How to work out the cause of a DMA Bus Error (QCI PXA320)?

Guennadi Liakhovetski [off-list ref] writes:

On Thu, 27 Aug 2009, Judith Baumgarten wrote:

quoted

Hi,

I'm still working on the pxa_camera driver for PXA320.

Is this a new driver or an extension to pxa270?

Good question :)

quoted

At the moment I can
capture some frames (RAW mode), but then I get a DMA Bus Error. Has anyone an
idea how to deal with that error or work out the specific cause?
The manual says, that if an error occures the channel is stopped until it's
reprogrammed and the corresponding RUN bit is set, but there are no RUN bits
in QCI DMA. So I don't see a chance to start it again...

I think we also had some DMA issues on pxa270, but we fixed them latest 
with these patches:

"V4L/DVB (11322): pxa_camera: Fix overrun condition on last buffer"
"V4L/DVB (11321): pxa_camera: Redesign DMA handling"

if you're writing a new driver, have a look t pxa270 and at those patches 
specifically.

You know, that little beast of PXA320 has a _dedicated_ 4-channel DMA controller
(with dedicated registers CIDADR, CIDSADR, ...). Maybe the rules we followed
for pxa270 are not all true anymore.

I would suggest to Judith to scan with care PXA3xx manual, volume III, chapter
3.4.5. To help debugging, you could add sysfs entries for CI* registers in
3.5.28.

Ah, and you're right, there is no "RUN" bit in CICMDx. I suspect, after a 30s
reading of the specification, that reloading CIDADR with a correct value will
trigger the DMA transfer (QCI DMA FIFO threshold -> DMA request line assert ->
DMA transfer).

Cheers.

--
Robert