-----Original Message-----
From: dev <redacted> On Behalf Of Bruce Richardson
Sent: Thursday, June 17, 2021 1:31 AM
To: fengchengwen <redacted>
Cc: thomas@monjalon.net; Yigit, Ferruh <redacted>;
dev@dpdk.org; nipun.gupta@nxp.com; hemant.agrawal@nxp.com;
maxime.coquelin@redhat.com; honnappa.nagarahalli@arm.com;
jerinj@marvell.com; david.marchand@redhat.com; jerinjacobk@gmail.com
Subject: Re: [dpdk-dev] [RFC PATCH] dmadev: introduce DMA device library

On Wed, Jun 16, 2021 at 05:41:45PM +0800, fengchengwen wrote:

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On 2021/6/16 0:38, Bruce Richardson wrote:

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On Tue, Jun 15, 2021 at 09:22:07PM +0800, Chengwen Feng wrote:

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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

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  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

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  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

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  APIs a lot more awkward to use with anything but mbufs or similar

buffers

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  where we already have a pre-computed physical address.

The iova is an hint to application, and widely used in DPDK.
If switch to void, how to pass the address (iova or just va ?) this
may introduce implementation dependencies here.

Or always pass the va, and the driver performs address translation,
and this translation may cost too much cpu I think.

On the latter point, about driver doing address translation I would agree.
However, we probably need more discussion about the use of iova vs just
virtual addresses. My thinking on this is that if we specify the API using iovas
it will severely hurt usability of the API, since it forces the user to take more
inefficient codepaths in a large number of cases. Given a pointer to the
middle of an mbuf, one cannot just pass that straight as an iova but must
instead do a translation into offset from mbuf pointer and then readd the
offset to the mbuf base address.

Agree. Vhost is one consumer of DMA devices. To support SW fallback
in case of DMA copy errors, vhost needs to pass VA for both DPDK mbuf
and guest buffer to the callback layer (a middle layer between vhost and
dma device). If DMA devices use iova, it will require the callback layer to
call rte_mem_virt2iova() to translate va to iova in data path, even if iova
is va in some cases. But if DMA devices claim to use va, device differences
can be hided inside driver, which makes the DMA callback layer simpler
and more efficient.

Thanks,
Jiayu

My preference therefore is to require the use of an IOMMU when using a
dmadev, so that it can be a much closer analog of memcpy. Once an iommu
is present, DPDK will run in VA mode, allowing virtual addresses to our
hugepage memory to be sent directly to hardware. Also, when using
dmadevs on top of an in-kernel driver, that kernel driver may do all iommu
management for the app, removing further the restrictions on what memory
can be addressed by hardware.

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* Use of id values rather than user-provided handles. Allowing the

user/app

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  to manage the amount of data stored per operation is a better solution,

I

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  feel than proscribing a certain about of in-driver tracking. Some apps

may

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  not care about anything other than a job being completed, while other

apps

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  may have significant metadata to be tracked. Taking the user-context
  handles out of the API also makes the driver code simpler.

The user-provided handle was mainly used to simply application
implementation, It provides the ability to quickly locate contexts.

The "use of id values" seem like the dma_cookie of Linux DMA engine
framework, user will get a unique dma_cookie after calling
dmaengine_submit(), and then could use it to call

dma_async_is_tx_complete() to get completion status.

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Yes, the idea of the id is the same - to locate contexts. The main difference is
that if we have the driver manage contexts or pointer to contexts, as well as
giving more work to the driver, it complicates the APIs for measuring
completions. If we use an ID-based approach, where the app maintains its
own ring of contexts (if any), it avoids the need to have an "out" parameter
array for returning those contexts, which needs to be appropriately sized.
Instead we can just report that all ids up to N are completed. [This would be
similar to your suggestion that N jobs be reported as done, in that no
contexts are provided, it's just that knowing the ID of what is completed is
generally more useful than the number (which can be obviously got by
subtracting the old value)]

We are still working on prototyping all this, but would hope to have a
functional example of all this soon.

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How about define the copy prototype as following:
  dma_cookie_t rte_dmadev_copy(uint16_t dev_id, xxx) while the
dma_cookie_t is int32 and is monotonically increasing, when >=0 mean
enqueue successful else fail.
when complete the dmadev will return latest completed dma_cookie, and
the application could use the dma_cookie to quick locate contexts.

If I understand this correctly, I believe this is largely what I was suggesting -
just with the typedef for the type? In which case it obviously looks good to
me.

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* 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.

The rte_ioat_completed_ops API is too complex, and consider some
applications may never copy fail, so split them as two API.
It's indeed not friendly to other scenarios that always require error

handling.

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I prefer use completed operations number as return value other than
the ID so that application could simple judge whether have new
completed operations, and the new prototype:
 uint16_t rte_dmadev_completed(uint16_t dev_id, dma_cookie_t *cookie,
uint32_t *status, uint16_t max_status, uint16_t *num_fails);

1) for normal case which never expect failed ops:
   just call: ret = rte_dmadev_completed(dev_id, &cookie, NULL, 0,
NULL);
2) for other case:
   ret = rte_dmadev_completed(dev_id, &cookie, &status, max_status,

&fails);

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   at this point the fails <= ret <= max_status

Completely agree that we need to plan for the happy-day case where all is
passing. Looking at the prototypes you have above, I am ok with returning
number of completed ops as the return value with the final completed cookie
as an "out" parameter.
For handling errors, I'm ok with what you propose above, just with one small
adjustment - I would remove the restriction that ret <= max_status.

In case of zero-failures, we can report as many ops succeeding as we like,
and even in case of failure, we can still report as many successful ops as we
like before we start filling in the status field. For example, if 32 ops are
completed, and the last one fails, we can just fill in one entry into status, and
return 32. Alternatively if the 4th last one fails we fill in 4 entries and return
32. The only requirements would be:
* fails <= max_status
* fails <= ret
* cookie holds the id of the last entry in status.

A further possible variation is to have separate "completed" and
"completed_status" APIs, where "completed_status" is as above, but
"completed" skips the final 3 parameters and returns -1 on error. In that case
the user can fall back to the completed_status call.

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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.

Agree, will fix in v2

Thanks. In parallel, we will be working on our prototype implementation too,
taking in the feedback here, and hopefully send it as an RFC soon.
Then we can look to compare and contrast and arrive at an agreed API. It
might also be worthwhile to set up a community call for all interested parties
in this API to discuss things with a more rapid turnaround. That was done in
the past for other new device class APIs that were developed, e.g. eventdev.

Regards,
/Bruce