From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mails.dpdk.org (mails.dpdk.org [217.70.189.124]) by inbox.dpdk.org (Postfix) with ESMTP id C2638A0C41; Wed, 23 Jun 2021 05:50:55 +0200 (CEST) Received: from [217.70.189.124] (localhost [127.0.0.1]) by mails.dpdk.org (Postfix) with ESMTP id 490AF4003F; Wed, 23 Jun 2021 05:50:55 +0200 (CEST) Received: from szxga03-in.huawei.com (szxga03-in.huawei.com [45.249.212.189]) by mails.dpdk.org (Postfix) with ESMTP id E778C4003E for ; Wed, 23 Jun 2021 05:50:52 +0200 (CEST) Received: from dggemv704-chm.china.huawei.com (unknown [172.30.72.53]) by szxga03-in.huawei.com (SkyGuard) with ESMTP id 4G8q2C5kd0z70jb; Wed, 23 Jun 2021 11:46:43 +0800 (CST) Received: from dggpeml500024.china.huawei.com (7.185.36.10) by dggemv704-chm.china.huawei.com (10.3.19.47) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Wed, 23 Jun 2021 11:50:50 +0800 Received: from [127.0.0.1] (10.40.190.165) by dggpeml500024.china.huawei.com (7.185.36.10) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2176.2; Wed, 23 Jun 2021 11:50:49 +0800 To: Jerin Jacob CC: Bruce Richardson , Thomas Monjalon , Ferruh Yigit , dpdk-dev , Nipun Gupta , Hemant Agrawal , Maxime Coquelin , Honnappa Nagarahalli , Jerin Jacob , David Marchand , "Satananda Burla" , Prasun Kapoor , =?UTF-8?Q?Morten_Br=c3=b8rup?= References: <1623763327-30987-1-git-send-email-fengchengwen@huawei.com> <25d29598-c26d-8497-2867-9b650c79df49@huawei.com> From: fengchengwen Message-ID: <3db2eda0-4490-2b8f-c65d-636bcf794494@huawei.com> Date: Wed, 23 Jun 2021 11:50:48 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.40.190.165] X-ClientProxiedBy: dggems705-chm.china.huawei.com (10.3.19.182) To dggpeml500024.china.huawei.com (7.185.36.10) X-CFilter-Loop: Reflected Subject: Re: [dpdk-dev] [RFC PATCH] dmadev: introduce DMA device library X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" On 2021/6/23 1:51, Jerin Jacob wrote: > On Fri, Jun 18, 2021 at 2:22 PM fengchengwen 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 >>>>>>>>> --- >>>>>>>> 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 >>> >>> . >>> >> > > . >