Re: [PATCH v2 2/2] ethdev: add new offload flag to keep CRC

From: Thomas Monjalon <thomas@monjalon.net>
To: Shahaf Shuler <shahafs@mellanox.com>
Cc: Ferruh Yigit <ferruh.yigit@intel.com>,
	Neil Horman <nhorman@tuxdriver.com>,
	John McNamara <john.mcnamara@intel.com>,
	Marko Kovacevic <marko.kovacevic@intel.com>,
	"dev@dpdk.org" <dev@dpdk.org>
Subject: Re: [PATCH v2 2/2] ethdev: add new offload flag to keep CRC
Date: Thu, 29 Mar 2018 09:43:12 +0200	[thread overview]
Message-ID: <15555561.lU3UqzoiBB@xps> (raw)
In-Reply-To: <DB7PR05MB4426A99C0E4907BBF0E24761C3A20@DB7PR05MB4426.eurprd05.prod.outlook.com>

29/03/2018 07:38, Shahaf Shuler:
> Wednesday, March 21, 2018 9:48 PM, Ferruh Yigit:
> > DEV_RX_OFFLOAD_KEEP_CRC offload flag added.
> > 
> > DEV_RX_OFFLOAD_CRC_STRIP flag will remain one more release but default
> > behavior in PMDs is to strip the CRC independent from this flag.
> > 
> > Until DEV_RX_OFFLOAD_CRC_STRIP flag is removed:
> > - Setting both KEEP_CRC & CRC_STRIP is INVALID
> > - Setting only CRC_STRIP PMD should strip the CRC
> > - Setting only KEEP_CRC PMD should keep the CRC
> > - Not setting both PMD should strip the CRC
> 
> We cannot have such default behavior, it may break existing applications.
> 
> The API of ethdev offloads says (even though it is not well documented) :  DEV_RX_OFFLOAD_CRC_STRIP (emphasis the STRIP). 
> meaning, if set -> STRIP, if not set -> don't strip.  I am aware to at least one application which wants to have the CRC, and for that purpose it naturally don't set the offload flag. 
> 
> The fact some PMDs lack the ability to toggle the CRC stripping should be exposed in the "limitations" section of their related guide. 
> 
> Up to here, this is the behavior as defined by the API. 
> 
> Now, we want to change it, and I think it makes sense. However I think we should take similar approach to what we did with the ethdev offloads API:
> 1. at first stage a new offload flag is exposed DEV_RX_OFFLOAD_KEEP_CRC and implemented on the PMDs.

This is what is proposed above, except that we change the default behaviour.
If we introduce a new flag which is the contrary of the old one, we need
to choose which one is the default, so which one has no effect.

> 2. there is a conversion function on ethdev. Which for example converts ~DEV_RX_OFFLOAD_CRC_STRIP -> DEV_RX_OFFLOAD_KEEP_CRC for the PMDs.
> 3. deprecation of DEV_RX_OFFLOAD_CRC_STRIP for applications and remove of the conversion functions. 