kernel after 5.0-rc2 may not boot using BBL

[Atish Patra <atish.patra@wdc.com>]

If you are building a larger kernel image, it may not boot using BBL 
after 5.0-rc2. It works fine with OpenSBI though.

Here is the reasoning in case anybody else is facing the same issue.

The following patch was added 5.0-rc2 which helps reduce the flat image 
size for Linux kernel.

"22e6a2e1 RISC-V: Make BSS section as the last section in vmlinux.lds.S"

As the BSS section is moved to the end of the ELF, flat image will not 
include it. As BBL copies the DTB at the next 2MB page boundary, BSS
section may overlap with DTB address. Kernel clears BSS section during 
early boot which may clear the DTB as well leading to a boot failure.

OpenSBI always copies the DTB far ahead in the memory (0x82200000). 
That's why you won't see any issue with OpenSBI.

Here is a BBL fix that can solve this problem. I will submit a PR to 
riscv-pk.

diff --git a/bbl/bbl.c b/bbl/bbl.c
index 1b96a9d5..0d448d82 100644
--- a/bbl/bbl.c
+++ b/bbl/bbl.c
@@ -14,7 +14,7 @@ static uintptr_t dtb_output()
  {
    extern char _payload_end;
    uintptr_t end = (uintptr_t) &_payload_end;
-  return (end + MEGAPAGE_SIZE - 1) / MEGAPAGE_SIZE * MEGAPAGE_SIZE;
+  return (end + 16 * MEGAPAGE_SIZE - 1) / MEGAPAGE_SIZE * MEGAPAGE_SIZE;
  }

Thanks Anup for the explanation and above fix.


Regards,
Atish

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