app: add CONFIG_SOF_OS_LINUX_COMPAT_PRIORITY

Add a new Kconfig option to select strict Linux compatibility mode
for SOF build. This is enabled when building with the Linux OS
overlay (app/os_linux_overlay.conf).

SOF is used in environments, where it is easier to force Linux
version upgrade, and in environments (like the typical Linux
desktop distribution), where kernel and SOF firwmare are integrated
separately, and it is common place for end user to run custom
kernels.

This option allows to cater for both type of usages. For Intel
platforms, we intend to use the strict compability approach
also in the future, for the SOF firmware binaries we release
via sof-bin.

Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>

Author: kv2019i

Kconfig.sof

@@ -47,6 +47,14 @@ config FAST_GET
 	  counting. Source is src/lib/fast-get.c. The option should be selected
 	  on platforms, where __cold_rodata is supported.
 
+config SOF_OS_LINUX_COMPAT_PRIORITY
+	bool "Prioritize backwards compatibility for old Linux kernels"
+	help
+	  Build option to maintain maximal backwards compatibility with old
+	  versions of Linux SOF driver. When this is not set, firmware may
+	  require newer version of host, and/or use features that are not
+	  available in stable Linux kernel trees.
+
 rsource "Kconfig.xtos"
 
 rsource "src/Kconfig"

app/os_linux_overlay.conf

@@ -3,6 +3,12 @@
 # with SOF driver in Linux kernel
 #
 
+# As a general rule, update of SOF firmware shall never
+# break existing systems by depending on Linux kernel patches
+# that have been upstreamed after commercial availability
+# of a product.
+CONFIG_SOF_OS_LINUX_COMPAT_PRIORITY=y
+
 # SOF Linux driver does not require FW to retain its
 # state, so context save can be disabled
 CONFIG_ADSP_IMR_CONTEXT_SAVE=n