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

[Chengchang Tang]

On 2021/2/8 16:40, Olivier Matz wrote:
> 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.
>>>
>>> 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
> 
> .

Thank you, Thomas and Olivier. It's really helpful.

Chengchang