[+cc Lukas]
On Tue, Aug 09, 2016 at 01:22:30PM +0200, Thomas Gleixner wrote:
> From: Benedikt Spranger <b.spran...@linutronix.de>
>
> PCI and PCIBIOS probing only scans devices at function number 0/8/16/...
> Subdevices (e.g. multiqueue) have function numbers which are not a
> multiple of 8.
>
> Simple hypervisors (e.g. Jailhouse) pass subdevices directly w/o providing
> virtual PCI mappings like KVM. As a consequence a simple PCI passthrough from
> Jailhouse to a linux guest is not able to detect such devices.
>
> Changing the probe functions to scan all function numbers makes it work. This
> has no side effects and there is no reason to force the 0/8/16... probing
> scheme.
"devfn" here is a 8-bit field (5 bits of device number and 3 bits of
function number), so incrementing by 8 is really a way of looking at
function 0 of each device number. I'm pretty sure this is based on
something in the spec that says a multi-function device must implement
function 0. Please look that up and include a reference in the
changelog so we have a more complete story here.
It's possible there are other assumptions in the code about
multi-function devices always having a function 0. It would take a
little more research to be certain that this wouldn't break anything.
As Lukas pointed out, it does increase the number of probe attempts by
a factor of 8. I don't know how much that will affect boot time, but
it's certainly something to consider and hopefully quantify.
> Signed-off-by: Benedikt Spranger <b.spran...@linutronix.de>
> Signed-off-by: Thomas Gleixner <t...@linutronix.de>
> ---
> arch/x86/pci/legacy.c | 2 +-
> drivers/pci/probe.c | 2 +-
> 2 files changed, 2 insertions(+), 2 deletions(-)
>
> --- a/arch/x86/pci/legacy.c
> +++ b/arch/x86/pci/legacy.c
> @@ -42,7 +42,7 @@ void pcibios_scan_specific_bus(int busn)
> if (pci_find_bus(0, busn))
> return;
>
> - for (devfn = 0; devfn < 256; devfn += 8) {
> + for (devfn = 0; devfn < 256; devfn++) {
> if (!raw_pci_read(0, busn, devfn, PCI_VENDOR_ID, 2, &l) &&
> l != 0x0000 && l != 0xffff) {
> DBG("Found device at %02x:%02x [%04x]\n", busn, devfn,
> l);
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -2063,7 +2063,7 @@ unsigned int pci_scan_child_bus(struct p
> dev_dbg(&bus->dev, "scanning bus\n");
>
> /* Go find them, Rover! */
> - for (devfn = 0; devfn < 0x100; devfn += 8)
> + for (devfn = 0; devfn < 0x100; devfn++)
> pci_scan_slot(bus, devfn);
>
> /* Reserve buses for SR-IOV capability. */
> --
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