Re: comedi_poll for cb_pcidas64

On Thu, 2005-04-07 at 11:10 -0400, Frank Mori Hess wrote:
> I think the smallest block size supported by the hardware is something like 
> 256 samples, as it only seemed to want to do a dma transfer when an entire 
> "segment" of the board's fifo was full.  The negative consequences are 
> that there is more cpu overhead (more interrupts) and the size of the 
> board's fifo is reduced when you make the block size smaller (making fifo 
> overruns slightly more probable).  Neither of these factors should be 
> noticeable at the speeds you are taking data.
> 
> You could set the TRIG_WAKE_EOS command flag, which will cause the driver 
> to turn off dma and do pio transfers at the end of each scan (even more 
> cpu overhead, but should be okay for 1kHz scans).

Ok, TRIG_WAKE_EOS works really well. 

htop reports at most 30% CPU usage on a Duron 800MHz; that should be
quite bearable - particularly as the target systems will be much more
up-to-date. 

For comparison we've just ordered a NI 6025E as that seems to support
polling as far as I can tell from the driver (we needed a second board
anyway), but I think we'll take the TRIG_WAKE_EOS route. 

> 
> >
> > Anyway, this would be a hack as COMEDI_POLL is exactly what I'd really
> > need.
> >
> > Frank: Do you have any PCI-DAS 60xx docs that indicate whether the
> > driver could implement POLL'ing on this hardware?
> 
> You can get register level programming information from the MC website.  
> You can also get a manual for the plx9080 pci chip on the web.  I don't 
> think it is possible to get polling to do anything useful though.  As I 
> recall, the samples just accumulate in the hardware fifo until a segment 
> is full, then they are all dma'd over to computer memory together.  I 
> couldn't figure out a way to make it initiate a dma transfer on fifo not 
> empty.  Or if you are using TRIG_WAKE_EOS then you don't need to poll, 
> unless you wanted to grab samples part-way through a channel scan.
> 

Thank you very much for your detailed explanation and all the insight.

Thanks,
Jan

Received on 2005-04-09Z13:19:59