Hi Vinod, On Wed, Mar 18, 2020 at 05:14:27PM +0200, Laurent Pinchart wrote: > On Wed, Mar 11, 2020 at 05:52:48PM +0200, Laurent Pinchart wrote: > > On Wed, Mar 04, 2020 at 09:54:26PM +0530, Vinod Koul wrote: > >> On 04-03-20, 18:00, Laurent Pinchart wrote: > >>> On Wed, Mar 04, 2020 at 09:07:18PM +0530, Vinod Koul wrote: > >>>> On 04-03-20, 10:01, Laurent Pinchart wrote: > >>>>> On Wed, Mar 04, 2020 at 10:43:01AM +0530, Vinod Koul wrote: > >>>>>> On 03-03-20, 21:22, Laurent Pinchart wrote: > >>>>>>> On Tue, Mar 03, 2020 at 10:02:54AM +0530, Vinod Koul wrote: > >>>>>>>> On 02-03-20, 09:37, Laurent Pinchart wrote: > >>>>>>>>>> I would be more comfortable in calling an API to do so :) > >>>>>>>>>> The flow I am thinking is: > >>>>>>>>>> > >>>>>>>>>> - prep cyclic1 txn > >>>>>>>>>> - submit cyclic1 txn > >>>>>>>>>> - call issue_pending() (cyclic one starts) > >>>>>>>>>> > >>>>>>>>>> - prep cyclic2 txn > >>>>>>>>>> - submit cyclic2 txn > >>>>>>>>>> - signal_cyclic1_txn aka terminate_cookie() > >>>>>>>>>> - cyclic1 completes, switch to cyclic2 (dmaengine driver) > >>>>>>>>>> - get callback for cyclic1 (optional) > >>>>>>>>>> > >>>>>>>>>> To check if hw supports terminate_cookie() or not we can check if the > >>>>>>>>>> callback support is implemented > >>>>>>>>> > >>>>>>>>> Two questions though: > >>>>>>>>> > >>>>>>>>> - Where is .issue_pending() called for cyclic2 in your above sequence ? > >>>>>>>>> Surely it should be called somewhere, as the DMA engine API requires > >>>>>>>>> .issue_pending() to be called for a transfer to be executed, otherwise > >>>>>>>>> it stays in the submitted but not pending queue. > >>>>>>>> > >>>>>>>> Sorry missed that one, I would do that after submit cyclic2 txn step and > >>>>>>>> then signal signal_cyclic1_txn termination > >>>>>>> > >>>>>>> OK, that matches my understanding, good :-) > >>>>>>> > >>>>>>>>> - With the introduction of a new .terminate_cookie() operation, we need > >>>>>>>>> to specify that operation for all transfer types. What's its > >>>>>>>> > >>>>>>>> Correct > >>>>>>>> > >>>>>>>>> envisioned semantics for non-cyclic transfers ? And how do DMA engine > >>>>>>>>> drivers report that they support .terminate_cookie() for cyclic > >>>>>>>>> transfers but not for other transfer types (the counterpart of > >>>>>>>>> reporting, in my proposition, that .issue_pending() isn't supported > >>>>>>>>> replace the current cyclic transfer) ? > >>>>>>>> > >>>>>>>> Typically for dmaengine controller cyclic is *not* a special mode, only > >>>>>>>> change is that a list provided to controller is circular. > >>>>>>> > >>>>>>> I don't agree with this. For cyclic transfers to be replaceable in a > >>>>>>> clean way, the feature must be specifically implemented at the hardware > >>>>>>> level. A DMA engine that supports chaining transfers with an explicit > >>>>>>> way to override that chaining, and without the logic to report if the > >>>>>>> inherent race was lost or not, really can't support this API. > >>>>>> > >>>>>> Well chaining is a typical feature in dmaengine and making last chain > >>>>>> point to first makes it circular. I have seen couple of engines and this > >>>>>> was the implementation in the hardware. > >>>>>> > >>>>>> There can exist special hardware for this purposes as well, but the > >>>>>> point is that the cyclic can be treated as circular list. > >>>>>> > >>>>>>> Furthemore, for non-cyclic transfers, what would .terminate_cookie() do > >>>>>>> ? I need it to be defined as terminating the current transfer when it > >>>>>>> ends for the cyclic case, not terminating it immediately. All non-cyclic > >>>>>>> transfers terminate by themselves when they end, so what would this new > >>>>>>> operation do ? > >>>>>> > >>>>>> I would use it for two purposes, cancelling txn but at the end of > >>>>>> current txn. I have couple of usages where this would be helpful. > >>>>> > >>>>> I fail to see how that would help. Non-cyclic transfers always stop at > >>>>> the end of the transfer. "Cancelling txn but at the end of current txn" > >>>>> is what DMA engine drivers already do if you call .terminate_cookie() on > >>>>> the ongoing transfer. It would thus be a no-op. > >>>> > >>>> Well that actually depends on the hardware, some of them support abort > >>>> so people cancel it (terminate_all approach atm) > >>> > >>> In that case it's not terminating at the end of the current transfer, > >>> but terminating immediately (a.k.a. aborting), right ? Cancelling at the > >>> end of the current transfer still seems to be a no-op to me for > >>> non-cyclic transfers, as that's what they do on their own already. > >> > >> Correct, it is abort for current txn. > >> > >>>>>> Second in error handling where some engines do not support > >>>>>> aborting (unless we reset the whole controller) > >>>>> > >>>>> Could you explain that one ? I'm not sure to understand it. > >>>> > >>>> So I have dma to a slow peripheral and it is stuck for some reason. I > >>>> want to abort the cookie and let subsequent ones runs (btw this is for > >>>> non cyclic case), so I would use that here. Today we terminate_all and > >>>> then resubmit... > >>> > >>> That's also for immediate abort, right ? > >> > >> Right > >> > >>> For this to work properly we need very accurate residue reporting, as > >>> the client will usually need to know exactly what has been transferred. > >>> The device would need to support DMA_RESIDUE_GRANULARITY_BURST when > >>> aborting an ongoing transfer. What hardware supports this ? > >> > >> git grep DMA_RESIDUE_GRANULARITY_BURST drivers/dma/ |wc -l > >> 27 > >> > >> So it seems many do support the burst reporting. > > > > Yes, but not all of those may support aborting a transfer *and* > > reporting the exact residue of cancelled transfers. We need both to > > implement your proposal. > > > >>>>>> But yes the .terminate_cookie() semantics should indicate if the > >>>>>> termination should be immediate or end of current txn. I see people > >>>>>> using it for both. > >>>>> > >>>>> Immediate termination is *not* something I'll implement as I have no > >>>>> good way to test that semantics. I assume you would be fine with leaving > >>>>> that for later, when someone will need it ? > >>>> > >>>> Sure, if you have hw to support please test. If not, you will not > >>>> implement that. > >>>> > >>>> The point is that API should support it and people can add support in > >>>> the controllers and test :) > >>> > >>> I still think this is a different API. We'll have > >>> > >>> 1. Existing .issue_pending(), queueing the next transfer for non-cyclic > >>> cases, and being a no-op for cyclic cases. > >>> 2. New .terminate_cookie(AT_END_OF_TRANSFER), being a no-op for > >>> non-cyclic cases, and moving to the next transfer for cyclic cases. > >>> 3. New .terminate_cookie(ABORT_IMMEDIATELY), applicable to both cyclic > >>> and non-cyclic cases. > >>> > >>> 3. is an API I don't need, and can't easily test. I agree that it can > >>> have use cases (provided the DMA device can abort an ongoing transfer > >>> *and* still support DMA_RESIDUE_GRANULARITY_BURST in that case). > >>> > >>> I'm troubled by my inability to convince you that 1. and 2. are really > >>> the same, with 1. addressing the non-cyclic case and 2. addressing the > >>> cyclic case :-) This is why I think they should both be implemeted using > >>> .issue_pending() (no other option for 1., that's what it uses today). > >>> This wouldn't prevent implementing 3. with a new .terminate_cookie() > >>> operation, that wouldn't need to take a flag as it would always operate > >>> in ABORT_IMMEDIATELY mode. There would also be no need to report a new > >>> capability for 3., as the presence of the .terminate_cookie() handler > >>> would be enough to tell clients that the API is supported. Only a new > >>> capability for 2. would be needed. > >> > >> Well I agree 1 & 2 seem similar but I would like to define the behaviour > >> not dependent on the txn being cyclic or not. That is my concern and > >> hence the idea that: > >> > >> 1. .issue_pending() will push txn to pending_queue, you may have a case > >> where that is done only once (due to nature of txn), but no other > >> implication > >> > >> 2. .terminate_cookie(EOT) will abort the transfer at the end. Maybe not > >> used for cyclic but irrespective of that, the behaviour would be abort > >> at end of cyclic > > > > Did you mean "maybe not used for non-cyclic" ? > > > >> 3. .terminate_cookie(IMMEDIATE) will abort immediately. If there is > >> anything in pending_queue that will get pushed to hardware. > >> > >> 4. Cyclic by nature never completes > >> - as a consequence needs to be stopped by terminate_all/terminate_cookie > >> > >> Does these rules make sense :) > > > > It's a set of rules that I think can handle my use case, but I still > > believe my proposal based on just .issue_pending() would be simpler, in > > line with the existing API concepts, and wouldn't preclude the addition > > of .terminate_cookie(IMMEDIATE) at a later point. It's your call though, > > especially if you provide the implementation :-) When do you think you > > will be able to do so ? > > Gentle ping :-) Any update ? > >>>>>> And with this I think it would make sense to also add this to > >>>>>> capabilities :) > >>>>> > >>>>> I'll repeat the comment I made to Peter: you want me to implement a > >>>>> feature that you think would be useful, but is completely unrelated to > >>>>> my use case, while there's a more natural way to handle my issue with > >>>>> the current API, without precluding in any way the addition of your new > >>>>> feature in the future. Not fair. > >>>> > >>>> So from API design pov, I would like this to support both the features. > >>>> This helps us to not rework the API again for the immediate abort. > >>>> > >>>> I am not expecting this to be implemented by you if your hw doesn't > >>>> support it. The core changes are pretty minimal and callback in the > >>>> driver is the one which does the job and yours wont do this > >>> > >>> Xilinx DMA drivers don't support DMA_RESIDUE_GRANULARITY_BURST so I > >>> can't test this indeed. > >> > >> Sure I understand that! Am sure folks will respond to CFT and I guess > >> Peter will also be interested in testing. > > > > s/testing/implementing it/ :-) > > > >>>>>>>> So, the .terminate_cookie() should be a feature for all type of txn's. > >>>>>>>> If for some reason (dont discount what hw designers can do) a controller > >>>>>>>> supports this for some specific type(s), then they should return > >>>>>>>> -ENOTSUPP for cookies that do not support and let the caller know. > >>>>>>> > >>>>>>> But then the caller can't know ahead of time, it will only find out when > >>>>>>> it's too late, and can't decide not to use the DMA engine if it doesn't > >>>>>>> support the feature. I don't think that's a very good option. > >>>>>> > >>>>>> Agreed so lets go with adding these in caps. > >>>>> > >>>>> So if there's a need for caps anyway, why not a cap that marks > >>>>> .issue_pending() as moving from the current cyclic transfer to the next > >>>>> one ? > >>>> > >>>> Is the overhead really too much on that :) If you like I can send the > >>>> core patches and you would need to implement the driver side? > >>> > >>> We can try that as a compromise. One of main concerns with developing > >>> the core patches myself is that the .terminate_cookie() API still seems > >>> ill-defined to me, so it would be much more efficient if you translate > >> > >> yeah lets take a stab at defining this and see if we come up with > >> something meaningful > >> > >>> the idea you have in your idea into code than trying to communicate it > >>> to me in all details (one of the grey areas is what should > >>> .terminate_cookie() do if the cookie passed to the function corresponds > >>> to an already terminated or, more tricky from a completion callback > >>> point of view, an issued but not-yet-started transfer, or also a > >>> submitted but not issued transfer). If you implement the core part, then > >>> that problem will go away. > >>> > >>> How about the implementation in virt-dma.[ch] by the way ? > >> > >> It needs to be comprehended and tested as well.. since these are simple > >> callbacks to driver, we should not need huge changes here (i need to > >> double check though) -- Regards, Laurent Pinchart
