dma_alloc_coherent versus streaming DMA, neither works satisfactory

[arnd@arndb.de (Arnd Bergmann)]

On Thursday 30 April 2015 15:50:15 Mike Looijmans wrote:
> 
> Just to give you a status update, I tried that too (by adding a 
> dma_mmap_coherent variant that omits the "prot" change, and some printks to 
> verify that it actually does as expected).
> 
> Current status is that the ACP behaves exactly like the HP port, which it 
> should not do. If I send data from logic via the ACP port through the L2 
> cache, using a version of dma_sync that just invalidates the cache could 
> (should?) result in data corruption. Instead, the data gets corrupted only if 
> you do not invalidate the line. This is what the (non-coherent) HP port 
> behaves like, as it writes directly to DDR.
> 
> Currently I'm assuming that the tools did something wrong in the bitstream, 
> for example, wiring the "AWCACHE" and similar signals on the ACP to logic "0" 
> instead of "1" while claiming to have wired them to "1" in the UI. A bug like 
> that would also explain the behaviour I'm seeing now.
> 
> I'll let you know once I find out more.
> 
> 

Ok, maybe you have to configure the SCU to include the ACP in the
cache coherency? That might not be done by default. I don't really
know anything about the SCU or the ACP, so I'm just taking wild
guesses here.

	Arnd