On Thu, 2011-04-21 at 00:34 -0700, Lennert Buytenhek wrote: > On Tue, Apr 19, 2011 at 06:36:41PM +0530, Nishant Sarmukadam wrote: > > > > > > > Timestamp tx packets using a HW micro-second timer. > > > > > > This timestamp will be compared to the current timestamp > > > > > > in the hardware and if the difference is greater than 500ms, > > > > > > the packet will be dropped. > > > > > > > > > > Why don't you do this entirely in the firmware? > > > > > > > > > > When a packet is put into the TX ring by the host, give it a timestamp > > > > > of zero. > > > > > > > > > > When the firmware gets a TX interrupt, have it walk the TX ring from > > > > > the last TX entry where a timestamp was written to (i.e. keep a separate > > > > > index for this), and timestamp all TX ring entries that are marked as > > > > > being FW OWNED but don't have a timestamp yet (and update your index). > > > > > > > > > > Then when the time comes to process the TX packet (i.e. it has reached > > > > > the head of the TX queue), compare the timestamp with the current time. > > > > > > > > Since firmware cannot access host memory directly, it needs additional > > > > dma operations to read all descriptors, then check the ownership bit and > > > > update the timestamp for the descriptors. It can easily take about 1us > > > > per descriptor for this and it can hit the peak throughput by about > > > > 2.5%. > > > > > > That DMA needs to happen anyway, whether you have the host timestamp > > > the packets or not, so this doesn't seem to be an applicable argument. > > > > Currently, firmware downloads a small fixed number of descriptors at a > > given time from any tx queue as it does not have memory to download all > > the descriptors from a host queue. > > > > Assume that the total tx buffers available in firmware is X and host tx > > queue size is 2X and that the host queue is completely backlogged. In > > this case, the firmware needs to download the descriptors from the head > > of the host queue to transmit corresponding packets and it needs to > > download the descriptors at the tail of the host queue to timestamp > > them. But, firmware cannot transmit the packets associated with > > descriptors from the tail since there are packets in host queue which > > were queued earlier to these packets. This effectively results in each > > descriptor being downloaded twice - once for timestamp and once for > > transmission. > > Point taken, but if the firmware has N buffers in its H/W TX ring, > does the host really need to have 2N buffers in its F/W TX ring? Yes, we have confirmed this and 2N buffers are needed in the host for peak throughput. > > There is another issue in that mwl8k H/W reports that a packet has been > transmitted OK even before the packet is attempted to be transmitted > (i.e. firmware marks the packet as transmitted as soon as the DMA for > the packet data completes), which sounds like a good candidate for > fixing at the same time. > Since, this issue is generic and needs to be addressed irrespective of the current patch set, we prefer handling this in a separate patch. > > > This will significantly affect the peak throughput performance. > > Does that mean that you've implemented this and tried it out? :-) Yes, we have recently done some profiling experiments in the firmware to confirm this. -- 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
