PCI: Default PCIe ASPM control to on and require !EMBEDDED to disable

The CONFIG_PCIEASPM option is confusing and potentially dangerous. ASPM is
a hardware mediated feature rather than one under direct OS control, and
even if the config option is disabled the system firmware may have turned
on ASPM on various bits of hardware. This can cause problems later -
various hardware that claims to support ASPM does a poor job of it and may
hang or cause other difficulties. The kernel is able to recognise this in
many cases and disable the ASPM functionality, but only if CONFIG_PCIEASPM
is enabled.

Given that in its default configuration this option will either leave the
hardware as it was originally or disable hardware functionality that may
cause problems, it should by default y. The only reason to disable it
ought to be to reduce code size, so make it dependent on CONFIG_EMBEDDED.

Signed-off-by: Matthew Garrett <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Jesse Barnes <[email protected]>

Author: Matthew Garrett

drivers/pci/pcie/Kconfig

@@ -31,14 +31,22 @@ source "drivers/pci/pcie/aer/Kconfig"
 # PCI Express ASPM
 #
 config PCIEASPM
-	bool "PCI Express ASPM support(Experimental)"
-	depends on PCI && EXPERIMENTAL && PCIEPORTBUS
-	default n
+	bool "PCI Express ASPM control" if EMBEDDED
+	depends on PCI && PCIEPORTBUS
+	default y
 	help
-	  This enables PCI Express ASPM (Active State Power Management) and
-	  Clock Power Management. ASPM supports state L0/L0s/L1.
+	  This enables OS control over PCI Express ASPM (Active State
+	  Power Management) and Clock Power Management. ASPM supports
+	  state L0/L0s/L1.
 
-	  When in doubt, say N.
+	  ASPM is initially set up the the firmware. With this option enabled,
+	  Linux can modify this state in order to disable ASPM on known-bad
+	  hardware or configurations and enable it when known-safe.
+
+	  ASPM can be disabled or enabled at runtime via
+	  /sys/module/pcie_aspm/parameters/policy
+
+	  When in doubt, say Y.
 config PCIEASPM_DEBUG
 	bool "Debug PCI Express ASPM"
 	depends on PCIEASPM