Question about BUILTIN_DTB support in RISCV

[Conor Dooley]

On Sat, Dec 02, 2023 at 03:45:00AM +0900, Masahiro Yamada wrote:
> Hi.
> 
> I have a question about CONFIG_BUILTIN_DTB for riscv.
> 
> Please see this commit history.
> 
> 
> [1]
> 2d2682512f0faf4d09a696184bf3c0bb6838baca
> added built-in DTB support, attempting
> to include multiple DTB into vmlinux
> by using SOC_BUILTIN_DTB_DECLARE() macro.
> 
> 
> [2]
> d5805af9fe9ffe4a9d975e9bc39496f57a161076
> pointed out that choosing the correct DTB
> is impossible. It fell back to the single
> built-in DTB support, like other architectures.
> 
> [3]
> 0ddd7eaffa644baa78e247bbd220ab7195b1eed6
> added BUILTIN_DTB support for sifive and microchip,
> while apparently multiple DTBs are embedded into vmlinux.

The thread for this one is interesting:
https://lore.kernel.org/all/20210604120639.1447869-1-alex@ghiti.fr/

None of the other !canaan platforms actually support this, so it is only
for PolarFire and the SiFive boards that this is enabled. It sounds,
from your comments below, that this does not even work. If it doesn't, I
would be inclined to just delete it.

If XIP is the user, we want to deprecate XIP anyway, so pointing out
that this has not worked properly in a long time for PolarFire and
?ever? for the SiFive unmatched is another nail in the coffin for that.

My vote would be for reverting [3] and removing support for XIP :)

> So, how does it work?
> 
> 
> With
> CONFIG_ARCH_MICROCHIP_POLARFIRE=y
> CONFIG_ARCH_SIFIVE=y
> CONFIG_BUILTIN_DTB=y
> 
> 7 DTB files are embedded in vmlinux.
> 
> 
> masahiro at zoe:~/ref/linux(master)$ riscv64-linux-gnu-nm -n vmlinux |
> grep -A15 dtb_start
> ffffffff82112620 D __dtb_start
> ffffffff82115a2c D __dtb_mpfs_icicle_kit_end
> ffffffff82115a40 D __dtb_mpfs_m100pfsevp_begin
> ffffffff82118b3b D __dtb_mpfs_m100pfsevp_end
> ffffffff82118b40 D __dtb_mpfs_polarberry_begin
> ffffffff8211b9c2 D __dtb_mpfs_polarberry_end
> ffffffff8211b9e0 D __dtb_mpfs_sev_kit_begin
> ffffffff8211e7bb D __dtb_mpfs_sev_kit_end
> ffffffff8211e7c0 D __dtb_mpfs_tysom_m_begin
> ffffffff8212162e D __dtb_mpfs_tysom_m_end
> ffffffff82121640 D __dtb_hifive_unleashed_a00_begin
> ffffffff821236af D __dtb_hifive_unleashed_a00_end
> ffffffff821236c0 D __dtb_hifive_unmatched_a00_begin
> ffffffff8212621b D __dtb_hifive_unmatched_a00_end
> ffffffff82126220 D __dtb_end
> 
> 
> In my understanding, the first one
> (mpfs-icicle-kit.dtb) is always used.
> 
> You cannot use the other 6 DTBs.
> Am I missing something?

Alex? I gave a cursory test and it does in fact behave like this.

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