Re: [BISECTED REGRESSION] b43legacy broken on G4 PowerBook

From: Larry Finger
Date: Thu Jun 06 2019 - 15:30:55 EST

On 6/6/19 6:43 AM, Christoph Hellwig wrote:
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
#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));
- max_zone_pfns[ZONE_DMA] = min(max_low_pfn, 0x7fffffffUL >> PAGE_SHIFT);
+ max_zone_pfns[ZONE_DMA] = min(max_low_pfn,
max_zone_pfns[ZONE_NORMAL] = max_low_pfn;

I am generating a test kernel with this patch.

FYI, the "free" command on my machine shows 1.5+ G of memory. That likely means I have 2G installed.

I have tested a patched kernel in which b43legacy falls back to a 31-bit DMA mask when the 32-bit one failed. That worked, but would likely kill the x86 version. Let me know if think a fix in the driver rather than the kernel would be better. I still need to understand why the same setup works in b43 and fails in b43legacy. :(