[PATCH] MIPS: Bcm47xx: Fix 128MB RAM support

[PATCH] MIPS: Bcm47xx: Fix 128MB RAM support

[Hauke Mehrtens]

Ignoring the last page when ddr size is 128M. Cached
accesses to last page is causing the processor to prefetch
using address above 128M stepping out of the ddr address
space.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
---
 arch/mips/bcm47xx/prom.c |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/arch/mips/bcm47xx/prom.c b/arch/mips/bcm47xx/prom.c
index 5efc995..0fa646c 100644
--- a/arch/mips/bcm47xx/prom.c
+++ b/arch/mips/bcm47xx/prom.c
@@ -141,6 +141,14 @@ static __init void prom_init_mem(void)
 			break;
 	}
 
+	/* Ignoring the last page when ddr size is 128M. Cached
+	 * accesses to last page is causing the processor to prefetch
+	 * using address above 128M stepping out of the ddr address
+	 * space.
+	 */
+	if (mem == 0x8000000)
+		mem -= 0x1000;
+
 	add_memory_region(0, mem, BOOT_MEM_RAM);
 }
 
-- 
1.6.3.3

Re: [PATCH] MIPS: Bcm47xx: Fix 128MB RAM support

[Ralf Baechle]

On Sat, Feb 20, 2010 at 07:51:20PM +0100, Hauke Mehrtens wrote:

> Ignoring the last page when ddr size is 128M. Cached
> accesses to last page is causing the processor to prefetch
> using address above 128M stepping out of the ddr address
> space.

Is this a hardware issue prefetching issue?  The kernel should not try
CPU prefetch instructions at all on non-coherent CPUs such as the
BCM47xx.

  Ralf

Re: [PATCH] MIPS: Bcm47xx: Fix 128MB RAM support

[Hauke Mehrtens]

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Ralf Baechle wrote:
> On Sat, Feb 20, 2010 at 07:51:20PM +0100, Hauke Mehrtens wrote:
> 
>> Ignoring the last page when ddr size is 128M. Cached
>> accesses to last page is causing the processor to prefetch
>> using address above 128M stepping out of the ddr address
>> space.
> 
> Is this a hardware issue prefetching issue?  The kernel should not try
> CPU prefetch instructions at all on non-coherent CPUs such as the
> BCM47xx.

This is a hardware issue on the bcm47xx when 128MB ram is present. This
workaround is out of broadcom's kernel sources and is included in
OpenWRT for some months. Without this patch the kernel does not even
print out anything and with this patch it is working.

Hauke


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Re: [PATCH] MIPS: Bcm47xx: Fix 128MB RAM support

[Ralf Baechle]

On Mon, Feb 22, 2010 at 09:25:11PM +0100, Hauke Mehrtens wrote:

> > Is this a hardware issue prefetching issue?  The kernel should not try
> > CPU prefetch instructions at all on non-coherent CPUs such as the
> > BCM47xx.
> 
> This is a hardware issue on the bcm47xx when 128MB ram is present. This
> workaround is out of broadcom's kernel sources and is included in
> OpenWRT for some months. Without this patch the kernel does not even
> print out anything and with this patch it is working.

Thanks!

I was asking to ensure this isn't a workaround for an already solved
kernel software issue.  Patch applied,

  Ralf