During PCH (platform/board) manufacturing process a global platform
reset has to be induced in order for the configuration changes take
the effect upon following platform reset. This is an internal platform
state and is not intended to be used in the regular platform resets.
The setting is exposed via ETR3 (Extended Test Mode Register 3).
After the manufacturing process is completed the register cannot be
written anymore and is hardware locked.
This setting was commonly done by accessing PMC registers via /dev/mem
but due to security concerns /dev/mem access is much more restricted,
hence the reason for exposing this setting via the dedicated sysfs
interface.
To prevent post manufacturing abuse the register is protected
by hardware locking and the file is set to read-only mode via is_visible
handler.

The register in MMIO space is defined for Cannon Lake and newer PCHs.

Cc: Hans de Goede <hdegoede@redhat.com>
Cc: David E Box <david.e.box@intel.com>
Reviewed-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: NTamar Mashiah <tamar.mashiah@intel.com>
Signed-off-by: NTomas Winkler <tomas.winkler@intel.com>
Link: https://lore.kernel.org/r/20210411141532.3004893-1-tomas.winkler@intel.comReviewed-by: NHans de Goede <hdegoede@redhat.com>
Signed-off-by: NHans de Goede <hdegoede@redhat.com>