linux-kernel - Re: [PATCH v2] PCI: dwc: Fix resume failure if no EP is connected at some platforms

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Message-ID: <20241106222933.GA1543549@bhelgaas>
Date: Wed, 6 Nov 2024 16:29:33 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Hongxing Zhu <hongxing.zhu@....com>
Cc: "kwilczynski@...nel.org" <kwilczynski@...nel.org>,
	"bhelgaas@...gle.com" <bhelgaas@...gle.com>,
	"lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>,
	Frank Li <frank.li@....com>, "mani@...nel.org" <mani@...nel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kernel@...gutronix.de" <kernel@...gutronix.de>,
	"imx@...ts.linux.dev" <imx@...ts.linux.dev>
Subject: Re: [PATCH v2] PCI: dwc: Fix resume failure if no EP is connected at
 some platforms

On Wed, Nov 06, 2024 at 01:59:41AM +0000, Hongxing Zhu wrote:
> > -----Original Message-----
> > From: Bjorn Helgaas <helgaas@...nel.org>
> > Sent: 2024年11月6日 7:27
> > To: Hongxing Zhu <hongxing.zhu@....com>
> > Cc: kwilczynski@...nel.org; bhelgaas@...gle.com;
> > lorenzo.pieralisi@....com; Frank Li <frank.li@....com>; mani@...nel.org;
> > linux-pci@...r.kernel.org; linux-arm-kernel@...ts.infradead.org;
> > linux-kernel@...r.kernel.org; kernel@...gutronix.de; imx@...ts.linux.dev
> > Subject: Re: [PATCH v2] PCI: dwc: Fix resume failure if no EP is connected at
> > some platforms
> > 
> > On Mon, Jul 22, 2024 at 02:15:13PM +0800, Richard Zhu wrote:
> > > The dw_pcie_suspend_noirq() function currently returns success
> > > directly if no endpoint (EP) device is connected. However, on some
> > > platforms, power loss occurs during suspend, causing dw_resume() to do
> > > nothing in this case.
> > > This results in a system halt because the DWC controller is not
> > > initialized after power-on during resume.

> > > @@ -933,23 +933,23 @@ int dw_pcie_suspend_noirq(struct dw_pcie *pci)
> > >  	if (dw_pcie_readw_dbi(pci, offset + PCI_EXP_LNKCTL) &
> > PCI_EXP_LNKCTL_ASPM_L1)
> > >  		return 0;
> > >
> > > -	if (dw_pcie_get_ltssm(pci) <= DW_PCIE_LTSSM_DETECT_ACT)
> > > -		return 0;
> > > -
> > > -	if (pci->pp.ops->pme_turn_off)
> > > -		pci->pp.ops->pme_turn_off(&pci->pp);
> > > -	else
> > > -		ret = dw_pcie_pme_turn_off(pci);
> > > +	if (dw_pcie_get_ltssm(pci) > DW_PCIE_LTSSM_DETECT_ACT) {
> > > +		/* Only send out PME_TURN_OFF when PCIE link is up */
> > > +		if (pci->pp.ops->pme_turn_off)
> > > +			pci->pp.ops->pme_turn_off(&pci->pp);
> > > +		else
> > > +			ret = dw_pcie_pme_turn_off(pci);
> > 
> > This looks possibly racy since the link can go down at any point.
>
> When link is down and without this commit changes,
> dw_pcie_suspend_noirq() return directly, and the PME_TURN_OFF
> wouldn't be kicked off.

Right, that's the code change.

> I change the behavior to issue the PME_TURN_OFF when link is up
> here.

But I don't think you responded to the race question.  What happens
here?

  if (dw_pcie_get_ltssm(pci) > DW_PCIE_LTSSM_DETECT_ACT) {
    --> link goes down here <--
    pci->pp.ops->pme_turn_off(&pci->pp);

You decide the LTSSM is active and the link is up.  Then the link goes
down.  Then you send PME_Turn_off.  Now what?

If it's safe to try to send PME_Turn_off regardless of whether the
link is up or down, there would be no need to test the LTSSM state.

Bjorn