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From: Michael Ellerman <mpe@ellerman.id.au>
To: Joel Savitz <jsavitz@redhat.com>, linux-kernel@vger.kernel.org
Cc: Joel Savitz <jsavitz@redhat.com>, Nicholas Piggin <npiggin@gmail.com>,
 Christophe Leroy <christophe.leroy@csgroup.eu>, "Aneesh Kumar K.V"
 <aneesh.kumar@kernel.org>, "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
 Benjamin Gray <bgray@linux.ibm.com>, Paul Mackerras <paulus@ozlabs.org>,
 linuxppc-dev@lists.ozlabs.org, Gonzalo Siero <gsierohu@redhat.com>
Subject: Re: [PATCH] powerpc: align memory_limit to 16MB in early_parse_mem
In-Reply-To: <20240301203023.2197451-1-jsavitz@redhat.com>
References: <20240301203023.2197451-1-jsavitz@redhat.com>
Date: Sat, 02 Mar 2024 10:23:36 +1100
Message-ID: <87cysdfsef.fsf@mail.lhotse>
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Hi Joel,

Joel Savitz <jsavitz@redhat.com> writes:
> On 64-bit powerpc, usage of a non-16MB-aligned value for the mem= kernel
> cmdline parameter results in a system hang at boot.

Can you give us any more details on that? It might be a bug we can fix.

> For example, using 'mem=4198400K' will always reproduce this issue.
>
> This patch fixes the problem by aligning any argument to mem= to 16MB
> corresponding with the large page size on powerpc.

The large page size depends on the MMU, with Radix it's 2MB or 1GB. So
depending on what's happening 16MB may not be enough.

What system are you testing on?

cheers

> Fixes: 2babf5c2ec2f ("[PATCH] powerpc: Unify mem= handling")
> Co-developed-by: Gonzalo Siero <gsierohu@redhat.com>
> Signed-off-by: Gonzalo Siero <gsierohu@redhat.com>
> Signed-off-by: Joel Savitz <jsavitz@redhat.com>
> ---
>  arch/powerpc/kernel/prom.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/kernel/prom.c b/arch/powerpc/kernel/prom.c
> index 0b5878c3125b..8cd3e2445d8a 100644
> --- a/arch/powerpc/kernel/prom.c
> +++ b/arch/powerpc/kernel/prom.c
> @@ -82,8 +82,12 @@ static int __init early_parse_mem(char *p)
>  {
>  	if (!p)
>  		return 1;
> -
> +#ifdef CONFIG_PPC64
> +	/* Align to 16 MB == size of ppc64 large page */
> +	memory_limit = ALIGN(memparse(p, &p), 0x1000000);
> +#else
>  	memory_limit = PAGE_ALIGN(memparse(p, &p));
> +#endif
>  	DBG("memory limit = 0x%llx\n", memory_limit);
>  
>  	return 0;
> -- 
> 2.43.0