On Mon, Jul 25, 2016 at 10:19:44AM -0400, Sinan Kaya wrote:
> >>
> >> It looks like I introduced a behavioral change while refactoring the code.
> >> The previous one would call the callback only if the transfer was successful
> >> but it would always call dma_cookie_complete.
> >>
> >> The new behavior is to call dma_cookie_complete only if the transfer is successful
> >> and it calls the callback even in the case of error cases. Then, the client has
> >> to query if transfer was successful.
> >>
> >> Which one is the correct behavior?
> >
> > Hi Sinan,
> >
> > Cookie is always completed. That simply means trasactions was completed and
> > has no indication if the transaction was successfull or not.
> >
> > Uptill now we had no way of reporting error, Dave's series adds that up, so
> > you can use it.
> >
> > Callback is optional for users. Again we didnt convey success of
> > transaction, but a callback for reporting that trasaction was completed. So
> > invoking callback makes sense everytime.
> >
>
> Let's put Dave's series aside for the moment and assume an error case where
> something bad happened during the transfer. I can add the error code once Dave's
> series get merged.
Fair enough..
> Here is the callback from dmatest.
>
> static void dmatest_callback(void *arg)
> {
> done->done = true;
> }
>
> Here is how the request is made.
>
> dma_async_issue_pending(chan);
>
> wait_event_freezable_timeout(done_wait, done.done,
> msecs_to_jiffies(params->timeout));
>
> status = dma_async_is_tx_complete(chan, cookie, NULL, NULL);
> if (!done.done) {
> timeout
> } else if (status != DMA_COMPLETE) {
> error
> }
>
> success.
>
> Based on what I see here, receiving callback all the time is OK. The client
> checks if the callback is received or not first.
Callback is optional from API PoV. Yes ppl do implement it :)
> Next, the client checks the status of the tx_status.
>
> In the error case mentioned, the callback will be executed. done.done will be true.
>
> If I set dma_cookie_complete(desc) in error case, it would be wrong to tell the client
> that the transfer is successful.
And here is the thing that you missed :)
Dmaengine tells transaction is complete. It does not say if the txn is
success or failure. It can transfer data and not say if data was
correct. A successful transaction implies data integrity as well, which
dmaengine can't provide.
> In my opinion, the new behavior is correct. Calling dma_cookie_complete(desc) all the time
> is not. Do you agree?
>
> If yes, I can divide this patch into two. One to correct the ordering. Another one
> for behavioral change.
See above..
A callback or tx_status will only tell you the txn is completed. That is
why we have DMA_COMPLETE and not DMA_SUCCESS.
So current order seems fine to me!
--
~Vinod
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