em network issues

[Scott Long]

Jack Vogel wrote:
> On 10/25/06, Doug Ambrisko <ambrisko at ambrisko.com> wrote:
> 
>>      3) In em_process_receive_interrupts/em_rxeof always decrement
>>         the count on every run through the loop.  If you notice
>>         count is an is an int that starts at the passed in value
>>         of -1.  It then count-- until count==0.  Doing -1, -2, -3
>>         takes awhile until the int rolls over to 0.   Passing 100
>>         limits it more :-)  So this can run 3 * 100 versuses
>>         infinite * int roll over assuming we don't skip a count--.
> 
> Been chatting with Jesse Brandeburg (one of our senior Linux guys) about
> receive side cleaning. Gave me a number of things to think about. First 
> off,
> this change you mention is problematic. The reason it doesnt increment
> every time thru the while loop is its meant as a packet counter, NOT a
> descriptor counter. If we just fix this number at 100, and have it only
> counting descriptors you could get all but the EOP descriptor of a packet
> and then exit without finishing it and calling the stack, not a good 
> tactic.
> 
> Having a limited count is still a good idea, but I think we still want 
> to base
> it on packets and not just descriptors.
> 
> Jesse also talked about their experience with the Linux driver, deciding
> where to update the RDT, my current code doesnt do it til after the whole
> while loop is completed (havent looked at CURRENT again today yet),
> Jesse suggested doing it but not EVERY pass in the loop, maybe making
> it mod the number of rx descriptors. Having that AND a fixed limit on the
> number of total packets cleaned in a pass might be good.

Good idea.  Though for 1518 byte frames, I think you'll only have one 
descriptor per packet.  Definitely need to do the right thing for jumbo
frames, though.

> 
> I was also thinking, maybe having the taskqueue code be selectable, but
> not just a POLL vs TASKQUEUE, rather keep a legacy intr option which
> has a POLL option within it.
> 
> My motivation for doing that is I can take the TASKQUEUE code into the
> Intel code base, but make it backward compatible, the default would have
> it optioned off.
> 
> Jack

I had it that way initially, and I think you commented that it was ugly
;-)