Re: [PATCH/RFT] iwlagn: fix RX skb alignment

[reinette chatre <reinette.chatre@intel.com>]

Johannes,

On Mon, 2008-11-17 at 16:47 -0800, Johannes Berg wrote:
> [patch below, sorry for the long list of CCs]
> 
> 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: Johannes Berg <johannes@sipsolutions.net>
> ---

Above you mention that this patch fixes bug #11596. Could you please
explain how this patch addresses that problem? A short summary of the
bug is that no data (ping packets) can be transmitted while station is
associated. We have, what appears to be, a similar bug in our bug
tracking system
(http://www.intellinuxwireless.org/bugzilla/show_bug.cgi?id=1807 ), but
this patch was not able to resolve this issue.

Bug #11596 was recently closed after the user tested with 2.6.27.4,
which does not have this patch. 

Thank you

Reinette