[PATCH v3 1/3] dma: Support multiple interleaved frames with non-contiguous memory

[Srikanth Thokala]

On Tue, Feb 18, 2014 at 10:20 PM, Jassi Brar <jaswinder.singh at linaro.org> wrote:
> On 18 February 2014 16:58, Srikanth Thokala <sthokal at xilinx.com> wrote:
>> On Mon, Feb 17, 2014 at 3:27 PM, Jassi Brar <jaswinder.singh at linaro.org> wrote:
>>> On 15 February 2014 17:30, Srikanth Thokala <sthokal at xilinx.com> wrote:
>>>> The current implementation of interleaved DMA API support multiple
>>>> frames only when the memory is contiguous by incrementing src_start/
>>>> dst_start members of interleaved template.
>>>>
>>>> But, when the memory is non-contiguous it will restrict slave device
>>>> to not submit multiple frames in a batch.  This patch handles this
>>>> issue by allowing the slave device to send array of interleaved dma
>>>> templates each having a different memory location.
>>>>
>>> How fragmented could be memory in your case? Is it inefficient to
>>> submit separate transfers for each segment/frame?
>>> It will help if you could give a typical example (chunk size and gap
>>> in bytes) of what you worry about.
>>
>> With scatter-gather engine feature in the hardware, submitting separate
>> transfers for each frame look inefficient. As an example, our DMA engine
>> supports up to 16 video frames, with each frame (a typical video frame
>> size) being contiguous in memory but frames are scattered into different
>> locations. We could not definitely submit frame by frame as it would be
>> software overhead (HW interrupting for each frame) resulting in video lags.
>>
> IIUIC, it is 30fps and one dma interrupt per frame ... it doesn't seem
> inefficient at all. Even poor-latency audio would generate a higher
> interrupt-rate. So the "inefficiency concern" doesn't seem valid to
> me.
>
> Not to mean we shouldn't strive to reduce the interrupt-rate further.
> Another option is to emulate the ring-buffer scheme of ALSA.... which
> should be possible since for a session of video playback the frame
> buffers' locations wouldn't change.
>
> Yet another option is to use the full potential of the
> interleaved-xfer api as such. It seems you confuse a 'video frame'
> with the interleaved-xfer api's 'frame'. They are different.
>
> Assuming your one video frame is F bytes long and Gk is the gap in
> bytes between end of frame [k] and start of frame [k+1] and  Gi != Gj
> for i!=j
> In the context of interleaved-xfer api, you have just 1 Frame of 16
> chunks. Each chunk is Fbytes and the inter-chunk-gap(ICG) is Gk  where
> 0<=k<15
> So for your use-case .....
>   dma_interleaved_template.numf = 1   /* just 1 frame */
>   dma_interleaved_template.frame_size = 16  /* containing 16 chunks */
>    ...... //other parameters
>
> You have 3 options to choose from and all should work just as fine.
> Otherwise please state your problem in real numbers (video-frames'
> size, count & gap in bytes).

Initially I interpreted interleaved template the same.  But, Lars corrected me
in the subsequent discussion and let me put it here briefly,

In the interleaved template, each frame represents a line of size denoted by
chunk.size and the stride by icg.  'numf' represent number of frames i.e.
number of lines.

In video frame context,
chunk.size -> hsize
chunk.icg -> stride
numf -> vsize
and frame_size is always 1 as it will have only one chunk in a line.

So, the API would not allow to pass multiple frames and we came up with a
resolution to pass an array of interleaved template structs to handle this.

Srikanth

>
> -Jassi
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