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      iwlagn: fix RX skb alignment · 4018517a
      Johannes Berg 提交于
      So I dug deeper into the DMA problems I had with iwlagn and a kind soul
      helped me in that he said something about pci-e alignment and mentioned
      the iwl_rx_allocate function to check for crossing 4KB boundaries. Since
      there's 8KB A-MPDU support, crossing 4k boundaries didn't seem like
      something the device would fail with, but when I looked into the
      function for a minute anyway I stumbled over this little gem:
      
      	BUG_ON(rxb->dma_addr & (~DMA_BIT_MASK(36) & 0xff));
      
      Clearly, that is a totally bogus check, one would hope the compiler
      removes it entirely. (Think about it)
      
      After fixing it, I obviously ran into it, nothing guarantees the
      alignment the way you want it,  because of the way skbs and their
      headroom are allocated. I won't explain that here nor double-check that
      I'm right, that goes beyond what most of the CC'ed people care about.
      
      So then I came up with the patch below, and so far my system has
      survived minutes with 64K pages, when it would previously fail in
      seconds. And I haven't seen a single instance of the TX bug either. But
      when you see the patch it'll be pretty obvious to you why.
      
      This should fix the following reported kernel bugs:
      
      http://bugzilla.kernel.org/show_bug.cgi?id=11596
      http://bugzilla.kernel.org/show_bug.cgi?id=11393
      http://bugzilla.kernel.org/show_bug.cgi?id=11983
      
      I haven't checked if there are any elsewhere, but I suppose RHBZ will
      have a few instances too...
      
      I'd like to ask anyone who is CC'ed (those are people I know ran into
      the bug) to try this patch.
      
      I am convinced that this patch is correct in spirit, but I haven't
      understood why, for example, there are so many unmap calls. I'm not
      entirely convinced that this is the only bug leading to the TX reply
      errors.
      Signed-off-by: NJohannes Berg <johannes@sipsolutions.net>
      Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>
      4018517a