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Date: Tue, 9 Aug 2016 14:13:20 +0200
From: Lukas Wunner <lukas@wunner.de>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
	Bjorn Helgaas <bhelgaas@google.com>, linux-pci@vger.kernel.org,
	Jan Kiszka <jan.kiszka@siemens.com>,
	Benedikt Spranger <b.spranger@linutronix.de>
Subject: Re: x86/PCI: Scan all functions during probing
Message-ID: <20160809121320.GA6518@wunner.de>
References: <alpine.DEB.2.20.1608091318530.5388@nanos>
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On Tue, Aug 09, 2016 at 01:22:30PM +0200, Thomas Gleixner wrote:
> From: Benedikt Spranger <b.spranger@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.

It does have the side effect that probing (and thus booting) is prolonged.
Depending on how much that is, it may be worth pondering if usage of the
smaller stride should be constrained to platforms that really need it
(assuming they can be detected/quirked).

Just claiming "has no side effects" is going out on a limb I think.

Thanks,

Lukas

> 
> Signed-off-by: Benedikt Spranger <b.spranger@linutronix.de>
> Signed-off-by: Thomas Gleixner <tglx@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. */