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Date:   Sat, 2 Dec 2023 11:45:44 +0000
From:   Conor Dooley <conor@kernel.org>
To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        "open list:SIFIVE DRIVERS" <linux-riscv@lists.infradead.org>,
        Alexandre Ghiti <alex@ghiti.fr>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Question about BUILTIN_DTB support in RISCV
Message-ID: <20231202-paparazzi-thievish-744c40e351be@spud>
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On Sat, Dec 02, 2023 at 03:45:00AM +0900, Masahiro Yamada wrote:
> Hi.
>=20
> I have a question about CONFIG_BUILTIN_DTB for riscv.
>=20
> Please see this commit history.
>=20
>=20
> [1]
> 2d2682512f0faf4d09a696184bf3c0bb6838baca
> added built-in DTB support, attempting
> to include multiple DTB into vmlinux
> by using SOC_BUILTIN_DTB_DECLARE() macro.
>=20
>=20
> [2]
> d5805af9fe9ffe4a9d975e9bc39496f57a161076
> pointed out that choosing the correct DTB
> is impossible. It fell back to the single
> built-in DTB support, like other architectures.
>=20
> [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,
=66rom 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?
>=20
>=20
> With
> CONFIG_ARCH_MICROCHIP_POLARFIRE=3Dy
> CONFIG_ARCH_SIFIVE=3Dy
> CONFIG_BUILTIN_DTB=3Dy
>=20
> 7 DTB files are embedded in vmlinux.
>=20
>=20
> masahiro@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
>=20
>=20
> In my understanding, the first one
> (mpfs-icicle-kit.dtb) is always used.
>=20
> 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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