linux-kernel - Re: [PATCH v5] powerpc/powernv: wire up rng during setup

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Date:   Wed, 22 Jun 2022 12:27:20 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        LKML <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH v5] powerpc/powernv: wire up rng during setup_arch

Christophe Leroy <christophe.leroy@...roup.eu> writes:
> Le 21/06/2022 à 20:47, Jason A. Donenfeld a écrit :
>> On Tue, Jun 21, 2022 at 06:33:11PM +0000, Christophe Leroy wrote:
>>> Le 21/06/2022 à 16:08, Jason A. Donenfeld a écrit :
>>>> The platform's RNG must be available before random_init() in order to be
>>>> useful for initial seeding, which in turn means that it needs to be
>>>> called from setup_arch(), rather than from an init call. Fortunately,
>>>> each platform already has a setup_arch function pointer, which means we
>>>> can wire it up that way. Complicating things, however, is that POWER8
>>>> systems need some per-cpu state and kmalloc, which isn't available at
>>>> this stage. So we split things up into an early phase and a later
>>>> opportunistic phase. This commit also removes some noisy log messages
>>>> that don't add much.
>>>
>>> Regarding the kmalloc(), I have not looked at it in details, but usually
>>> you can use memblock_alloc() when kmalloc is not available yet.
>> 
>> That seems a bit excessive, especially as those allocations are long
>> lived. And we don't even *need* it that early, but just before
>> random_init(). Michael is running this v5 on the test rig overnight, so
>> we'll learn in the Australian morning whether this finally did the trick
>> (I hope).
>
> The fact that they are long lived make them a good candidate for 
> memblock_alloc().
>
> But fair enough, if they are not required that early then just do it later.

memblock works but then we trip on ioremap vs early_ioremap.

Fixing that is a bit of a pain as we'd have to stop using of_iomap() and
we'd also need to switch the mappings to ioremap() later in boot.

We'd also have to defer the percpu initialisation.

So it's all just a bit of a pain when we actually only need to get the
hook ready before random_init() which is called much later in boot when
slab/ioremap/percpu are all ready.

cheers