Re: [RFC] PCI/PM: Keep runtime PM enabled for unbound PCI devices

[Huang Ying <ying.huang@intel.com>]

On Mon, 2012-11-19 at 23:00 +0100, Rafael J. Wysocki wrote:
> On Monday, November 19, 2012 11:31:01 AM Alan Stern wrote:
> > On Mon, 19 Nov 2012, Huang Ying wrote:
> > 
> > > For unbound PCI devices, what we need is:
> > > 
> > >  - Always in D0 state, because some devices does not work again after
> > >    being put into D3 by the PCI bus.
> > > 
> > >  - In SUSPENDED state if allowed, so that the parent devices can still
> > >    be put into low power state.
> > > 
> > > To satisfy these requirement, the runtime PM for the unbound PCI
> > > devices are disabled and set to SUSPENDED state.  One issue of this
> > > solution is that the PCI devices will be put into SUSPENDED state even
> > > if the SUSPENDED state is forbidden via the sysfs interface
> > > (.../power/control) of the device.  This is not an issue for most
> > > devices, because most PCI devices are not used at all if unbounded.
> > > But there are exceptions.  For example, unbound VGA card can be used
> > > for display, but suspend its parents make it stop working.
> > > 
> > > To fix the issue, we keep the runtime PM enabled when the PCI devices
> > > are unbound.  But the runtime PM callbacks will do nothing if the PCI
> > > devices are unbound.  This way, we can put the PCI devices into
> > > SUSPENDED state without put the PCI devices into D3 state.
> > > 
> > > Known issue: after some changing, pci_dev->driver is used to indicate
> > > whether the PCI devices are bound and whether the runtime PM callbacks
> > > should do nothing.  Maybe it is better to use a dedicated flag such as
> > > .skip_rtpm_callbacks.  That may improve code readability.
> > 
> > I think it's okay like this, especially if you add a comment in 
> > pci_runtime_suspend, pci_runtime_resume, and pci_runtime_idle 
> > explaining that when pci_dev->driver isn't set, the device should 
> > always remain in D0 regardless of the runtime status.
> 
> Yes, I agree with Alan.  Please add comments as Alan's suggesting and it
> should be fine.

Sure.  Will do that.

Best Regards,
Huang Ying