[Intel-wired-lan] fm10k driver - flow control question

[Keller, Jacob E]

> -----Original Message-----
> From: Intel-wired-lan [mailto:intel-wired-lan-bounces at osuosl.org] On Behalf Of
> Keller, Jacob E
> Sent: Thursday, October 11, 2018 9:31 AM
> To: Jags N <jagsnn at gmail.com>; Wyborny, Carolyn <carolyn.wyborny at intel.com>
> Cc: intel-wired-lan at osuosl.org
> Subject: Re: [Intel-wired-lan] fm10k driver - flow control question
> 
> Hi,
> 
> > -----Original Message-----
> > From: Jags N [mailto:jagsnn at gmail.com]
> > Sent: Wednesday, October 10, 2018 10:50 PM
> > To: Wyborny, Carolyn <carolyn.wyborny at intel.com>
> > Cc: Alexander Duyck <alexander.duyck at gmail.com>; intel-wired-lan at osuosl.org;
> > Keller, Jacob E <jacob.e.keller at intel.com>
> > Subject: Re: [Intel-wired-lan] fm10k driver - flow control question
> >
> > Thanks Alex, Carolyn.
> >
> > Hi Jake,
> >
> > The question I have is on fm10k pcie host interface to control host congestion,
> >
> > "PCIE_RXDCTL.DropOnEmpty" if not set, can hold the packet till
> > "PCIE_DMA_CTRL.MaxHoldTime" expires, after which the queue is set to DROP to
> > avoid future time out on this queue.
> >
> > "PCIE_DMA_CTRL.MaxHoldTime" is defined as log2() of maximum waiting time for
> > descriptors to become available.The maximum time is 2^^31 * PCLK.
> >
> 
> Yes.
> 
> > With PCLK at 500 MHz for Gen3.0, is the following calculation and understanding
> > correct,
> > (2^^31 * PCLK) means, 2^^31 * 2ns = 4294967296 ns = 4 sec, is the maximum hold
> > time that the back pressure on the internal switch will be present after which the
> > queue is set to Drop.
> 
> The 4 second time is correct.
> 
> > Kindly advice.
> 
> However, I do not know how this backpressures (if any) to the switch. The RRC has an
> internal cache of descriptors for which it will hold packets. Eventually these will fill up,
> even as it waits for Rx descriptors from the host interface software memory. I do not
> know if the switch internals actually do any "back pressuring" here.
> 
> I'm going to ask somoene internally who knows the switch part better than I do. I
> hope to have an answer.
> 
> >
> > Regards,
> > Jagdish
> >

To follow up on this. I talked to one of the engineers who knows more about the switch hardware. Basically, there is no method to flow control or equivalent between the PEP (host interface) and the switch.

Thanks,
Jake