linux-kernel - Re: [PATCH v1 05/10] powerpc/mm: Do early ioremaps from top to bottom on PPC64 too.

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Date:   Tue, 13 Aug 2019 23:14:20 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Christoph Hellwig <hch@...radead.org>,
        linux-kernel@...r.kernel.org, npiggin@...il.com,
        Paul Mackerras <paulus@...ba.org>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v1 05/10] powerpc/mm: Do early ioremaps from top to
 bottom on PPC64 too.

On Wed, Aug 14, 2019 at 08:10:59AM +0200, Christophe Leroy wrote:
> > Note that while a few other architectures have a magic hack like powerpc
> > to make ioremap work before vmalloc, the normal practice would be
> > to explicitly use early_ioremap.  I guess your change is fine for now,
> > but it might make sense convert powerpc to the explicit early_ioremap
> > scheme as well.
> > 
> 
> I've been looking into early_ioremap(), but IIUC early_ioremap() is for
> ephemeral mappings only, it expects all early mappings to be gone at the end
> of init.

Yes.

> PPC installs definitive early mappings (for instance for PCI). How does that
> have to be handled ?

Good question, and no good answer.  I've just been looking at a generic
ioremap for simple architectures, and been finding all kinds of crap
and inconsistencies, and this is one of the things I noticed.