PCI: add kernel parameter to override devid<->driver mapping.

Scanning a lot of devices during boot requires a lot of time. On other scenarios there is a need to bind a driver to a specific slot.

Binding devices to pci-stub driver does not work, as it will not differentiate between devices of the same type. Using some start scripts is error prone.

The solution leverages driver_override functionality introduced by

	commit: 782a985d7af26db39e86070d28f987cad21313c0
	Author: Alex Williamson <[email protected]>
	Date:   Tue May 20 08:53:21 2014 -0600

    	PCI: Introduce new device binding path using pci_dev.driver_override

In order to bind PCI slots to specific drivers use:

	pci=driver[xxxx:xx:xx.x]=foo,driver[xxxx:xx:xx.x]=bar,...

---
v1 -> v2:
- Addressed Michael S. Tsirkin comments
- Removed 32 slots limitation
- Better handling of memory allocation failures (preferred BUG_ON over error messages)
- Addressed Alex Williamson's comments:
- Modified commit message to show parameter usage more clear.
- I preferred to re-use parse_args instead of manually using strstr in order to better comply with command line parsing rules.
- I didn't use any locking when parsing the command line args (see parse_done usage) assuming that first call will be early in system boot and no race can occur. Please correct me if I am wrong.

Notes:
- I have further ideas on top of this patch based on your reviews. I thought of:
- Use wildcards to specify entire buses/devices, something like:

     	driver[0001:02:*.*]=pci-stub
   - Use comma to separate several devices:
     	driver[0001:02:03.4,0001:02:04.0,...]=pci-stub

- Make domain optional:
driver[00:03.0]=pci-stub

Comments will be appreciated,
Thanks,
Marcel

 Documentation/kernel-parameters.txt |  4 ++
 drivers/pci/bus.c                   | 86 +++++++++++++++++++++++++++++++++++++
 drivers/pci/pci.c                   |  2 +
 3 files changed, 92 insertions(+)

index 5ae8608..c1cbb4c 100644
@@ -2631,6 +2631,10 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 		pcie_scan_all	Scan all possible PCIe devices.  Otherwise we
 				only look for one device below a PCIe downstream
 				port.
+		driver		Provide an override to the devid<->driver mapping
+				for a specific slot.
+				Bind PCI slot 0001:02:03.4 to pci-stub by:
+					driver[0001:02:03.4]=pci-stub
 
 	pcie_aspm=	[PCIE] Forcibly enable or disable PCIe Active State Power
 			Management.

index b7678be..6bd7844 100644
@@ -4468,6 +4468,8 @@ static int __init pci_setup(char *str)
 				pcie_bus_config = PCIE_BUS_PEER2PEER;
 			} else if (!strncmp(str, "pcie_scan_all", 13)) {
 				pci_add_flags(PCI_SCAN_ALL_PCIE_DEVS);
+			} else if (!strncmp(str, "driver", 6)) {
+				/* lazy evaluation by the pci subsystem */
 			} else {
 				printk(KERN_ERR "PCI: Unknown option `%s'\n",
 						str);

1.8.3.1