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

[Bharat Kumar Gogada]

> 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.
> > +	return true;
> > +}
> > +
> > +/**
> > + * xilinx_cpm_pcie_map_bus - Get configuration base
> > + * @bus: PCI Bus structure
> > + * @devfn: Device/function
> > + * @where: Offset from base
> > + *
> > + * Return: Base address of the configuration space needed to be
> > + *	   accessed.
> > + */
> > +static void __iomem *xilinx_cpm_pcie_map_bus(struct pci_bus *bus,
> > +					     unsigned int devfn, int where) {
> > +	struct xilinx_cpm_pcie_port *port = bus->sysdata;
> > +	int relbus;
> > +
> > +	if (!xilinx_cpm_pcie_valid_device(bus, devfn))
> > +		return NULL;
> > +
> > +	relbus = (bus->number << ECAM_BUS_NUM_SHIFT) |
> > +		 (devfn << ECAM_DEV_NUM_SHIFT);
> > +
> > +	return port->reg_base + relbus + where; }
> > +
> > +/* PCIe operations */
> > +static struct pci_ops xilinx_cpm_pcie_ops = {
> > +	.map_bus = xilinx_cpm_pcie_map_bus,
> > +	.read	= pci_generic_config_read,
> > +	.write	= pci_generic_config_write,
> > +};

Regards,
bharat