Re: [PATCH v2] PCI/portdrv: Do not require an interrupt for all AER capable ports

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Dec 08, 2022 at 07:58:42AM +0200, Mika Westerberg wrote:
> On Wed, Dec 07, 2022 at 04:35:37PM -0600, Bjorn Helgaas wrote:
> > On Wed, Dec 07, 2022 at 10:41:05AM +0200, Mika Westerberg wrote:
> > > Only Root Ports and Event Collectors use MSI for AER. PCIe Switch ports
> > > or endpoints on the other hand only send messages (that get collected by
> > > the former). For this reason do not require PCIe switch ports and
> > > endpoints to use interrupt if they support AER.
> > > 
> > > This allows portdrv to attach PCIe switch ports of Intel DG1 and DG2
> > > discrete graphics cards. These do not declare MSI or legacy interrupts.
> > 
> > Help me understand more about this situation.  I guess we want portdrv
> > to attach not to a GPU itself, but to a switch port on the card that
> > *leads* to the GPU?
> 
> Yes correct.
> 
> > From the patch, it looks like the only PCIe port service this switch
> > port advertises is AER (not PME, DPC, hotplug, etc), and it doesn't
> > have MSI or MSI-X.
> 
> Correct.
> 
> > So aerdriver should be able to register for PCIE_PORT_SERVICE_AER, but
> > aer_probe() ignores everything except Root Ports and RCECs.  What's
> > the benefit then?  I must be missing something.
> 
> The portdrv is needed for power management and everything else PCI even
> if there is no actual "service" attached.

Thanks!  I'm trying to connect the dots to get to the specific bug fix
or improvement made by this patch.

The pcie_pme_driver itself doesn't seem involved because it registers
for PCIE_PORT_SERVICE_PME, and that's only set for Root Ports and
RCECs.

The pcie_portdrv_pm_ops would be another possibility, but with the
exception of pcie_port_runtime_idle(), all those ops just iterate over
service drivers, which aren't involved.

So I'm guessing this has to do with setting
DPM_FLAG_NO_DIRECT_COMPLETE and DPM_FLAG_SMART_SUSPEND in
pcie_portdrv_probe().  Does the lack of portdrv mean the GPU can't
suspend?  Does this reduce power consumption?  How would a user know
this change would help their system?

Bjorn