ppc405 dma - vm - cache question

[Smith, Gene]

Feng, Shuchen wrote, On 10/27/2004 10:25 AM:

> Gene Smith wrote :
> 
> 
> 
>>I have a question for ppc experts in rtems land. I am trying to port the
> 
> 
>>emac ethernet driver from netbsd to rtems. In the driver code I see all 
>>kinds of calls to things like this:
> 
> 
>>bus_dmamap_create();
>>bus_dmamem_alloc();
>>bus_dmamap_load();
>>bus_dmamap_sync(); <-- called a lot in tx/rx ISRs.
> 
> 
> 
>>I think that these are used to reconcile virtual memory in unix to 
>>physical memory addressed by devices (emac or mal device on ppc405).
> 
> 
> My understanding is that they do software snoop to ensure cache 
> coherency.
> 
> 
>>My assumption is that in rtems I can pretty much ignore these or make my 
>>own versions since rtems always uses real mode; anywhere memory is 
>>needed for dma purposes by the ethernet device and/or driver I can just 
>>malloc it and use the returned address for dma verbatim.
> 
> 
> Either hardware snoop or software snoop is required if the ethernet
> controller
> accesses SDRAM space that is also cacheable by the CPU.
> 

"snoop of hardware or software" is a new term to me. Could you explain 
what you mean by this? Yes, I can see cache would be a concern like when 
tcp/ip writes to mbufs in data cache and the device does not see the 
real data when it tries to send the "same" mbufs using dma. I think this 
requires the sync and/or eieio instructions but not sure. I am still 
trying to learn this stuff. Thanks.