Hi Vinod, 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 ? > >>>> 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
