[PATCH 00/20] ARM: pxa: move core and drivers to dmaengine

Hi Robert,

On 10.08.2013 00:50, Robert Jarzmik wrote:

Daniel Mack [off-list ref] writes:

quoted

 * camera driver:
     I started the transition, but I'm not sure how much sense that
     makes without access to the hardware. I'd much appreciate if
     anyone could volunteer for this piece; I'll happily share what
     I got so far. Sascha, Sachin, Guennadi?

Hi Daniel,

Do you mean this driver ? :
drivers/media/platform/soc_camera/pxa_camera.c

Yes, exactly.

In that case I might help. But before I can do that, I have to be convinced that
dmaengine can deal with this driver. I'm thinking in particular of :
 - "hot running DMA" queuing
 - multiple DMA channel synchronization (ie. 3 channel sync)

All that is described in there :
  Documentation/video4linux/pxa_camera.txt

Yes, I've seen that, and while the documentation about all that is
excellent, I lack an explanation why things are so complicated for this
application, and why a simple cyclic DMA approach does not suffice here.
I'm sure there's a reason though.

There might be need to teach the dmaengine core more functionality, but
in order to do that, we first need to understand the exact requirements.

If someone with dmaengine knowledge could have a look at pxa_camera.txt (maybe
Vinod ?) and tell me that dma_engine framework fullfills the 2 requirements,
then I'll be happy to help.

Yes, Vinod and and Dan are certainly the best ones to comment on that I
think.

One minor point though is that I'm working on pxa27x. If the serie is not
compatible in a way with pxa27x, I won't be able to do anything.

No, it should work just fine on pxa27x.

Another point I'd like to know, is what is the performance penalty in using
dmaengine, and do you have any figures ?

The DMA transfers themselves certainly perform equally well, and the
framework is just a thin layer. Where would you expect performance penalty?

Lastly, they was debug information to debug descriptors chaining, channel
statuses, requestors. I didn't see where these had gone, could you point me to
the right file ?

Such a debug interface is not part of the mmp-pdma implementation at
this point, and the core doesn't have a generic debugfs feature either.
If you need that, we'd have to add it back.

FWIW, I attached my work-in-progress patch for this driver which just
does some basic dmaengine preparations. Be aware that this does not even
compile, it's really just a snapshot.


Thanks,
Daniel

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