, 13 Nov 2025 11:22:50 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Manivannan Sadhasivam <mani@...nel.org>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@....qualcomm.com>,
	lpieralisi@...nel.org, kwilczynski@...nel.org, bhelgaas@...gle.com,
	will@...nel.org, linux-pci@...r.kernel.org,
	linux-kernel@...r.kernel.org, robh@...nel.org,
	linux-arm-msm@...r.kernel.org, zhangsenchuan@...incomputing.com,
	vincent.guittot@...aro.org
Subject: Re: [PATCH v2 2/3] PCI: qcom: Check for the presence of a device
 instead of Link up during suspend

On Thu, Nov 13, 2025 at 10:24:17PM +0530, Manivannan Sadhasivam wrote:
> On Thu, Nov 13, 2025 at 10:41:47AM -0600, Bjorn Helgaas wrote:
> > On Fri, Nov 07, 2025 at 10:13:18AM +0530, Manivannan Sadhasivam wrote:
> > > The suspend handler checks for the PCIe Link up to decide when to turn off
> > > the controller resources. But this check is racy as the PCIe Link can go
> > > down just after this check.
> > > 
> > > So use the newly introduced API, pci_root_ports_have_device() that checks
> > > for the presence of a device under any of the Root Ports to replace the
> > > Link up check.
> > 
> > Why is pci_root_ports_have_device() itself not racy?
> 
> Because it is very uncommon for the 'pci_dev' to go away during the
> host controller suspend. It might still be possible in edge cases,
> but very common as the link down. I can reword it.

I guess it's better to acknowledge replacing one race with another
than it would be to suggest that this *removes* a race.

But I don't understand the point of this.  Is
pci_root_ports_have_device() *less* racy than the
qcom_pcie_suspend_noirq() check?  Why would that be?

I'm kind of skeptical about adding pci_root_ports_have_device() at
all.  It seems like it just encourages racy behavior in drivers.

> > > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@....qualcomm.com>
> > > ---
> > >  drivers/pci/controller/dwc/pcie-qcom.c | 6 ++++--
> > >  1 file changed, 4 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/pci/controller/dwc/pcie-qcom.c b/drivers/pci/controller/dwc/pcie-qcom.c
> > > index 805edbbfe7eb..b2b89e2e4916 100644
> > > --- a/drivers/pci/controller/dwc/pcie-qcom.c
> > > +++ b/drivers/pci/controller/dwc/pcie-qcom.c
> > > @@ -2018,6 +2018,7 @@ static int qcom_pcie_probe(struct platform_device *pdev)
> > >  static int qcom_pcie_suspend_noirq(struct device *dev)
> > >  {
> > >  	struct qcom_pcie *pcie;
> > > +	struct dw_pcie_rp *pp;
> > >  	int ret = 0;
> > >  
> > >  	pcie = dev_get_drvdata(dev);
> > > @@ -2053,8 +2054,9 @@ static int qcom_pcie_suspend_noirq(struct device *dev)
> > >  	 * powerdown state. This will affect the lifetime of the storage devices
> > >  	 * like NVMe.
> > >  	 */
> > > -	if (!dw_pcie_link_up(pcie->pci)) {
> > > -		qcom_pcie_host_deinit(&pcie->pci->pp);
> > > +	pp = &pcie->pci->pp;
> > > +	if (!pci_root_ports_have_device(pp->bridge->bus)) {
> > > +		qcom_pcie_host_deinit(pp);
> > >  		pcie->suspended = true;
> > >  	}
> > >  
> > > -- 
> > > 2.48.1
> > > 
> 
> -- 
> மணிவண்ணன் சதாசிவம்