x86/iommu: Clean up the CONFIG_GART_IOMMU config option a bit

Improve the explanation of this config option.

Cc: Andi Kleen <ak@linux.intel.com>
Cc: Borislav Petkov <borislav.petkov@amd.com>
Link: http://lkml.kernel.org/n/tip-gUMmysvsbl3mccbyf6olmxqg@git.kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/Kconfig

@@ -760,15 +760,21 @@ config GART_IOMMU
 	select SWIOTLB
 	depends on X86_64 && PCI && AMD_NB
 	---help---
-	  Support for full DMA access of devices with 32bit memory access only
-	  on systems with more than 3GB. This is usually needed for USB,
-	  sound, many IDE/SATA chipsets and some other devices.
-	  Provides a driver for the older AMD Athlon64/Opteron/Turion/Sempron GART
-	  based hardware IOMMU.
-	  Newer systems typically have a better AMD IOMMU.
-	  The code is only active when needed (enough memory and limited
-	  device) unless CONFIG_IOMMU_DEBUG or iommu=force is specified
-	  too.
+	  Provides a driver for older AMD Athlon64/Opteron/Turion/Sempron
+	  GART based hardware IOMMUs.
+
+	  The GART supports full DMA access for devices with 32-bit access
+	  limitations, on systems with more than 3 GB. This is usually needed
+	  for USB, sound, many IDE/SATA chipsets and some other devices.
+
+	  Newer systems typically have a modern AMD IOMMU, supported via
+	  the CONFIG_AMD_IOMMU=y config option.
+
+	  In normal configurations this driver is only active when needed:
+	  there's more than 3 GB of memory and the system contains a
+	  32-bit limited device.
+
+	  If unsure, say Y.
 
 config CALGARY_IOMMU
 	bool "IBM Calgary IOMMU support"