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RISC-V: handle DTB aligned to less than 2MB

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By default OpenSBI and BBL will pass the DTB at a 2MB-aligned address.
However, by default there are no 2MB aligned regions between the SBI and
the kernel, so we have to choose a 2MB aligned region after the kernel.
OpenSBI defaults to placing the DTB 32MB after the start of the kernel but
this is not sufficient for a kernel with a large MFS embedded.
We could increase this offset to a larger number (e.g. 64/128/256) but that
imposes restrictions on the minimum RAM size.
Another solution would be to place the DTB between OpenSBI and the kernel
at 1MB alignment, but current locore.S code assumes 2MB alignment.

With this change I can now boot on QEMU with an OpenSBI configured to
store the DTB at an offset of 1MB.

See also https://github.com/riscv/opensbi/issues/169

Reviewed By:	mhorne
Differential Revision: https://reviews.freebsd.org/D25151
This commit is contained in:
Alex Richardson 2020-06-08 08:51:52 +00:00
parent 0e89af3b88
commit c714d79726
1 changed files with 6 additions and 0 deletions
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6
sys/riscv/riscv/locore.S
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@ -139,6 +139,8 @@ _start:
lla s1, pagetable_l2_devmap
mv s2, a1
srli s2, s2, PAGE_SHIFT
/* Mask off any bits that aren't aligned */
andi s2, s2, ~((1 << (PTE_PPN1_S - PTE_PPN0_S)) - 1)
li t0, (PTE_KERN)
slli t2, s2, PTE_PPN0_S /* << PTE_PPN0_S */
@ -214,6 +216,10 @@ va:
sd t0, RISCV_BOOTPARAMS_KERN_STACK(sp)
li t0, (VM_EARLY_DTB_ADDRESS)
/* Add offset of DTB within superpage */
li t1, (L2_OFFSET)
and t1, a1, t1
add t0, t0, t1
sd t0, RISCV_BOOTPARAMS_DTBP_VIRT(sp)
mv a0, sp