[PATCH] b43: A fix for DMA transmission sequence errors

[Michael Büsch]

On Sat, 23 Mar 2013 00:27:30 +0100
Rafał Miłecki <zajec5 at gmail.com> wrote:

> Today I've plugged my 14e4:4315 and (unfortunately?) it's working
> pretty well. I hoped to reproduce some problems but failed to do so. I
> was transmitting for an hour with average speed 11MiB/s and didn't
> notice any DMA issues.
> 
> I was using iperf with interval of 60 seconds and only 3 results
> showed some problems (8.5MiB/s, 2.5MiB/s, 4.5MiB/s). No disconnections
> however and no DMA errors. I just got "Group rekeying completed..." in
> wpa_supplicant.
> 
> So as I can't reproduce this, I can't find any other fix for this
> issue, and there's no reason to stop this workaround. I'll just apply
> it and test over weekend to check for any regressions, but they are
> highly unlikely.

I don't really believe in this being a firmware bug.

Some b43 DMA engines (all?) have some alignment and page-boundary-crossing
constraints. I would rather guess that on some kernels with some options
turned on, alignment and/or boundary constraints are violated every
now and then. (and thus the packet never reaches the firmware).

I don't remember the details, though. Too long since I worked on that.
But a few sanity checks could probably be added to the code to check
this hypothesis.

Does the failing kernel/machine have any special things w.r.t. memory?
Like iommu, hugepages, whetever...

-- 
Michael
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