Documentation: Update 4.22 release notes with x86 .data section

Change-Id: I2d6d611df8930ad0c473489eacee9019cbdacb9e
Signed-off-by: Jeremy Compostella <[email protected]>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/78000
Reviewed-by: Bora Guvendik <[email protected]>
Reviewed-by: Martin L Roth <[email protected]>
Tested-by: build bot (Jenkins) <[email protected]>

Documentation/releases/coreboot-4.22-relnotes.md

@@ -19,6 +19,46 @@ Update this document with changes that should be in the release notes.
 * This section should have full descriptions and can or should have
   a link to the referenced commits.
 
+### x86: support .data section for pre-memory stages
+
+x86 pre-memory stages did not support the `.data` section and as a
+result developers were required to include runtime initialization code
+instead of relying on C global variable definition.
+
+Other platforms do not have that limitation. Hence, resolving it helps
+to align code and reduce compilation based restriction (cf. the use of
+`ENV_HAS_DATA_SECTION` compilation flag in various places of coreboot
+code).
+
+There were three types of binary to consider:
+1. eXecute-In-Place pre-memory stages
+2. `bootblock` stage is a bit different as it uses Cache-As-Ram but
+   the memory mapping and its entry code different
+3. pre-memory stages loaded in and executed from Cache-As-RAM
+   (cf. `CONFIG_NO_XIP_EARLY_STAGES`).
+
+eXecute-In-Place pre-memory stages (#1) relies on a new ELF segment as
+the code segment Virtual Memory Address and Load Memory Address are
+identical but the data needs to be linked in cache-As-RAM (VMA) but to
+be stored right after the code (LMA).
+
+`bootblock` (#2) also uses this new segment to store the data right
+after the code and it loads it to Cache-As-RAM at runtime. However,
+the code involved is different.
+
+Not eXecute-In-Place pre-memory stages (#3) did not really need any
+special work other than enabling a .data section as the code and data
+VMA / LMA translation vector is the same.
+
+Related important commits:
+
+- c9cae530e5ac54c5b3639d0d555966ca5cad65ff ("cbfstool: Make add-stage
+  support multiple ignore sections")
+- 79f2e1fc8b6192e96f99c05f71baeb77d4633d40 ("cbfstool: Make add-stage
+  support multiple loadable segments")
+- b7832de0260b042c25bf8f53abcb32e20a29ae9c ("x86: Add .data section
+  support for pre-memory stages")
+
 ### Toolchain updates
 
 * Upgrade GMP from 6.2.1 to 6.3.0