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Subject: Re: [BISECTED REGRESSION] b43legacy broken on G4 PowerBook
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
 Christoph Hellwig <hch@lst.de>
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From: Larry Finger <Larry.Finger@lwfinger.net>
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Date: Tue, 11 Jun 2019 20:52:42 -0500
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On 6/11/19 5:46 PM, Benjamin Herrenschmidt wrote:
> On Tue, 2019-06-11 at 17:20 -0500, Larry Finger wrote:
>> b43-pci-bridge 0001:11:00.0: dma_direct_supported: failed (mask =
>> 0x3fffffff,
>> min_mask = 0x5ffff000/0x5ffff000, dma bits = 0x1f
> 
> Ugh ? A mask with holes in it ? That's very wrong... That min_mask is
> bogus.

I agree, but that is not likely serious as most systems will have enough memory 
that the max_pfn term will be much larger than the initial min_mask, and 
min_mask will be unchanged by the min function. In addition, min_mask is not 
used beyond this routine, and then only to decide if direct dma is supported. 
The following patch generates masks with no holes, but I cannot see that it is 
needed.

diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
index 2c2772e9702a..e3edd4f29e80 100644
--- a/kernel/dma/direct.c
+++ b/kernel/dma/direct.c
@@ -384,7 +384,8 @@ int dma_direct_supported(struct device *dev, u64 mask)
         else
                 min_mask = DMA_BIT_MASK(32);

-       min_mask = min_t(u64, min_mask, (max_pfn - 1) << PAGE_SHIFT);
+       min_mask = min_t(u64, min_mask, ((max_pfn - 1) << PAGE_SHIFT) |
+                                        DMA_BIT_MASK(PAGE_SHIFT));

         /*
          * This check needs to be against the actual bit mask value, so


Larry