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

On Wed, Jun 16, 2021 at 3:47 PM fengchengwen [off-list ref] wrote:

On 2021/6/16 15:09, Morten Brørup wrote:

quoted

quoted

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:

quoted

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 <redacted>
---

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.

2) I assume current data plane APIs are not thread-safe. Is it right?


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

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