On 19 February 2012 14:31, Yiannis Yiakoumis <gyiakoumis@xxxxxxxxx> wrote: > 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? I'll have to do some digging. I recall reading about this at some stage but I can't for the life of me remember the details. The first couple of bits of each DCU D_MISC register controls when the DCU starts arbitration for the PCU. The PCU is what does the DMA. That _may_ begin the process of doing frame DMA early? As I said, I'd hav eto check. > So, does DMA take place on a per packet basis? If so, what does start_tx_dma do? The driver just tickles the hardware to say "start DMA when you're ready." It just enables the DMA to occur at some point in the future. May reasons may stop the DMA from occuring at exactly the time you flip on the QCU TxE bit - for example, you may flip on the TX DMA when the unit is currently RX'ing, so there's no way the frame will go out immediately. Or it's a beacon queue, and although you flip on TxE, the QCU won't allow the frame to go out until the relevant beacon interval DMA timer fires. The QCU has a variety of frame scheduling policies - ASAP, CBR, DBA-gated (ie, the beacon gating), TIM gated, beacon-sent gated. So no, tickling "start DMA" doesn't necessarily start DMA. > 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? I can't really say. I'd guess it's a mix - ie, the hardware is likely free to fill the TX FIFO as it needs to. It may decide to DMA the next frame whilst the current frame is going out. I'm not sure of all the hairy MAC DMA details just yet. > When all 10 packets have been through tx_dma I start getting > interrupts. The first is a TXEOL and the rest TXDESC. That's because the MAC can be told to fire off certain kinds of interrupts, based on: * the contents of the AR_ISR and AR_ISR_S* registers - you can request that you be notified of TXDESC, TXEOL, TXURN, etc. * the contents of the per-descriptor "interrupt me!" bit. > 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? It won't be _greater_ than that (unless it failed - ie, it didn't go out at all.) But, it is likely less than that. Poking the MAC and saying "DMA me!" is not as accurate a starting point as you'd like it to be. You don't AFAIK get notified by the hardware when the DMA _occurs_, only that it's completed the transfer, and you are provided a TSF snapshot in the TX completion part of the TX descriptor. I can't recall whether it's the TSF value at the head or the end of the (successful) packet transmission. But that value is there. Why don't you ask the list for help about what it is you're exactly trying to achieve? :-) Adrian -- 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
