Hi Laurent, 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. > > >> 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 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 :) > > >> 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. > > >>>> 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) -- ~Vinod
