Re: [dpdk-dev] [RFC PATCH] dmadev: introduce DMA device library

[Bruce Richardson <bruce.richardson@intel.com>]

On Wed, Jun 16, 2021 at 08:07:26PM +0530, Jerin Jacob wrote:
> On Wed, Jun 16, 2021 at 3:47 PM fengchengwen <fengchengwen@huawei.com> wrote:
> >
> > On 2021/6/16 15:09, Morten Brørup wrote:
> > >> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Bruce Richardson
> > >> Sent: Tuesday, 15 June 2021 18.39
> > >>
> > >> On Tue, Jun 15, 2021 at 09:22:07PM +0800, Chengwen Feng wrote:
> > >>> This patch introduces 'dmadevice' which is a generic type of DMA
> > >>> device.
> > >>>
> > >>> The APIs of dmadev library exposes some generic operations which can
> > >>> enable configuration and I/O with the DMA devices.
> > >>>
> > >>> Signed-off-by: Chengwen Feng <fengchengwen@huawei.com>
> > >>> ---
> > >> Thanks for sending this.
> > >>
> > >> Of most interest to me right now are the key data-plane APIs. While we
> > >> are
> > >> still in the prototyping phase, below is a draft of what we are
> > >> thinking
> > >> for the key enqueue/perform_ops/completed_ops APIs.
> > >>
> > >> Some key differences I note in below vs your original RFC:
> > >> * Use of void pointers rather than iova addresses. While using iova's
> > >> makes
> > >>   sense in the general case when using hardware, in that it can work
> > >> with
> > >>   both physical addresses and virtual addresses, if we change the APIs
> > >> to use
> > >>   void pointers instead it will still work for DPDK in VA mode, while
> > >> at the
> > >>   same time allow use of software fallbacks in error cases, and also a
> > >> stub
> > >>   driver than uses memcpy in the background. Finally, using iova's
> > >> makes the
> > >>   APIs a lot more awkward to use with anything but mbufs or similar
> > >> buffers
> > >>   where we already have a pre-computed physical address.
> > >> * Use of id values rather than user-provided handles. Allowing the
> > >> user/app
> > >>   to manage the amount of data stored per operation is a better
> > >> solution, I
> > >>   feel than proscribing a certain about of in-driver tracking. Some
> > >> apps may
> > >>   not care about anything other than a job being completed, while other
> > >> apps
> > >>   may have significant metadata to be tracked. Taking the user-context
> > >>   handles out of the API also makes the driver code simpler.
> > >> * I've kept a single combined API for completions, which differs from
> > >> the
> > >>   separate error handling completion API you propose. I need to give
> > >> the
> > >>   two function approach a bit of thought, but likely both could work.
> > >> If we
> > >>   (likely) never expect failed ops, then the specifics of error
> > >> handling
> > >>   should not matter that much.
> > >>
> > >> For the rest, the control / setup APIs are likely to be rather
> > >> uncontroversial, I suspect. However, I think that rather than xstats
> > >> APIs,
> > >> the library should first provide a set of standardized stats like
> > >> ethdev
> > >> does. If driver-specific stats are needed, we can add xstats later to
> > >> the
> > >> API.
> > >>
> > >> Appreciate your further thoughts on this, thanks.
> > >>
> > >> Regards,
> > >> /Bruce
> > >
> > > I generally agree with Bruce's points above.
> > >
> > > I would like to share a couple of ideas for further discussion:
> 
> 
> I believe some of the other requirements and comments for generic DMA will be
> 
> 1) Support for the _channel_, Each channel may have different
> capabilities and functionalities.
> Typical cases are, each channel have separate source and destination
> devices like
> DMA between PCIe EP to Host memory, Host memory to Host memory, PCIe
> EP to PCIe EP.
> So we need some notion of the channel in the specification.
>

Can you share a bit more detail on what constitutes a channel in this case?
Is it equivalent to a device queue (which we are flattening to individual
devices in this API), or to a specific configuration on a queue?
 
> 2) I assume current data plane APIs are not thread-safe. Is it right?
> 
Yes.

> 
> 3) Cookie scheme outlined earlier looks good to me. Instead of having
> generic dequeue() API
> 
> 4) Can split the rte_dmadev_enqueue_copy(uint16_t dev_id, void * src,
> void * dst, unsigned int length);
> to two stage API like, Where one will be used in fastpath and other
> one will use used in slowpath.
> 
> - slowpath API will for take channel and take other attributes for transfer
> 
> Example syantx will be:
> 
> struct rte_dmadev_desc {
>            channel id;
>            ops ; // copy, xor, fill etc
>           other arguments specific to dma transfer // it can be set
> based on capability.
> 
> };
> 
> rte_dmadev_desc_t rte_dmadev_preprare(uint16_t dev_id,  struct
> rte_dmadev_desc *dec);
> 
> - Fastpath takes arguments that need to change per transfer along with
> slow-path handle.
> 
> rte_dmadev_enqueue(uint16_t dev_id, void * src, void * dst, unsigned
> int length,  rte_dmadev_desc_t desc)
> 
> This will help to driver to
> -Former API form the device-specific descriptors in slow path  for a
> given channel and fixed attributes per transfer
> -Later API blend "variable" arguments such as src, dest address with
> slow-path created descriptors
> 

This seems like an API for a context-aware device, where the channel is the
config data/context that is preserved across operations - is that correct?
At least from the Intel DMA accelerators side, we have no concept of this
context, and each operation is completely self-described. The location or
type of memory for copies is irrelevant, you just pass the src/dst
addresses to reference.

> The above will give better performance and is the best trade-off
> between performance and per transfer variables.

We may need to have different APIs for context-aware and context-unaware
processing, with which to use determined by the capabilities discovery.
Given that for these DMA devices the offload cost is critical, more so than
any other dev class I've looked at before, I'd like to avoid having APIs
with extra parameters than need to be passed about since that just adds
extra CPU cycles to the offload.

/Bruce