Re: [ath5k-devel] [support] ath5k contention windows

[Jonathan Guerin <jonathan@guerin.id.au>]

On Wed, Sep 15, 2010 at 10:58 AM, Bruno Randolf <br1@einfach.org> wrote:
> On Wed September 15 2010 08:07:36 Jonathan Guerin wrote:
>> I'm capturing using a third-party station. I've had to use a Madwifi
>> station, as ath5k throws a lot of spurious timestamps up, which make
>> calculating this very, very difficult.
>
> ah, that's interesting and something we should check, too...
>
>> Actually, I've found that they were exactly one slot time negative
>> (give or take 1us). That said, I was doing these calculations using
>> the 802.11a OFDM spec timings. I looked at the driver and realised
>> that SIFS is actually being initialised to 14us, whereas it should be
>> set to 16us. This will also affect DIFS. Once I take these into
>> consideration, they come up as approximately 4-5us out. Now that I
>> think about it, it could make sense.
>
> hmm, we have a function ath5k_hw_get_default_sifs() which returns 16 for
> 802.11a, but it's not used for setting the IFS...
>
> also i think current ath5k_hw_reset_tx_queue() is buggy and sets the AIFS
> wrongly. it uses ah->ah_aifs + tq->tqi_aifs which is 2 + AR5K_TXQ_USEDEFAULT
> which results in 2 + (-1). that does not make sense to me and i will post a
> patch fixing up some of that confusion (but not the SIFS issue) today.

I'm getting these values from: initval.c
/* Initial mode-specific settings for AR5212 (Written before ar5212_ini) */
static const struct ath5k_ini_mode ar5212_ini_mode_start[] = {
<...>
	{ AR5K_DCU_GBL_IFS_SIFS,
	/*	a/XR	   aTurbo	  b	   g (DYN)     gTurbo     */
	   { 0x00000230, 0x000001e0, 0x000000b0, 0x00000160, 0x000001e0 } },
	{ AR5K_DCU_GBL_IFS_SLOT,
	   { 0x00000168, 0x000001e0, 0x000001b8, 0x0000018c, 0x000001e0 } },
<...>

The IFS_SLOT time is correct:
0x168/0x28 (clock multiplier) = 9us

SIFS is coming up short:
ox230/0x28 = 14us

I assume that the card calculates the DIFS from these two values, as
described in Section 9.2.10 DCF timing relations in the spec?

By the way, I may be completely wrong about some of these things, I'm
just going off the understanding I have regarding the register values.

>
> bruno
>

Thanks for looking into this!

Jonathan