[dpdk-dev] [PATCH] compressdev: implement API

[Ahmed Mansour]

>> Hi Fiona,
>>
>> Thanks for starting this discussion. In the current API the user must
>> make 12 API calls just to get information to compress. Maybe there is a
>> way to simplify. At least for some use cases (stateless). I think a call
>> sometime next week would be good to help clarify coalesce some of the
>> complexity.
> [Fiona] Would 10:30 GMT on Wednesday 28th Feb suit?
[Ahmed] I am in Ottawa Canada so Wednesday 12:00 or 1:00 GMT would be
better. Does that work?
>> I added specific comments inline.
>>
>> Thanks,
>>
>> Ahmed
>>
>> On 2/21/2018 2:12 PM, Trahe, Fiona wrote:
>>> We've been struggling with the idea of session in compressdev.
>>>
>>> Is it really a session?
>>>  - It's not in the same sense as cryptodev where it's used to hold a key, and maps to a Security
>> Association.
>>>  - It's a set of immutable data that is needed with the op and stream to perform the operation.
>>>  - It inherited from cryptodev the ability to be set up for multiple driver types and used across any
>>>     devices of those types. For stateful ops this facility can't be used.
>>>     For stateless we don't think it's important, and think it's unlikely to be used.
>>>  - Drivers use it to prepare private data, set up resources, do pre-work, so there's
>>>     less work to be done on the data path. Initially we didn't have a stream, we do now,
>>>     this may be a better alternative place for that work.
>>> So we've been toying with the idea of getting rid of the session.
>> [Ahmed] In our proprietary API the stream and session are one. A session
>> holds many properties like the op-type, instead of having this
>> information in the op itself.  This way we lower the per op setup cost.
>> This also allows rapid reuse of stateful infrastructure, once a stream
>> is closed on a stateful session, the next op (stream) on this session
>> reuses the stateful storage. Obviously if a stream is in "pause mode" on
>> a session, all following ops that may be unrelated to this
>> stream/session must also wait until this current stream is closed or
>> aborted before the infrastructure can be reused.
>>> We also struggle with the idea of setting up a stream for stateless ops.
>>>   - Well, really I just think the name is misleading, i.e. there's no problem with setting
>>>     up some private PMD data to use with stateless operations, just calling it a
>>>     stream doesn't seem right.
>> [Ahmed] I agree. The op has all the necessary information to process it
>> in the current API? Both the stream and the op are one time use. We
>> can't attach multiple similar ops to a single stream/session and rely on
>> their properties to simplify op setup, so why the hassle.
>>> So putting above thoughts together I want to propose:
>>> -	Removal of the session and all associated APIs.
>>> -	Passing in one of three data types in the rte_comp_op
>>>
>>>     union {
>>>         struct rte_comp_xform *xform;
>>>         /**< Immutable compress/decompress params */
>>>         void *pmd_stateless_data;
>>>         /**< Stateless private PMD data derived from an rte_comp_xform
>>>          * rte_comp_stateless_data_init() must be called on a device
>>>          * before sending any STATELESS operations. If the PMD returns a non-NULL
>>>          * value the handle must be attached to subsequent STATELESS operations.
>>>          * If a PMD returns NULL, then the xform should be passed directly to each op
>>>          */
>>>         void *stream;
>>>         /* Private PMD data derived initially from an rte_comp_xform, which holds state
>>>          * and history data and evolves as operations are processed.
>>>          * rte_comp_stream_create() must be called on a device for all STATEFUL
>>>          * data streams and the resulting stream attached
>>>          * to the one or more operations associated with the data stream.
>>>          * All operations in a stream must be sent to the same device.
>>>          */
>>>     }
>> [Ahmed] I like this setup, but I am not sure in what cases the xform
>> immutable would be used. I understand the other two.
> [Fiona] The xform is there because I don't know yet what limitations may apply to the
> pmd_stateless_data. If it has no limitation and once set up once on a device 
> can be attached simultaneously to any op sent to any qp on that device
> then we don't need the xform. But I understood from Shally's earlier request for
> setting up a stream on a stateless request that some resources are
> allocated, so we may need to document some limitations.
> In this case the xform may be a better path for PMDs which don't have the same
> limitations.  
>
>
>>> Notes:
>>> 1. Internally if a PMD wants to use the exact same data structure for both it can do,
>>>      just on the API I think it's better if they're named differently with
>>>      different comments.
>>> 2. I'm not clear of the constraints if any, which attach to the pmd_stateless_data
>>>      For our PMD it would only hold immutable data as the session did, and so
>>>      could be attached to many ops in parallel.
>>>      Is this true for all PMDs or are there constraints which should be called out?
>>>      Is it limited to a specific device, qp, or to be used on one op at a time?
>>> 3. Am open to other naming suggestions, just trying to capture the essence
>>>     of these data structs better than our current API does.
>>>
>>> We would put some more helper fns and structure around the above code if people
>>> are in agreement, just want to see if the concept flies before going further?
>>>
>>> Fiona
>>>
>>>
>>>
>