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

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

On Thu, Jun 17, 2021 at 12:02:00PM +0100, Bruce Richardson wrote:
> On Thu, Jun 17, 2021 at 05:48:05PM +0800, fengchengwen wrote:
> > On 2021/6/17 1:31, Bruce Richardson wrote:
> > > 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.
> > 
> > Some DMA devices many don't support IOMMU or IOMMU bypass default, so driver may
> > should call rte_mem_virt2phy() do the address translate, but the rte_mem_virt2phy()
> > cost too many CPU cycles.
> > 
> > If the API defined as iova, it will work fine in:
> > 1) If DMA don't support IOMMU or IOMMU bypass, then start application with
> >    --iova-mode=pa
> > 2) If DMA support IOMMU, --iova-mode=pa/va work both fine
> > 
> 
> I suppose if we keep the iova as the datatype, we can just cast "void *"
> pointers to that in the case that virtual addresses can be used directly. I
> believe your RFC included a capability query API - "uses void * as iova" 
> should probably be one of those capabilities, and that would resolve this.
> If DPDK is in iova=va mode because of the presence of an iommu, all drivers
> could report this capability too.
> 
> > > 
> > >>> * 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.
> > 
> > I think we understand the same:
> > 
> > The fails <= ret <= max_status include following situation:
> > 1) If max_status is 32, and there are 32 completed ops, then the ret will be 32
> > no matter which ops is failed
> > 2) If max_status is 33, and there are 32 completed ops, then the ret will be 32
> > 3) If max_status is 16, and there are 32 completed ops, then the ret will be 16
> > 
> > and the cookie always hold the id of the last returned completed ops, no matter
> > it's completed successful or failed
> > 
> 
> I actually disagree on the #3. If max_status is 16, there are 32 completed
> ops, and *no failures* the ret will be 32, not 16, because we are not
> returning any status entries so max_status need not apply. Keeping that
> same scenario #3, depending on the number of failures and the point of
> them, the return value may similarly vary, for example:
> * if job #28 fails, then ret could still be 32, cookie would be the cookie
>   for that job, "fails" parameter would return as 4, with status holding the
>   failure of 28 plus the succeeded status of jobs 29-31, i.e. 4 elements.
> * if job #5 fails, then we can't fit the status list from 5 though 31 in an
>   array of 16, so "fails" == 16(max_status) and status contains the 16
>   statuses starting from #5, which means that cookie contains the value for
>   job #20 and ret is 21.
> 
> In other words, ignore max_status and status parameters *unless we have an
> error to return*, meaning the fast-path/happy-day case works as fast as
> possible. You don't need to worry about sizing your status array to be big,
> and you always get back a large number of completions when available. Your
> fastpath code only need check the "fails" parameter to see if status needs
> to ever be consulted, and in normal case it doesn't.
> 
> If this is too complicated, maybe we can simplify a little by returning just
> one failure at a time, though at the cost of making error handling slower?
> 
> rte_dmadev_completed(dev_id, &cookie, &failure_status)
> 
> In this case, we always return the number of completed ops on success,
> while on failure, we return the first error code. For a single error, this
> works fine, but if we get a burst of errors together, things will work
> slower - which may be acceptable if errors are very rare. However, for idxd 
> at least if a fence occurs after a failure all jobs in the batch after the
> fence would be skipped, which would lead to the "burst of errors" case.
> Therefore, I'd prefer to have the original suggestion allowing multiple
> errors to be reported at a time.
> 
> /Bruce

Apologies for self-reply, but thinking about it more, a combination of
normal-case and error-case APIs may be just simpler:

int rte_dmadev_completed(dev_id, &cookie)

returns number of items completed and cookie of last item. If there is an
error, returns all successfull values up to the error entry and returns -1
on subsequent call.

int rte_dmadev_completed_status(dev_id, &cookie, max_status, status_array,
	&error_count)

this is a slower completion API which behaves like you originally said
above, returning number of completions x, 0 <= x <= max_status, with x
status values filled into array, and the number of unsuccessful values in
the error_count value.

This would allow code to be written in the application to use
rte_dmadev_completed() in the normal case, and on getting a "-1" value, use
rte_dmadev_completed_status() to get the error details. If strings of
errors might be expected, the app can continually use the
completed_status() function until error_count returns 0, and then switch
back to the faster/simpler version.

This two-function approach also allows future support for other DMA
functions such as comparison, where a status value is always required. Any
apps using that functionality would just always use the "_status" function
for completions.

/Bruce