Re: [PATCH V2 4/6] powerpc/44x: don't use tlbivax on AMP systems

[Josh Boyer <jwboyer@linux.vnet.ibm.com>]

On Tue, Feb 01, 2011 at 12:48:44PM -0600, Dave Kleikamp wrote:
>diff --git a/arch/powerpc/mm/tlb_nohash.c b/arch/powerpc/mm/tlb_nohash.c
>index 2a030d8..b33c5e6 100644
>--- a/arch/powerpc/mm/tlb_nohash.c
>+++ b/arch/powerpc/mm/tlb_nohash.c
>@@ -35,6 +35,7 @@
> #include <linux/preempt.h>
> #include <linux/spinlock.h>
> #include <linux/memblock.h>
>+#include <linux/of_fdt.h>
>
> #include <asm/tlbflush.h>
> #include <asm/tlb.h>
>@@ -153,6 +154,17 @@ EXPORT_SYMBOL(local_flush_tlb_page);
>  */
> #ifdef CONFIG_SMP
>
>+static int amp;
>+
>+#ifdef CONFIG_44x
>+void __init early_init_mmu_44x(void)
>+{
>+	unsigned long root = of_get_flat_dt_root();
>+	if (of_flat_dt_is_compatible(root, "ibm,47x-AMP"))
>+		amp = 1;
>+}
>+#endif /* CONFIG_44x */

Something aside from the property thing sits weirdly with me on this as
well.

We have this guarded by CONFIG_44x but also CONFIG_SMP, and we're doing
476 specific checks (for now).  There is at least one 44x board that has
dual-CPUs (AMCC Arches, iirc) that can theoretically be run in AMP mode.
However, it won't be using an SMP kernel because it's a single core per CPU.
Admittedly I don't think it supports the tlbivax instruction either so
the patch as it stands doesn't impact that theoretical scenario much.

I do wonder if we really need to guard the call to this behind
CONFIG_SMP though.  Maybe a slight performance increase I suppose, but
if we wind up using the AMP check elsewhere then it might be needed
anyway.  Something to think about.

Oh, and I agree 'cooperative-partition' or something would be a better
check.

josh