PCI: Provide Kconfig option for lockless config space accessors

The generic PCI configuration space accessors are globally serialized via
pci_lock. On larger systems this causes massive lock contention when the
configuration space has to be accessed frequently. One such access pattern
is the Intel Uncore performance counter unit.

Provide a kernel config option which can be selected by an architecture
when the low level PCI configuration space accessors in the architecture
use their own serialization or can operate completely lockless.

Signed-off-by: Thomas Gleixner <[e-mail]>
Acked-by: Bjorn Helgaas <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Stephane Eranian <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Thomas Gleixner <[e-mail]>

drivers/pci/Kconfig

···
 config PCI_ECAM
 	bool
 
+config PCI_LOCKLESS_CONFIG
+	bool
+
 config PCI_IOV
 	bool "PCI IOV support"
 	depends on PCI

drivers/pci/access.c

···
 #define PCI_word_BAD (pos & 1)
 #define PCI_dword_BAD (pos & 3)
 
+#ifdef CONFIG_PCI_LOCKLESS_CONFIG
+# define pci_lock_config(f)	do { (void)(f); } while (0)
+# define pci_unlock_config(f)	do { (void)(f); } while (0)
+#else
+# define pci_lock_config(f)	raw_spin_lock_irqsave(&pci_lock, f)
+# define pci_unlock_config(f)	raw_spin_unlock_irqrestore(&pci_lock, f)
+#endif
+
 #define PCI_OP_READ(size, type, len) \
 int pci_bus_read_config_##size \
 	(struct pci_bus *bus, unsigned int devfn, int pos, type *value)	\
···
 	unsigned long flags;						\
 	u32 data = 0;							\
 	if (PCI_##size##_BAD) return PCIBIOS_BAD_REGISTER_NUMBER;	\
-	raw_spin_lock_irqsave(&pci_lock, flags);			\
+	pci_lock_config(flags);						\
 	res = bus->ops->read(bus, devfn, pos, len, &data);		\
 	*value = (type)data;						\
-	raw_spin_unlock_irqrestore(&pci_lock, flags);		\
+	pci_unlock_config(flags);					\
 	return res;							\
 }
 
···
 	int res;							\
 	unsigned long flags;						\
 	if (PCI_##size##_BAD) return PCIBIOS_BAD_REGISTER_NUMBER;	\
-	raw_spin_lock_irqsave(&pci_lock, flags);			\
+	pci_lock_config(flags);						\
 	res = bus->ops->write(bus, devfn, pos, len, value);		\
-	raw_spin_unlock_irqrestore(&pci_lock, flags);		\
+	pci_unlock_config(flags);					\
 	return res;							\
 }