Re: [dpdk-dev] [PATCH v2] doc: plan splitting the ethdev ops struct

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 2/25/2020 3:51 PM, Andrew Rybchenko wrote:
> On 2/25/20 3:44 PM, Ferruh Yigit wrote:
>> For the ABI compatibility it is better to hide internal data structures
>> from the application as much as possible. But because of some inline
>> functions 'struct eth_dev_ops' can't be hidden completely.
>>
>> Plan is to split the 'struct eth_dev_ops' into two as ones used by
>> inline functions and ones not used, and hide the second part that not
>> used by inline functions completely to the application.
>>
>> Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
>> Acked-by: Jerin Jacob <jerinj@marvell.com>
>> ---
>> Cc: David Marchand <david.marchand@redhat.com>
>> Cc: Thomas Monjalon <thomas@monjalon.net>
>> Cc: Andrew Rybchenko <arybchenko@solarflare.com>
>>
>> v2:
>> * Add target date for the work
>> * Give more detail on what will be done
>> ---
>>  doc/guides/rel_notes/deprecation.rst | 11 +++++++++++
>>  1 file changed, 11 insertions(+)
>>
>> diff --git a/doc/guides/rel_notes/deprecation.rst b/doc/guides/rel_notes/deprecation.rst
>> index 99d81564a..ff612a615 100644
>> --- a/doc/guides/rel_notes/deprecation.rst
>> +++ b/doc/guides/rel_notes/deprecation.rst
>> @@ -86,6 +86,17 @@ Deprecation Notices
>>    In 19.11 PMDs will still update the field even when the offload is not
>>    enabled.
>>  
>> +* ethdev: Split the ``struct eth_dev_ops`` struct to hide it as much as possible.
>> +  Currently the ``struct eth_dev_ops`` struct is accessible by the application
>> +  because some inline functions, like ``rte_eth_tx_descriptor_status()``,
>> +  access the struct directly. The struct will be separate in two, the ops used
>> +  by inline functions still will be accessible to user but rest will be hidden.
>> +  Initial split will be done in 20.05 with adding reserved fields for the struct
>> +  used by inline functions, and by putting new struct reference into public one
>> +  to not increase the size of ``struct rte_eth_dev``, proper split will be done
>> +  in 20.11 by moving inline dev_ops function to next to Rx/Tx burst functions and
>> +  hiding rest.
>> +
>>  * cryptodev: support for using IV with all sizes is added, J0 still can
>>    be used but only when IV length in following structs ``rte_crypto_auth_xform``,
>>    ``rte_crypto_aead_xform`` is set to zero. When IV length is greater or equal
> 
> I'd like to understand why do we need an intermediate solution first.

We don't need it really, we may prefer to postpone the update to 20.11.

> Also rereading above few times I've failed to fully understand what
> will be done and why it does not break API/ABI.
> 

Agree it wasn't very clear, but I mean something like below, I hope it clarifies:

Previously:

struct rte_eth_dev

+--------+
|        |    *dev_ops    struct eth_dev_ops
|        |
|        +-------------->+--------+
|        |               |        |
|        |               |        |
|        |               |        |
|        |               |        |
+--------+               |        |
                         |        |
                         |        |
                         |        |
                         +--------+


Proposed:

struct rte_eth_dev

+--------+
|        |                struct eth_dev_ops
|        |
|*dev_ops+-------------->+--------+
|        |               |        |
|        |               | Reserv |
|        |               |        |
|        |               |        |
+--------+               |        |
                         +--------+
                         | inline |
                         | de^_ops|               struct eth_dev_ops
                         +--------+
                         | *priv  +------------->+--------+
                         +--------+              |        |
                                                 |        |
                                                 |        |
                                                 |        |
                                                 |        |
                                                 |        |
                                                 |        |
                                                 |        |
                                                 +--------+

This is only to keep ABI compatibility [1] while separating the struct.

[1]
- The offset of some functions in the dev_ops struct should be same
- The size of the "struct rte_eth_dev" should be same