dma_alloc_coherent versus streaming DMA, neither works satisfactory

Arnd Bergmann
2015-04-29T10:41:30Z

On Wednesday 29 April 2015 12:33:00 Mike Looijmans wrote:
> On 29-04-15 12:07, Arnd Bergmann wrote:
> > On Wednesday 29 April 2015 11:47:37 Mike Looijmans wrote:
> >> On 29-04-15 11:17, Russell King - ARM Linux wrote:
> >>> The buffer should also be mapped into userspace with the same memory
> >>> type and cache attributes as the kernel side mapping. If using ACP,
> >>> then you probably want "normal memory, cacheable, writeback, read
> >>> allocate" or in the case of SMP, the same but "read/write allocate".
> >
> > If my suspicion is correct, we should either change arm_coherent_dma_ops()
> > to refer to a different mmap() callback that does the right thing for
> > coherent devices, or change arm_dma_mmap() to look at dev->is_coherent.
>
> Following the route, arch/arm/mm/dma-mapping.c uses pgprot_dmacoherent() which
> is defined in arch/arm/include/asm/pgtable.h and that just returns uncached
> memory.
>
> If you can give me some hints as to what the correct flags would be, I can
> patch my kernel and test it.

Use the flags that Russell listed above.

Arnd