Hi,

On Tue, Jun 4, 2013 at 8:37 PM, Oleksij Rempel <li...@rempel-privat.de> wrote:
> This error seems to be really rare, and we do not know real couse of it.
> But, in any case, we should check size of head before reducing it.

We had a similar issue in rt2x00 quite some time ago.

In general mac80211 should always reserve enough headroom as requested by
the driver in hw->extra_tx_headroom. However, there is a chance that a frame is
send to the driver again (see ieee80211_handle_filtered_frame). But if the frame
payload (or head) was moved due to padding and was not restored before calling
ieee80211_tx_status by the driver the second trip through the driver has reduced
headroom and could lead to such an error.

Quickly checking ath9k_htc it seems as if ath9k_htc_tx adds some padding
but ath9k_htc_tx_process does not remove the padding when passing the frame
back to mac80211.

Helmut
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