Re: UBIFS with dma on 4.6 kernel is not working

[Richard Weinberger]

Christoph,

On 21.10.2016 15:15, Christoph Hellwig wrote:
> On Fri, Oct 21, 2016 at 03:07:57PM +0200, Richard Weinberger wrote:
>> Hmm, thought this is still problematic on VIVT architectures.
>> Boris tried to provide a solution for that some time ago:
>> http://www.spinics.net/lists/arm-kernel/msg494025.html
> 
> Things have been working fine for approx 10 years when using
> flush_kernel_vmap_range before doing I/O using the physical addresses and
> then invalidate_kernel_vmap_range when completing the I/O and going back
> to using the virtual mapping for XFS.
> 
> Of course all this assumes at least the higher level that did the
> vm_map_ram operation knows about this dance between virtually mapped and
> physiscal addresses. 

Good to know, I was clearly wrong.

Let's see whether the costs of flush_kernel_vmap_range and invalidate_kernel_vmap_range
are smaller than the speedup by DMA on embedded platforms.
We'll have to test it.

Thanks,
//richard