Re: [PATCH v2 2/2] PCI/DPC: Allow Native DPC Host Bridges to use DPC

["Derrick, Jonathan" <jonathan.derrick@intel.com>]

Hi Sathyanarayanan,

On Sat, 2020-04-25 at 13:46 -0700, Kuppuswamy, Sathyanarayanan wrote:
> 
> On 4/23/20 8:11 AM, Derrick, Jonathan wrote:
> > Hi Sathyanarayanan,
> > 
> > On Wed, 2020-04-22 at 15:50 -0700, Kuppuswamy, Sathyanarayanan wrote:
> > > On 4/20/20 2:37 PM, Jon Derrick wrote:
> > > > The existing portdrv model prevents DPC services without either OS
> > > > control (_OSC) granted to AER services, a Host Bridge requesting Native
> > > > AER, or using one of the 'pcie_ports=' parameters of 'native' or
> > > > 'dpc-native'.
> > > > 
> > > > The DPC port service driver itself will also fail to probe if the kernel
> > > > assumes the port is using Firmware-First AER. It's a reasonable
> > > > expectation that a port using Firmware-First AER will also be using
> > > > Firmware-First DPC, however if a Host Bridge requests Native DPC, the
> > > > DPC driver should allow it and not fail to bind due to AER capability
> > > > settings.
> > > > 
> > > > Host Bridges which request Native DPC port services will also likely
> > > > request Native AER, however it shouldn't be a requirement. This patch
> > > > allows ports on those Host Bridges to have DPC port services.
> > > > 
> > > > This will avoid the unlikely situation where the port is Firmware-First
> > > > AER and Native DPC, and a BIOS or switch firmware preconfiguration of
> > > > the DPC trigger could result in unhandled DPC events.
> > > > 
> > > > Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
> > > > ---
> > > >    drivers/pci/pcie/dpc.c          | 3 ++-
> > > >    drivers/pci/pcie/portdrv_core.c | 3 ++-
> > > >    2 files changed, 4 insertions(+), 2 deletions(-)
> > > > 
> > > > diff --git a/drivers/pci/pcie/dpc.c b/drivers/pci/pcie/dpc.c
> > > > index 7621704..3f3106f 100644
> > > > --- a/drivers/pci/pcie/dpc.c
> > > > +++ b/drivers/pci/pcie/dpc.c
> > > > @@ -284,7 +284,8 @@ static int dpc_probe(struct pcie_device *dev)
> > > >    	int status;
> > > >    	u16 ctl, cap;
> > > >    
> > > > -	if (pcie_aer_get_firmware_first(pdev) && !pcie_ports_dpc_native)
> > > > +	if (pcie_aer_get_firmware_first(pdev) && !pcie_ports_dpc_native &&
> > > > +	    !pci_find_host_bridge(pdev->bus)->native_dpc)
> > > Why do it in probe as well ? if host->native_dpc is not set then the
> > > device DPC probe it self won't happen right ?
> > 
> > Portdrv only enables the interrupt and allows the probe to occur.
> 
> Please check the following snippet of code (from portdrv_core.c).
> 
> IIUC, pcie_device_init() will not be called if PCIE_PORT_SERVICE_DPC is
> not set in capabilities. Your change in portdrv_core.c already
> selectively enables the PCIE_PORT_SERVICE_DPC service based on
> native_dpc value.
> 
That's right. So pcie_device_init registers the port service driver
allowing the services enumeration to occur.

> So IMO, adding native_dpc check in dpc_probe() is redundant.
> 
> int pcie_port_device_register(struct pci_dev *dev)
> 	/* Allocate child services if any */
> 	status = -ENODEV;
> 	nr_service = 0;
> 	for (i = 0; i < PCIE_PORT_DEVICE_MAXSERVICES; i++) {
> 		int service = 1 << i;
> 		if (!(capabilities & service))
> 			continue;
> 		if (!pcie_device_init(dev, service, irqs[i]))
> 			nr_service++;
> 	}
> 
This is the tricky part
There's still a check in dpc_probe for AER FFS or pcie_ports=dpc-
native:

if (pcie_aer_get_firmware_first(pdev) && !pcie_ports_dpc_native)
	return -ENOTSUPP;

One option is to move that to get_port_device_capability and remove the
dpc_probe check

> > The probe itself will still fail if there's a mixed-mode _OSC
> > negotiated AER & DPC, due to pcie_aer_get_firmware_first returning 1
> > for AER and no check for DPC.
> > 
> > I don't know if such a platform will exist, but the kernel is already
> > wired for 'dpc-native' so it makes sense to extend it for this..
> > 
> > This transform might be more readable:
> > 	if (pcie_aer_get_firmware_first(pdev) &&
> > 	    !(pcie_ports_dpc_native || hb->native_dpc))
> > 
> > 
> > 
> > > >    		return -ENOTSUPP;
> > > >    
> > > >    	status = devm_request_threaded_irq(device, dev->irq, dpc_irq,
> > > > diff --git a/drivers/pci/pcie/portdrv_core.c b/drivers/pci/pcie/portdrv_core.c
> > > > index 50a9522..f2139a1 100644
> > > > --- a/drivers/pci/pcie/portdrv_core.c
> > > > +++ b/drivers/pci/pcie/portdrv_core.c
> > > > @@ -256,7 +256,8 @@ static int get_port_device_capability(struct pci_dev *dev)
> > > >    	 */
> > > >    	if (pci_find_ext_capability(dev, PCI_EXT_CAP_ID_DPC) &&
> > > >    	    pci_aer_available() &&
> > > > -	    (pcie_ports_dpc_native || (services & PCIE_PORT_SERVICE_AER)))
> > > > +	    (pcie_ports_dpc_native || host->native_dpc ||
> > > > +	     (services & PCIE_PORT_SERVICE_AER)))
> > > >    		services |= PCIE_PORT_SERVICE_DPC;
> > > >    
> > > >    	if (pci_pcie_type(dev) == PCI_EXP_TYPE_DOWNSTREAM ||
> > > >