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

[fengchengwen <fengchengwen@huawei.com>]

OK, thank Bruce

How about next week ?
PS: I still working on V2 and hope it as a basis for discussion.

On 2021/6/23 22:56, Bruce Richardson wrote:
> This is developing into quite a long discussion with multiple threads
> ongoing at the same time. Since it's getting relatively hard to follow (at
> least for me), can I suggest that we actually hold a call to discuss
> "dmadev" and to move things along. Since most of the dicussion participants
> I believe are in the eastern timezones can I suggest 8AM UTC as a suitable
> timeslot (which would be 9AM Irish time, 1:30PM India and 4PM PRC time).
> Would 8AM UTC on Friday suit people? The usual tool for such community
> discussion is Jitsi (meet.jit.si/DPDK), so I would suggest re-using that
> for this discussion.
> 
> Can anyone interested in participating in this discussion let me know
> [offlist, so we don't spam everyone], and I can try and co-ordinate if
> everyone is ok with above suggested timeslot and send out calendar invite.
> 
> Regards,
> /Bruce
> 
> On Wed, Jun 23, 2021 at 11:50:48AM +0800, fengchengwen wrote:
>> On 2021/6/23 1:51, Jerin Jacob wrote:
>>> On Fri, Jun 18, 2021 at 2:22 PM fengchengwen <fengchengwen@huawei.com> wrote:
>>>>
>>>> On 2021/6/17 22:18, Bruce Richardson wrote:
>>>>> 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 simplify the status_array's maintenance because we don't need init it to zero.
>>>> I think it's a good trade-off between performance and rich error info (status code).
>>>>
>>>> Here I'd like to discuss the 'burst size', which is widely used in DPDK application (e.g.
>>>> nic polling or ring en/dequeue).
>>>> Currently we don't define a max completed ops in rte_dmadev_completed() API, the return
>>>> value may greater than 'burst size' of application, this may result in the application need to
>>>> maintain (or remember) the return value of the function and special handling at the next poll.
>>>>
>>>> Also consider there may multiple calls rte_dmadev_completed to check fail, it may make it
>>>> difficult for the application to use.
>>>>
>>>> So I prefer following prototype:
>>>>   uint16_t rte_dmadev_completed(uint16_t dev_id, dma_cookie_t *cookie, uint16_t nb_cpls, bool *has_error)
>>>>     -- nb_cpls: indicate max process operations number
>>>>     -- has_error: indicate if there is an error
>>>>     -- return value: the number of successful completed operations.
>>>>     -- example:
>>>>        1) If there are already 32 completed ops, and 4th is error, and nb_cpls is 32, then
>>>>           the ret will be 3(because 1/2/3th is OK), and has_error will be true.
>>>>        2) If there are already 32 completed ops, and all successful completed, then the ret
>>>>           will be min(32, nb_cpls), and has_error will be false.
>>>>        3) If there are already 32 completed ops, and all failed completed, then the ret will
>>>>           be 0, and has_error will be true.
>>>>   uint16_t rte_dmadev_completed_status(uint16_t dev_id, dma_cookie_t *cookie, uint16_t nb_status, uint32_t *status)
>>>>     -- return value: the number of failed completed operations.
>>>
>>>
>>>
>>> In typical storage use cases etc, Sometimes application need to
>>> provide scatter-gather list,
>>> At least in our hardware sg list gives a "single completion result"
>>> and it stops on the first failure to restart
>>> the transfer by application. Have you thought of scatter-gather use
>>> case and how it is in other  HW?
>>
>> cookie and request are in a one-to-one correspondence, whether the request is a single or sg-list.
>> Kunpeng9x0 don't support sg-list, I'm still investigating other hardware.
>>
>> The above 'restart the transfer by application' mean re-schedule request (and have one new cookie) or
>> just re-enable current failed request (this may introduce new API) ?
>>
>>>
>>> prototype like the following works for us:
>>> rte_dmadev_enq_sg(void **src, void **dest, unsigned int **length, int
>>> nb_segments, cookie, ,,,)
>>
>> OK, we could define one scatter-list struct to wrap src/dest/length.
>>
>>>
>>>
>>>>
>>>> The application use the following invocation order when polling:
>>>>   has_error = false; // could be init to false by dmadev API, we need discuss
>>>>   ret = rte_dmadev_completed(dev_id, &cookie, bust_size, &has_error);
>>>>   // process successful completed case:
>>>>   for (int i = 0; i < ret; i++) {
>>>>   }
>>>>   if (unlikely(has_error)) {
>>>>     // process failed completed case
>>>>     ret = rte_dmadev_completed_status(dev_id, &cookie, burst_size - ret, status_array);
>>>>     for (int i = 0; i < ret; i++) {
>>>>       // ...
>>>>     }
>>>>   }
>>>>
>>>>
>>>>>
>>>>> 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
>>>>>
>>>>> .
>>>>>
>>>>
>>>
>>> .
>>>
>>
> 
> .
>