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

["hch@lst.de" <hch@lst.de>]

On Tue, Jan 15, 2019 at 06:03:39PM +0000, Thomas Hellstrom wrote:
> 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)

And there is nothing preventing dma-buf sharing of these buffers.
Unlike the get_sgtable mess it can actually work reliably on
architectures that have virtually tagged caches and/or don't
guarantee cache coherency with mixed attribute mappings.

> 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.

I know of no architectures that can do cache maintainance on a less
than cache line basis.  Either the instructions require you to
specifcy cache lines, or they do sometimes more, sometimes less
intelligent rounding up.

Note that as long dma non-coherent buffers are devices owned it
is up to the device and the user space driver to take care of
coherency, the kernel very much is out of the picture.