linux-kernel - Re: [BUGFIX 3/4] PCI/PM: Fix config reg access for D3cold and bridge suspending

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Date:	Sat, 4 Aug 2012 23:44:54 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Huang Ying <ying.huang@...el.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: Re: [BUGFIX 3/4] PCI/PM: Fix config reg access for D3cold and bridge suspending

On Saturday, August 04, 2012, Rafael J. Wysocki wrote:
> On Friday, August 03, 2012, Alan Stern wrote:
> > On Fri, 3 Aug 2012, Huang Ying wrote:
> > 
> > > This patch fixes the following bug:
> > > 
> > > http://marc.info/?l=linux-pci&m=134338059022620&w=2
> > > 
> > > Where lspci does not work properly if a device and the corresponding
> > > parent bridge (such as PCIe port) is suspended.  This is because the
> > > device configuration space registers will be not accessible if the
> > > corresponding parent bridge is suspended or the device is put into
> > > D3cold state.
> > > 
> > > To solve the issue, the bridge/PCIe port connected to the device is
> > > put into active state before read/write configuration space registers.
> > > If the device is in D3cold state, it will be put into active state
> > > too.
> > > 
> > > To avoid resume/suspend PCIe port for each configuration register
> > > read/write, a small delay is added before the PCIe port to go
> > > suspended.
> > 
> > 
> > > +static void
> > > +pci_config_pm_runtime_put(struct pci_dev *pdev)
> > > +{
> > > +	struct device *dev = &pdev->dev;
> > > +	struct device *parent = dev->parent;
> > > +
> > > +	pm_runtime_put(dev);
> > > +	if (parent)
> > > +		pm_runtime_put(parent);
> > > +}
> > 
> > This is just the sort of thing Rafael and I have been talking about.  
> > Why do an asynchronous put, going to all the trouble of using the 
> > workqueue, if the idle routine is just going to call 
> > pm_schedule_suspend()?
> 
> If that's PCI, it will call pm_runtime_suspend().  That probably _should_ be
> pm_schedule_suspend(), but it isn't at the moment.
> 
> > Why not call pm_runtime_put_sync() instead?
> 
> I guess because the caller doesn't care whether or not the devices will be
> suspended immediately and we seem to have agreed already that the added
> workqueue overhead is minimal.
> 
> If the _idle() routine were to call pm_schedule_suspend(), though, I'd
> agree that the overhead would be absolutely unnecessary.

Sorry, I should have had a closer look at pcie_port_runtime_idle() before
replying.

You're right, pm_runtime_put_sync() should be used for the parent.

Thanks,
Rafael
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