Re: [PATCH v2 8/8] rt2x00: Properly request tx headroom for alignment operations.

[Ivo van Doorn <ivdoorn@gmail.com>]

On Tuesday 24 November 2009, Gertjan van Wingerde wrote:
> On 11/24/09 18:19, Ivo van Doorn wrote:
> > On Monday 23 November 2009, Gertjan van Wingerde wrote:
> >> Current rt2x00 drivers may result in a "ieee80211_tx_status: headroom too
> >> small" error message when a frame needs to be properly aligned before
> >> transmitting it.
> >> This is because the space needed to ensure proper alignment isn't
> >> requested from mac80211.
> >> Fix this by adding sufficient amount of alignment space to the amount
> >> of headroom requested for TX frames.
> >>
> >> Reported-by: David Ellingsworth <david@identd.dyndns.org>
> >> Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
> >> ---
> >>  drivers/net/wireless/rt2x00/rt2x00.h    |    6 ++++++
> >>  drivers/net/wireless/rt2x00/rt2x00dev.c |   12 +++++++++++-
> >>  2 files changed, 17 insertions(+), 1 deletions(-)
> >>
> >> diff --git a/drivers/net/wireless/rt2x00/rt2x00.h b/drivers/net/wireless/rt2x00/rt2x00.h
> >> index 4d841c0..dcfc8c2 100644
> >> --- a/drivers/net/wireless/rt2x00/rt2x00.h
> >> +++ b/drivers/net/wireless/rt2x00/rt2x00.h
> >> @@ -113,6 +113,12 @@
> >>  	(  ((unsigned long)((__skb)->data + (__header))) & 3 )
> >>  
> >>  /*
> >> + * Constants for extra TX headroom for alignment purposes.
> >> + */
> >> +#define RT2X00_ALIGN_SIZE	4 /* Only whole frame needs alignment */
> >> +#define RT2X00_L2PAD_SIZE	8 /* Both header & payload need alignment */
> > 
> > Now that I think of it, why do we need 8 byte for L2 padding?
> > Shouldn't we need the same size for regular alignment, L2 padding only indicates
> > the padding occurs between header and payload rather then before the header only.
> > So unless I am mistaken about how I coded the header & payload moving in rt2x00lib,
> > you always need at most 4 bytes.
> > 
> 
> Nope. If I read the code right then the header can be moved by more than 4 bytes, namely
> when the payload needs to be shifted more than the header. In that case the header is
> shifted an extra 4 bytes to make room for the payload. However, 8 is the obvious maximum.

Ah right, well 8bytes is ok then. :)

Acked-by: Ivo van Doorn <IvDoorn@gmail.com>