The backlight class does all the locking needed for sysfs access, but
offers no API to interface to that locking without an layer violation.
Since we need to mutex-lock procfs access, implement in-driver locking for
brightness. It will go away the day thinkpad-acpi procfs goes away, or the
backlight class gives us a way to use its locks without a layer violation.
Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>
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