Re: [rt2x00-users] [PATCH v3 0/4] Further L2 padding fixes.

[Benoit PAPILLAULT <benoit.papillault@free.fr>]

Gertjan van Wingerde a écrit :
> On 12/01/09 00:46, Benoit PAPILLAULT wrote:
>   
>> Gertjan van Wingerde a écrit :
>>     
>>> The L2 padding fixes patch has grown a bit and now consists of 4 separate
>>> patches to clean the L2 padding code up and to fix a number of bugs at the
>>> same time.
>>>
>>> 	1. rt2x00: Further L2 padding fixes.
>>> 	2. rt2x00: Remove SKBDESC_L2_PADDED flag.
>>> 	3. rt2x00: Reorganize L2 padding inserting function.
>>> 	4. rt2x00: Only remove L2 padding in received frames if there is payload.
>>>
>>> ---
>>> Gertjan.
>>>       
>> Thanks Gertjan for the patches, I know that padding is a bit of
>> nightware. I've testing your tree and here are the results:
>>
>> 1. On TX : It fails for control frames with hdrlen=10 since it will
>> produce l2pad = 2 in your case, where it should be 0. In all other
>> cases, it works!
>>
>>     
>
> Hmm. That's strange, as patch 3 of the series should have handled this. Basically this patch
> checks if a frame actually has payload, and refrains from inserting l2pad and payload alignment
> in that case.
>
> How did you detect this failure?
>   
I sent every possible frame type in monitor and compared with what ath9k 
in monitor mode received.
> What did you observe on the "wire", i.e. how was the frame malformed because of this?
>   
For instance, i sent :
c400c400c40000000000 (10 bytes)

and I received :
c400ca00c400000000000000 (12 bytes)

You can see the 2 bytes added at the end (bytes 2-3 are modified by the 
HW since it contains the duration_id)
> Also, would it be possible to get some of the skb details of a frame that fails?
>   
I'll redo the test. What details do you want?
>
>   
>> Solution : padding is only needed for data frames for rt28x devices, so
>> I think it's better to something like rt2xqueue_padpos
>> [http://git.popipo.fr/?p=rt2x00.git;a=commitdiff;h=95ddf076c13062d1026025d97ba511f880a1792d]
>>
>>     
>
> I'm still not fond of using detailed ieee 802.11 frame format knowledge to do this. I prefer
> to use standard mac80211 functions with a bit of detection on the actual skb.
>   

If you want to use mac80211 functions, we can do something like that : 
/*
 * Determine the L2 padding size based on the 802.11 header length
 *
 * Exact formula is (4 - (header_length%4))%4 but since header_length is
 * always a multiple of 2, we could simplify to (header_length&3).
 *
 * L2PAD is only present for data frame and an easy way to check for that is
 * to compare header_length with 24 bytes.
 */
#define L2PAD_SIZE(__header) \
       ((__header)<24 ? 0 : ((4 - ((__header)%4))%4))

Regards,
Benoit