i.MX 6 and PCIe DMA issues

[michael.moese at men.de]

On Tue, Jul 11, 2017 at 03:50:19PM +0100, Robin Murphy wrote:
> I don't much like the sound of that "and" there - coherent DMA
> allocations are, as the name implies, already coherent for CPU and DMA
> accesses, and require no maintenance; the streaming DMA API
> (dma_{map,unmap,sync}_*) on the other hand is *only* for use on
> kmalloced memory.
Ok, I hope I am correct. I alloc my memory using dma_alloc_coherent()
once, the dma_handle is passed to the device, with no other dma_map*()
or dma_sync_*() calls needed?

Yesterday I observed some strange behavior  when I did some debug prints in 
the driver, printing me the result from phys_to_virt() of my
virtual address and the dma_handle. I know these may be different, but,
when I dump the memory (from userspace using mmap), I can see the data
at the adress of my dma_handle, but the memory the driver has the
pointer for has different contents. I don't understand this.

 
> The PL310 does have more than its fair share of wackiness, but unless
> you also see DMA going wrong for the on-chip peripherals, the problem is
> almost certainly down to the driver itself rather than the cache
> configuration.
Well, I think I need do dive into this as well, my former co-worker
disabled DMA for SPI, for example, in the device tree. That may be
another hint. I think I will need to find out what is setup here.


Thank you for your advise, I will try to find out more.

Michael