Re: [PATCH v2] dma: Add Xilinx AXI Video Direct Memory Access Enginedriver support

From: Lars-Peter Clausen
Date: Wed Feb 05 2014 - 11:30:28 EST

On 02/05/2014 05:25 PM, Srikanth Thokala wrote:
On Fri, Jan 31, 2014 at 12:21 PM, Srikanth Thokala <sthokal@xxxxxxxxxx> wrote:
Hi Vinod,

On Tue, Jan 28, 2014 at 8:43 AM, Vinod Koul <vinod.koul@xxxxxxxxx> wrote:
On Mon, Jan 27, 2014 at 06:42:36PM +0530, Srikanth Thokala wrote:
Hi Lars/Vinod,
The question here i think would be waht this device supports? Is the hardware
capable of doing interleaved transfers, then would make sense.

The hardware does 2D transfers. The parameters for a transfer are height,
width and stride. That's only a subset of what interleaved transfers can be
(xt->num_frames must be one for 2d transfers). But if I remember correctly
there has been some discussion on this in the past and the result of that
discussion was that using interleaved transfers for 2D transfers is
preferred over adding a custom API for 2D transfers.

I went through the prep_interleaved_dma API and I see only one descriptor
is prepared per API call (i.e. per frame). As our IP supports upto 16 frame
buffers (can be more in future), isn't it less efficient compared to the
prep_slave_sg where we get a single sg list and can prepare all the descriptors
(of non-contiguous buffers) in one go? Correct me, if am wrong and let me
know your opinions.
Well the descriptor maybe one, but that can represent multiple frames, for
example 16 as in your case. Can you read up the documentation of how multiple
frames are passed. Pls see include/linux/dmaengine.h

* Interleaved Transfer Request
* ----------------------------
* A chunk is collection of contiguous bytes to be transfered.
* The gap(in bytes) between two chunks is called inter-chunk-gap(ICG).
* ICGs may or maynot change between chunks.
* A FRAME is the smallest series of contiguous {chunk,icg} pairs,
* that when repeated an integral number of times, specifies the transfer.
* A transfer template is specification of a Frame, the number of times
* it is to be repeated and other per-transfer attributes.
* Practically, a client driver would have ready a template for each
* type of transfer it is going to need during its lifetime and
* set only 'src_start' and 'dst_start' before submitting the requests.
* | Frame-1 | Frame-2 | ~ | Frame-'numf' |
* |====....==.===...=...|====....==.===...=...| ~ |====....==.===...=...|
* == Chunk size
* ... ICG

Yes, it can handle multiple frames specified by 'numf' each of size
'frame_size * sgl[0].size'.
But, I see it only works if all the frames' memory is contiguous and
in this case we
can just increment 'src_start' by the total frame size 'numf' number
of times to fill in
for each HW descriptor (each frame is one HW descriptor). So, there
is no issue when the
memory is contiguous. If the frames are non contiguous, we have to
call this API for each
frame (hence for each descriptor), as the src_start for each frame is
different. Is it correct?

FYI: This hardware has an inbuilt Scatter-Gather engine.


If you want to submit multiple frames at once I think you should look at how the current dmaengine API can be extended to allow that. And also provide an explanation on how this is superior over submitting them one by one.

- Lars

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