Hi Adrian, Thanks for the explanation. I am a bit confused though. You are saying that the DMA starts when there is a txop, is this correct? So, does DMA take place on a per packet basis? If so, what does start_tx_dma do? To trace things, I am sending a 10-packet train through a raw socket. All packets go to the tx queue (length reaches 11) and then I start receiving TX interrupts. Does each DMA request gets completed before the next come, or tx_dma just posts the requests and these are processed later? When all 10 packets have been through tx_dma I start getting interrupts. The first is a TXEOL and the rest TXDESC. My understanding is that the time between the tx_dma and the TXDESC of a packet, captures both DMA and wireless transmission - could somebody comment on that? Thanks, Yiannis On Tue, Feb 14, 2012 at 9:46 PM, Adrian Chadd <adrian@xxxxxxxxxxx> wrote: > Hi, > > The way it works is pretty straightforward (from the surface.) > > * The radio signals whether the air is free or busy; > * The QCU handles the frame queuing; > * The DCU handles arbitrating between different TX queues, based on > whether the NIC is currently RXing or whether the air is busy; > * There's a bunch of timers in each DCU to implement > contention/backoff (see AR_D_* registers in the FreeBSD/madwifi HAL, I > don't know what they're called for ath5k.) > * The DCU then gates the QCU and kicks off DMA. > > So the hardware is what decides when to trigger the TX DMA. In fact, > for later NICs, they may start DMAing the frame _just before_ the > IFS/backoff counters reach 0, just in case it wins the opportunity to > talk over the air. > > You just setup everything to allow the NIC to begin TX DMA when it's > ready to send things. :-) > > HTH, > > > Adrian > > > On 14 February 2012 12:06, Yiannis Yiakoumis <gyiakoumis@xxxxxxxxx> wrote: >> Hi, >> >> I am reading the code and trying to understand what forces the packet >> out of the card, or to be more clear, what runs CSMA/CA and when >> detects a tx opportunity pushes the packet down to the phy. I can >> track the packet all the way until it gets to start_tx_dma(), but I am >> not clear what is the actual call that sends it out, and how this >> relates with QCU (I am on a 5212). I suspect that it's within the >> tasklet_schedule but wasn't able to find the details. >> Also, does QCU get involved on a packet-per-packet basis, or does it >> just set the initial values to the device's registers? >> >> Any help is much appreciated.. >> >> Thanks, >> -- >> Yiannis . >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-wireless" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html -- Yiannis . -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
