linux-kernel - Re: [PATCH] powerpc/40x: Map 32Mbytes of memory at startup

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Date:   Tue, 28 Sep 2021 02:28:02 +0200
From:   cp <carlojpisani@...il.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc/40x: Map 32Mbytes of memory at startup

> I guess we're not expecting systems with less than 32MB, so making it
> unconditional is OK?

hi
it's not 32MB of total physical ram, but rather using 32MB of the
total physical ram to correctly boot a kernel bigger than 8MB.

The problem is complex:
- u-boot loads things at 0x0200.0000 (in my case)
- then cuboot copies the kernel to 0x0090.0000, and appends the flat
device tree to the end of kernel
- then cuboot copies again the kernel to 0x0000.0000, and invokes head.S

at this point head.S needs to initialize the virtual memory, and needs
to have 32MB mapped in order to correctly gets the flat device tree

if the kernel is smaller than 8MB, then cuboot copies it to
0x0080.0000, and head.S only need to have 16MB mapped in order to gets
the flat device tree

Cheers