Re: [PATCH] drm/xen-front: Make shmem backed display buffer coherent

[Oleksandr Andrushchenko]

On 12/20/18 8:38 PM, Christoph Hellwig wrote:
> On Thu, Dec 20, 2018 at 07:35:15PM +0100, Daniel Vetter wrote:
> > > Err, with streaming DMA buffer sharing is trivial.  The coherent DMA
> > > allocator is what causes all kinds of horrible hacks that can't actually
> > > work on various platforms.
> > Hm, I thought the streaming dma api is the one that causes bounce
> > buffers and all that fun. If you're unlucky at least.
> Yes it may.  But even if that happens everything will actually work,
> just slower.  While the dma coherent API is simply broken.
>
> But if you don't want bounce buffering you need to use the dma
> noncoherent allocator as proposed here:
>
> 	https://lists.linuxfoundation.org/pipermail/iommu/2018-December/031982.html
>
> which combines allocating memory that doesn't need to be bounce
> buffered with a sharing scheme that can actually work.
So, the bottom line will be: I can use DMA API for what I need, but:
1. I need to remove GFP_USER
2. No need for DMA32 (so no chance for bouncing to step in)
3. I may need to check if mapping and unmapping of the buffer
at once will also help, e.g. no need to have the buffer mapped until
it is destroyed
Did I get it all right?

Thank you,
Oleksandr