This configuration option had no help text, so add it.

CONFIG_EXPERT is enabled on some distribution kernels, so people using a
distribution kernel's configuration as a starting point will see this
option.

Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: x86@kernel.org
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>

---
I would like someone to confirm that PCI Express is required for
MMCONFIG support.
---
 arch/x86/Kconfig | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 31c2c574c67d..fdfe8b7e895a 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2712,6 +2712,19 @@ config PCI_MMCONFIG
 	default y
 	depends on PCI && (ACPI || JAILHOUSE_GUEST)
 	depends on X86_64 || (PCI_GOANY || PCI_GOMMCONFIG)
+	help
+	  Add support for accessing the PCI configuration space as a memory
+	  mapped area. It is the recommended method if the system supports
+	  this (it must have PCI Express and ACPI for it to be available).
+
+	  In the unlikely case that enabling this configuration option causes
+	  problems, the mechanism can be switched off with the 'pci=nommconf'
+	  command line parameter.
+
+	  Say 'n' only if you are sure that your platform does not support this
+	  access method or you have problems caused by it.
+
+	  Say 'y' otherwise.
 
 config PCI_OLPC
 	def_bool y
-- 
2.25.1