risc-v/mpfs: clear L2 before use

SiFive document: "ECC Error Handling Guide" states:

"Any SRAM block or cache memory containing ECC functionality needs to be
initialized prior to use. ECC will correct defective bits based on memory
contents, so if memory is not first initialized to a known state, then ECC
will not operate as expected. It is recommended to use a DMA, if available,
to write the entire SRAM or cache to zeros prior to enabling ECC reporting.
If no DMA is present, use store instructions issued from the processor."

Clean the cache at this early stage so no ECC errors will be flooding later.

Signed-off-by: Eero Nurkkala <eero.nurkkala@offcode.fi>

arch/risc-v/src/mpfs/mpfs_head.S

@@ -166,6 +166,16 @@ __start:
 
 .continue_boot:
 
+  /* L2 needs to be zeroed before ECC (error correction) is enabled later. */
+
+  la  a4, __l2lim_start
+  la  a5, __l2lim_end
+
+.clear_l2lim:
+  sd   x0, 0(a4)
+  add  a4, a4, 8
+  blt  a4, a5, .clear_l2lim
+
   /* Clear PMP */
 
   csrw pmpcfg0, zero

boards/risc-v/mpfs/icicle/scripts/ld-envm-opensbi.script

@@ -32,6 +32,9 @@ ENTRY(_stext)
 EXTERN(__start)
 SECTIONS
 {
+    PROVIDE(__l2lim_start = ORIGIN(l2lim));
+    PROVIDE(__l2lim_end   = ORIGIN(l2lim) + LENGTH(l2lim));
+
     .text.sbi : {
       */sbi*
       */riscv_atomic*

boards/risc-v/mpfs/icicle/scripts/ld-envm.script

@@ -35,6 +35,9 @@ ENTRY(_stext)
 EXTERN(__start)
 SECTIONS
 {
+    PROVIDE(__l2lim_start = ORIGIN(lim));
+    PROVIDE(__l2lim_end   = ORIGIN(lim) + LENGTH(lim));
+
     .text : {
         _stext = ABSOLUTE(.);
         *(.start .start.*)