On Mon, Aug 08, 2016 at 02:25:28PM +0200, Lars-Peter Clausen wrote: > On 08/08/2016 11:08 AM, Vinod Koul wrote: > > On Thu, Aug 04, 2016 at 05:59:30PM +0200, Lars-Peter Clausen wrote: > >> On 08/04/2016 05:38 PM, Russell King - ARM Linux wrote: > >> [...] > >>> What you instead need to do is to find some way to record in your > >>> driver that transaction 2 failed, and when dma_cookie_status() says > >>> that a transaction has DMA_COMPLETE status, you need to look up to > >>> see whether it failed. > >> > >> In my opinion this is where the current API is broken by design. For each > >> transfer that fails you need to store the cookie associated with that > >> transfer in some kind of lookup table. Since there is no lifetime > >> associated > >> with a cookie entries in this table would need to be retained forever and > >> it > >> will grow unbound. > > > > And how many drivers can report errors? And how many drivers can guarantee > > DMA_COMPLETE implies transaction was succesful. > > The former just a handful, the later hopefully all. > > > > >> Ideally we'd mark error reporting through this interface as deprecated and > >> discourage new users of the interface. As far as I can see most of the few > >> drivers that do return DMA_ERROR get it wrong anyway, e.g. return it > >> unconditionally for all cookies when an error occurred for any of them. > > > > Error reporting is quite tricky as detection is a problem. So yes if you > > can do so, it is highly encouraged to report using new interface which is > > better than client checking after callback. > > > > Btw what is the behaviour after error? I would think that client will see an > > error and report to upper layer while initiaite closure of transaction. So > > does driver need to keep the state for a longer time :-) > > The problem is that this is not really clearly defined. > > 1) What should be done when multiple descriptors are queued and an error is > encountered on one of them. Should the descriptors that are after the one in > the queue that caused the error be discarded or should they be executed as > normal?
That is client's call. But a reasonable way would be for client to propagate those errors up, so it can terminate. > 2) How long does a error result need to be retained. Can it be discarded > when the terminate_all() is called, or can it be discarded when the next > issue_pending() is called or should it be retained forever? Uptill next terminate_all() > Unless we can clearly define the semantics of error reporting it is very > difficult for drivers to use it. Which is probably one of the reasons why > there are only very few DMAengine consumers that do actual error checking. Yes agreed, but most reasonable behaviour is to terminate. Also I would expect the error reporting to be done thru new API and explcitly told that we found error (if we can). - ~Vinod
