Re: [PATCH] lib/scatterlist: Provide a DMA page iterator

Thomas Hellstrom
2019-01-15T18:03:39Z

On Tue, 2019-01-15 at 16:20 +0100, hch@lst.de wrote:
> On Tue, Jan 15, 2019 at 03:24:55PM +0100, Christian König wrote:
> > Yeah, indeed. Bounce buffers are an absolute no-go for GPUs.
> >
> > If the DMA API finds that a piece of memory is not directly
> > accessible by
> > the GPU we need to return an error and not try to use bounce
> > buffers behind
> > the surface.
> >
> > That is something which always annoyed me with the DMA API, which
> > is
> > otherwise rather cleanly defined.
>
> That is exactly what I want to fix with my series to make
> DMA_ATTR_NON_CONSISTENT more useful and always available:
>
> https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.linuxfoundation.org%2Fpipermail%2Fiommu%2F2018-December%2F031985.html&amp;data=02%7C01%7Cthellstrom%40vmware.com%7Cb1799c4073024a824f9408d67afcfcea%7Cb39138ca3cee4b4aa4d6cd83d9dd62f0%7C0%7C0%7C636831624340834140&amp;sdata=JiRBfzZMvN3joJ4vKiErbzXAHaNzuBcLapRJDL%2Bt6Hc%3D&amp;reserved=0
>
> With that you allocate the memory using dma_alloc_attrs with
> DMA_ATTR_NON_CONSISTENT, and use dma_sync_single_* to transfer
> ownership to the cpu and back to the device, with a gurantee that
> there won't be any bouncing. So far the interest by the parties that
> requested the feature has been rather lacklustre, though.

In the graphics case, it's probably because it doesn't fit the graphics
use-cases:

1) Memory typically needs to be mappable by another device. (the "dma-
buf" interface)
2) DMA buffers are exported to user-space and is sub-allocated by it.
Mostly there are no GPU user-space kernel interfaces to sync / flush
subregions and these syncs may happen on a smaller-than-cache-line
granularity.

So to help the graphics driver, that coherent flag would be much more
useful.

/Thomas