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

[Vlad Zolotarov]

On 10/27/15 21:10, Ananyev, Konstantin wrote:
> Hi lads,
>
>> -----Original Message-----
>> From: Vlad Zolotarov [mailto:vladz at cloudius-systems.com]
>> Sent: Tuesday, October 27, 2015 6:48 PM
>> To: Thomas Monjalon; Ananyev, Konstantin; Zhang, Helin
>> Cc: dev at 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.

+1

> 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
>>>