Re: [dpdk-dev] [PATCH v4] ixgbe_pmd: enforce RS bit on every EOP descriptor for devices newer than 82598

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi lads,

> -----Original Message-----
> From: Vlad Zolotarov [mailto:vladz@cloudius-systems.com]
> Sent: Tuesday, October 27, 2015 6:48 PM
> To: Thomas Monjalon; Ananyev, Konstantin; Zhang, Helin
> Cc: dev@dpdk.org; Kirsher, Jeffrey T; Brandeburg, Jesse
> Subject: Re: [dpdk-dev] [PATCH v4] ixgbe_pmd: enforce RS bit on every EOP descriptor for devices newer than 82598
> 
> 
> 
> On 10/27/15 20:09, Thomas Monjalon wrote:
> > Any Follow-up to this discussion?
> > Should we mark this patch as rejected?
> 
> Hmmm... This patch fixes an obvious spec violation. Why would it be
> rejected?

No I don't think we can reject the patch:
There is a reproducible  TX hang on ixgbe PMD on described conditions.
Though, as I explained here:
http://dpdk.org/ml/archives/dev/2015-September/023574.html
Vlad's patch would cause quite a big slowdown.
We are still in the process to get an answer from HW guys are there any
alternatives that will allow to fix the problem and avoid the slowdown.
Konstantin  

> 
> >
> > 2015-08-24 11:11, Vlad Zolotarov:
> >> On 08/20/15 18:37, Vlad Zolotarov wrote:
> >>> According to 82599 and x540 HW specifications RS bit *must* be
> >>> set in the last descriptor of *every* packet.
> >>>
> >>> Before this patch there were 3 types of Tx callbacks that were setting
> >>> RS bit every tx_rs_thresh descriptors. This patch introduces a set of
> >>> new callbacks, one for each type mentioned above, that will set the RS
> >>> bit in every EOP descriptor.
> >>>
> >>> ixgbe_set_tx_function() will set the appropriate Tx callback according
> >>> to the device family.
> >> [+Jesse and Jeff]
> >>
> >> I've started to look at the i40e PMD and it has the same RS bit
> >> deferring logic
> >> as ixgbe PMD has (surprise, surprise!.. ;)). To recall, i40e PMD uses a
> >> descriptor write-back
> >> completion mode.
> >>
> >>   From the HW Spec it's unclear if RS bit should be set on *every* descriptor
> >> with EOP bit. However I noticed that Linux driver, before it moved to
> >> HEAD write-back mode, was setting RS
> >> bit on every EOP descriptor.
> >>
> >> So, here is a question to Intel guys: could u, pls., clarify this point?
> >>
> >> Thanks in advance,
> >> vlad
> >
> >