Re: [PATCH v4 2/2] PCI: xilinx-cpm: Add Versal CPM Root Port driver

[Bjorn Helgaas]

On Wed, Jan 29, 2020 at 11:16:47AM +0000, Bharat Kumar Gogada wrote:
> > Subject: Re: [PATCH v4 2/2] PCI: xilinx-cpm: Add Versal CPM Root Port driver
> > 
> > On Tue, Jan 28, 2020 at 06:14:43PM +0530, Bharat Kumar Gogada wrote:
> > > - Add support for Versal CPM as Root Port.
> > > - The Versal ACAP devices include CCIX-PCIe Module (CPM). The integrated
> > >   block for CPM along with the integrated bridge can function
> > >   as PCIe Root Port.
> > > - CPM Versal uses GICv3 ITS feature for achieving assigning MSI/MSI-X
> > >   vectors and handling MSI/MSI-X interrupts.
> > > - Bridge error and legacy interrupts in Versal CPM are handled using
> > >   Versal CPM specific MISC interrupt line.
> > >
> > > Changes v4:
> > > - change commit subject.
> > > - Remove unnecessary comments and type cast.
> > > - Added comments for CPM block register access using readl/writel.
> > >
> > > Signed-off-by: Bharat Kumar Gogada <bharat.kumar.gogada@xxxxxxxxxx>
> > > ...
> > 
> > > +static bool xilinx_cpm_pcie_valid_device(struct pci_bus *bus,
> > > +					 unsigned int devfn)
> > > +{
> > > +	struct xilinx_cpm_pcie_port *port = bus->sysdata;
> > > +
> > > +	/* Only one device down on each root port */
> > > +	if (bus->number == port->root_busno && devfn > 0)
> > > +		return false;
> > 
> > This whole *_valid_device() thing is a mess.  We shouldn't need it at all.  But if
> > we *do* need it, I don't think you should check the entire devfn because that
> > means you can't attach a multifunction device.
> > 
> > Several other drivers with similar *_valid_device() implementations check only
> > PCI_SLOT():
> > 
> >   dw_pcie_valid_device()
> >   advk_pcie_valid_device()
> >   pci_dw_valid_device()
> >   altera_pcie_valid_device()
> >   mobiveil_pcie_valid_device()
> >   rockchip_pcie_valid_device()
> > 
> > Even checking just PCI_SLOT() is problematic because I think an ARI device with
> > more than 8 functions will not work correctly.
> > 
> > What exactly happens if you omit this function, i.e., if we just go ahead and
> > attempt config accesses when the device is not present?  We
> > *should* get something like an Unsupported Request completion, and that
> > *should* be a recoverable error.  Most hardware turns this error into read data
> > of 0xffffffff.  The OS should be able to figure out that there's no device there
> > and continue with no ill effects.
> > 
> Thanks Bjorn. I did test and I do not see any issue without this. 
> Will resend patch with this change.

That's great, thanks for testing that!  I wonder how many other
drivers could just drop that code.

Bjorn