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Date:   Thu, 6 Jun 2019 13:43:25 +0200
From:   Christoph Hellwig <hch@....de>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Aaro Koskinen <aaro.koskinen@....fi>,
        Christoph Hellwig <hch@....de>,
        Christian Zigotzky <chzigotzky@...osoft.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Larry Finger <Larry.Finger@...inger.net>,
        linuxppc-dev@...ts.ozlabs.org, linux-wireless@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [BISECTED REGRESSION] b43legacy broken on G4 PowerBook

On Thu, Jun 06, 2019 at 08:57:49PM +1000, Benjamin Herrenschmidt wrote:
> > Wow... that's an odd amount. One thing we could possibly do is add code
> > to limit the amount of RAM when we detect that device....
> 
> Sent too quickly... I mean that *or* force swiotlb at 30-bits on those systems based
> on detecting the presence of that device in the device-tree.

swiotlb doesn't really help you, as these days swiotlb on matters for
the dma_map* case.  What would help is a ZONE_DMA that covers these
devices.  No need to do the 24-bit x86 does, but 30-bit would do it.

WIP patch for testing below:

diff --git a/arch/powerpc/include/asm/page.h b/arch/powerpc/include/asm/page.h
index b8286a2013b4..7a367ce87c41 100644
--- a/arch/powerpc/include/asm/page.h
+++ b/arch/powerpc/include/asm/page.h
@@ -319,6 +319,10 @@ struct vm_area_struct;
 #endif /* __ASSEMBLY__ */
 #include <asm/slice.h>
 
+#if 1 /* XXX: pmac?  dynamic discovery? */
+#define ARCH_ZONE_DMA_BITS 30
+#else
 #define ARCH_ZONE_DMA_BITS 31
+#endif
 
 #endif /* _ASM_POWERPC_PAGE_H */
diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
index cba29131bccc..2540d3b2588c 100644
--- a/arch/powerpc/mm/mem.c
+++ b/arch/powerpc/mm/mem.c
@@ -248,7 +248,8 @@ void __init paging_init(void)
 	       (long int)((top_of_ram - total_ram) >> 20));
 
 #ifdef CONFIG_ZONE_DMA
-	max_zone_pfns[ZONE_DMA]	= min(max_low_pfn, 0x7fffffffUL >> PAGE_SHIFT);
+	max_zone_pfns[ZONE_DMA]	= min(max_low_pfn,
+			((1UL << ARCH_ZONE_DMA_BITS) - 1) >> PAGE_SHIFT);
 #endif
 	max_zone_pfns[ZONE_NORMAL] = max_low_pfn;
 #ifdef CONFIG_HIGHMEM

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