Hi Vinod,
On Fri, Jan 24, 2020 at 10:50:51AM +0200, Laurent Pinchart wrote:
> On Fri, Jan 24, 2020 at 11:40:47AM +0530, Vinod Koul wrote:
> > On 23-01-20, 14:23, Laurent Pinchart wrote:
> >>>>>> @@ -701,6 +702,10 @@ struct dma_filter {
> >>>>>> * The function takes a buffer of size buf_len. The callback function will
> >>>>>> * be called after period_len bytes have been transferred.
> >>>>>> * @device_prep_interleaved_dma: Transfer expression in a generic way.
> >>>>>> + * @device_prep_interleaved_cyclic: prepares an interleaved cyclic transfer.
> >>>>>> + * This is similar to @device_prep_interleaved_dma, but the transfer is
> >>>>>> + * repeated until a new transfer is issued. This transfer type is meant
> >>>>>> + * for display.
> >>>>>
> >>>>> I think capture (camera) is another potential beneficiary of this.
> >>
> >> Possibly, although in the camera case I'd rather have the hardware stop
> >> if there's no more buffer. Requiring a buffer to always be present is
> >> annoying from a userspace point of view. For display it's different, if
> >> userspace doesn't submit a new frame, the same frame should keep being
> >> displayed on the screen.
> >>
> >>>>> So you don't need to terminate the running interleaved_cyclic and start
> >>>>> a new one, but prepare and issue a new one, which would
> >>>>> terminate/replace the currently running cyclic interleaved DMA?
> >>
> >> Correct.
> >>
> >>>> Why not explicitly terminate the transfer and start when a new one is
> >>>> issued. That can be common usage for audio and display..
> >>>
> >>> Yes, this is what I'm asking. The cyclic transfer is running and in
> >>> order to start the new transfer, the previous should stop. But in cyclic
> >>> case it is not going to happen unless it is terminated.
> >>>
> >>> When one would want to have different interleaved transfer the display
> >>> (or capture )IP needs to be reconfigured as well. The the would need to
> >>> be terminated anyways to avoid interpreting data in a wrong way.
> >>
> >> The use case here is not to switch to a new configuration, but to switch
> >> to a new buffer. If the transfer had to be terminated manually first,
> >> the DMA engine would potentially miss a frame, which is not acceptable.
> >> We need an atomic way to switch to the next transfer.
> >
> > So in this case you have, let's say a cyclic descriptor with N buffers
> > and they are cyclically capturing data and providing to client/user..
>
> For the display case it's cyclic over a single buffer that is repeatedly
> displayed over and over again until a new one replaces it, when
> userspace wants to change the content on the screen. Userspace only has
> to provide a new buffer when content changes, otherwise the display has
> to keep displaying the same one.
Is the use case clear enough, or do you need more information ? Are you
fine with the API for this kind of use case ?
> For cameras I don't think cyclic makes too much sense, except when the
> DMA engine can't work in single-shot mode and always requires a buffer
> to write into. That shouldn't be the norm.
>
> > So why would you like to submit again...? Once whole capture has
> > completed you would terminate, right...
> >
> > Sorry not able to wrap my head around why new submission is required and
> > if that is the case why previous one cant be terminated :)
--
Regards,
Laurent Pinchart
