Re: [dpdk-dev] [Question about 'rte_eth_tx_prepare']

[Olivier Matz <olivier.matz@6wind.com>]

Hi,

On Mon, Feb 08, 2021 at 09:07:39AM +0100, Thomas Monjalon wrote:
> 08/02/2021 07:29, Chengchang Tang:
> > Hi, Thomas Monjalon and Ferruh Yigit and others.
> > 
> > I have some questions about 'rte_eth_tx_prepare'.
> > 
> > When I use TSO offload in bond mode, the checksum error occurs. It is
> > because the bond PMD does not implement 'tx_prepare'. So, it will not
> > invoke the 'tx_prepare' of each PMDs to do prepare for the PMDs. I am
> > not sure whether to add the 'tx_preapre' implementation for the bond
> > PMD or put the process of pseudo header in the apps.
> > 
> > And we are now designing the outer UDP cksum offload for HNS3 PMD.
> > I find that many PMDs process these pseudo headers in 'tx_prepare', but
> > does not process the pseudo header for outer UDP checksum offload.
> > Instead, it is processed in csum forward mode of testpmd. Does this mean
> > that the pseudo header should be completed by the apps and the apps does
> > not need to call 'tx_prepare' to avoid repeated processing? (it seems
> > not transplantable) If so, it seems that PMDs need to avoid doing this
> > in 'tx_prepare'.
> > 
> > Here are two questions:
> > 1. What functions should be included in the 'tx_prepare' for PMDs?
> > 2. Whether an app must invoke 'rte_eth_tx_prepare' or under which
> > conditions an app must invoke the 'rte_eth_tx_prepare'?
> 
> I would say by default the app should prepare the checksums,
> except if there is an explicit offload request (DEV_TX_OFFLOAD_*).
> I think the tx_prepare should only prepare the HW Tx offload
> if the offload is not entirely done in HW.

On our side, we call tx_prepare() on a tx bulk if:

1/ a Tx offload is enabled on the port (DEV_TX_OFFLOAD_*)
2/ at least one mbuf in the bulk has an offload flag among
   PKT_TX_OFFLOAD_MASK

The purpose of tx_prepare() is to check that offload can be done in hw,
and to prepare the packet data for the hw. For instance, niantics NICs
require that the L4 checksum in the packet is set to the pseudo-header
checksum.

This could have be done in the tx() function, but it is a separated
function for 2 reasons:
- the application does not need to call it if it does not do hw tx offload
- the tx_prepare() function may modify packet data, which should not be the
  case for tx(). In a pipeline model, these 2 functions may be called on
  different cores.

In my opinion, the tx_prepare() function should deal with outer checksums
preparation as well.

Olivier