This linker voodoo clones the Earlycon drivers from the kernel into
the decompressor, selecting everything in the designated ELF sections
(.text.earlycon.info and .init.earlycon.info).

My linker script skills are quite primitive so the code is manually relocated
by the decompressor using information left by the kernel linker script.

So we have some constrains for the Earlycom drivers:

 - no global or static variables (mind the strings)
 - no dynamic memory allocation (but the decompressor reserves 1K of RAM
   for the driver)
 - no function calls outside of .text.earlycon.info and the restricted
   namespace provided by struct earlycon_drv
 - calls to normal functions in .text.earlycon.info must go through manual
   relocation (see RELOC_FUN() later on), inline functions do not need
   this extra step
 - drivers shall not bloat, all this stuff is stored uncompressed together
   with the decompressor and friends
 
Signed-off-by: Domenico Andreoli <domenico.andreoli@linux.com>

---
 arch/arm/boot/Makefile                  |    6 +++++-
 arch/arm/boot/compressed/vmlinux.lds.in |   16 +++++++++++++++-
 arch/arm/kernel/vmlinux.lds.S           |   10 ++++++++++
 3 files changed, 30 insertions(+), 2 deletions(-)

Index: b/arch/arm/boot/Makefile
===================================================================
--- a/arch/arm/boot/Makefile
+++ b/arch/arm/boot/Makefile
@@ -48,7 +48,11 @@ $(obj)/Image: vmlinux FORCE
 	$(call if_changed,objcopy)
 	@echo '  Kernel: $@ is ready'
 
-$(obj)/compressed/vmlinux: $(obj)/Image FORCE
+OBJCOPYFLAGS_earlycon.o = -O elf32-littlearm -j .init.earlycon.info -j .text.earlycon.info -j .ARM.attributes
+$(obj)/compressed/earlycon.o: vmlinux FORCE
+	$(call if_changed,objcopy)
+
+$(obj)/compressed/vmlinux: $(obj)/Image $(obj)/compressed/earlycon.o FORCE
 	$(Q)$(MAKE) $(build)=$(obj)/compressed $@
 
 $(obj)/zImage:	$(obj)/compressed/vmlinux FORCE
Index: b/arch/arm/kernel/vmlinux.lds.S
===================================================================
--- a/arch/arm/kernel/vmlinux.lds.S
+++ b/arch/arm/kernel/vmlinux.lds.S
@@ -111,6 +111,10 @@ SECTIONS
 		*(.got)			/* Global offset table		*/
 			ARM_CPU_KEEP(PROC_INFO)
 	}
+	.text.earlycon.info : {
+		__earlycon_text_begin = .;
+		*(.earlycon.info.text)
+	}
 
 	RO_DATA(PAGE_SIZE)
 
@@ -150,6 +154,12 @@ SECTIONS
 		*(.arch.info.init)
 		__arch_info_end = .;
 	}
+	.init.earlycon.info : {
+		LONG(__earlycon_text_begin)
+		__earlycon_desc_begin = .;
+		*(.earlycon.info.desc)
+		__earlycon_desc_end = .;
+	}
 	.init.tagtable : {
 		__tagtable_begin = .;
 		*(.taglist.init)
Index: b/arch/arm/boot/compressed/vmlinux.lds.in
===================================================================
--- a/arch/arm/boot/compressed/vmlinux.lds.in
+++ b/arch/arm/boot/compressed/vmlinux.lds.in
@@ -30,6 +30,9 @@ SECTIONS
     _start = .;
     *(.start)
     *(.text)
+    . = ALIGN(4);
+    __earlycon_text_begin = .;
+    *(.text.earlycon.info)
     *(.text.*)
     *(.fixup)
     *(.gnu.warning)
@@ -38,6 +41,11 @@ SECTIONS
   }
   .rodata : {
     *(.rodata)
+    . = ALIGN(4);
+    __earlycon_text_base = .;
+    __earlycon_desc_begin = . + 4;
+    *(.init.earlycon.info)
+    __earlycon_desc_end = .;
     *(.rodata.*)
   }
   .piggydata : {
@@ -59,7 +67,13 @@ SECTIONS
 
   . = BSS_START;
   __bss_start = .;
-  .bss			: { *(.bss) }
+  .bss : {
+    *(.bss)
+    . = ALIGN(4);
+    __earlycon_drvdata_begin = .;
+    . = . + 1K;
+    __earlycon_drvdata_end = .;
+  }
   _end = .;
 
   . = ALIGN(8);		/* the stack must be 64-bit aligned */