Hi Vinod, 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. > 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. > 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 ? > 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, 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 ? -- Regards, Laurent Pinchart
