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

[fengchengwen <fengchengwen@huawei.com>]

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
>>>
>>> .
>>>
>>
> 
> .
>