Re: UBIFS with dma on 4.6 kernel is not working

[Richard Weinberger]

Christoph,

On 21.10.2016 14:53, Christoph Hellwig wrote:
> On Fri, Oct 21, 2016 at 11:29:16AM +0200, Richard Weinberger wrote:
>> DMA to vmalloced memory not good, it may work by chance if you transfer
>> less than PAGE_SIZE.
>> Especially on ARM.
> 
> DMA to vmalloc'ed or vmap memory is perfectly fine, you just have to be
> very careful.
> 
> I would suggest to not expose the vmalloc address to the lower layers
> that do DMA, but instead expose the pages, either as an array or
> scatterlist.  Either allocate the pages using the normal page allocator
> and then use vm_map_ram to generate a virtual address for them (that
> is what XFS does for it's large metadata objects for example).  Or if
> you can't do that iterate over the vmalloc address in page size chunks
> and use vmalloc_to_page (we still also do that for one piece of legacy
> cruft in XFS, but I'd rather avoid that for new designs).
> 

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

Thanks,
//richard