Re: 8139_tx_interrupt - Tx time

[Zwane Mwaikambo]

On Fri, 10 Oct 2003, Adam Flizikowski wrote:

> Hi,
> 
> I am trying to find out how long it takes to push packet off the NIC in
> 8139_tx_interrupt(){} (kernel 2.4.20)
> 
> but i don't know how to identify packet in this loop - i know that "entry"
> stands for the packet but in the loop underneath:
> /* code snippet for 8139_tx_interrupt(){}*/
> 
> 1726         while (tx_left > 0) {
> 1727                 int entry = dirty_tx % NUM_TX_DESC;
> 1728                 int txstatus;
> 
> ...
> 
> 1762
> 1763                 dirty_tx++;
> 1764                 tx_left--;    // <- is single packet sent just
> here??????
> 1765         }
> 
> what "tx_left" and "dirty_tx" variables stand for? Is single iteration
> through the loop (lines 1726-1765) related to SINGLE packet transmission?

No, that simply checks tx status, the transmission is done elsewhere.

> I wanted to put some probes inside tx_interrupt but not sure where it should
> start/stop measuring to show single packet transmission time.

tx_interrupt is normally triggered on transmission completion, you 
probably want to sample from;

rtl8139_start_xmit {
...
	RTL_W32_F (TxStatus0 + (entry * sizeof (u32)),
		   tp->tx_flag | max(len, (unsigned int)ETH_ZLEN));
...
}

That is when the packet hits the hardware, you could keep per tx 
descriptor start/end timestamps with end being in tx_interrupt.

Hope that gets you somewhere.

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