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

["Trahe, Fiona" <fiona.trahe@intel.com>]

> -----Original Message-----
> From: Ahmed Mansour [mailto:ahmed.mansour@nxp.com]
> Sent: Saturday, February 24, 2018 1:17 AM
> To: Trahe, Fiona <fiona.trahe@intel.com>; dev@dpdk.org; Shally.Verma@cavium.com
> Cc: De Lara Guarch, Pablo <pablo.de.lara.guarch@intel.com>; Athreya, Narayana Prasad
> <NarayanaPrasad.Athreya@cavium.com>; Gupta, Ashish <Ashish.Gupta@cavium.com>; Sahu, Sunila
> <Sunila.Sahu@cavium.com>; Challa, Mahipal <Mahipal.Challa@cavium.com>; Jain, Deepak K
> <deepak.k.jain@intel.com>; Hemant Agrawal <hemant.agrawal@nxp.com>; Roy Pledge
> <roy.pledge@nxp.com>; Youri Querry <youri.querry_1@nxp.com>
> Subject: Re: [dpdk-dev] [PATCH] compressdev: implement API
> 
> 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?
> 
> 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
> >
> >
> >