Re: Getting alot of zero values from ads7846 using atmel_spi

[Niclas Karlsson <niclas.karlsson-h3aQMNCITb4@public.gmane.org>]

tis 2011-04-19 klockan 14:40 -0600 skrev Kenneth Heitke:
> On 04/18/2011 03:21 AM, Niclas Karlsson wrote:
> > tor 2011-03-10 klockan 15:41 +0100 skrev David Lamparter:
> >> On Tue, Mar 08, 2011 at 10:44:56AM +0100, Niclas Karlsson wrote:
> >>> Do anyone know why I read so many zeroes?
> >>
> >> You're probably reading values faster than the chip supports. Check the
> >> chip documentation for a maximum readout frequency or a ready flag.
> >>> I have made some printk in ads7846.c to trace the problem. It seemed to
> >>> read more correct values when I added the printks.
> >>
> >> A printk takes quite some time on an ARM like yours. The throttle makes
> >> it work...
> >
> > I've made som further investigations. The atmel_spi.c driver utilize PDC
> > (Peripheral DMA Controller) and when I was tracing the problem I
> > discovered that the interrupt indicating the PDC has written the data to
> > memory is fired before the data actually is written to memory. If a
> > small delay is added after the interrupt (like a trace printk("The value
> > is\n")) the PDC have enough time to write the data to the memory and I
> > get correct values from the ADC connected to the SPI.
> > Thoughts how to solve this?
> 
> Is it really the interrupt firing early or is it possible that you are 
> still reading cached data.  I'm not familiar with the device or driver 
> so I'm just throwing that out as a possibility.
I have thought about it but I don't know how to test it. I disabled the
 whole d-cache in the kernel configuration, but everything ran so slow
that I don't know if it was the disabled of d-cache or if the timing
became so much different.
If I want to bring the d-cache up to sync, how do I do that?
Is it possible to allocate memory which never will be stored in d-cache?
/Niclas
> >>
> >>
> >> David
> >>
> >> P.S.: you're lucky to not get crap values - seems they got the chip
> >> design right...
> >>
> > /Niclas
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> 


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Benefiting from Server Virtualization: Beyond Initial Workload 
Consolidation -- Increasing the use of server virtualization is a top
priority.Virtualization can reduce costs, simplify management, and improve 
application availability and disaster protection. Learn more about boosting 
the value of server virtualization. http://p.sf.net/sfu/vmware-sfdev2dev