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

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

On Wed, Jun 16, 2021 at 05:41:45PM +0800, fengchengwen wrote:
> On 2021/6/16 0:38, Bruce Richardson wrote:
> > 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.
> 
> 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.

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.

> > * 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.
> 
> 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.
> 

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.

> 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.

> > * 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.
> 
> 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);
>    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.

> > 
> > 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