i915 and swiotlb_max_segment

[Konrad Rzeszutek Wilk]

On Mon, May 10, 2021 at 05:25:25PM +0200, Christoph Hellwig wrote:
> Hi all,
> 
> swiotlb_max_segment is a rather strange "API" export by swiotlb.c,
> and i915 is the only (remaining) user.
> 
> swiotlb_max_segment returns 0 if swiotlb is not in use, 1 if
> SWIOTLB_FORCE is set or swiotlb-zen is set, and the swiotlb segment
> size when swiotlb is otherwise enabled.
> 
> i915 then uses it to:
> 
>  a) decided on the max order in i915_gem_object_get_pages_internal
>  b) decide on a max segment size in i915_sg_segment_size
> 
> for a) it really seems i915 should switch to dma_alloc_noncoherent
> or dma_alloc_noncontigous ASAP instead of using alloc_page and
> streaming DMA mappings.  Any chance I could trick one of the i915
> maintaines into doing just that given that the callchain is not
> exactly trivial?
> 
> For b) I'm not sure swiotlb and i915 really agree on the meaning
> of the value.  swiotlb_set_max_segment basically returns the entire
> size of the swiotlb buffer, while i915 seems to use it to limit
> the size each scatterlist entry.  It seems like dma_max_mapping_size
> might be the best value to use here.

Yes. The background behind that was SWIOTLB would fail because well, the
size of the sg was too large. And some way to limit it to max size
was needed - the dma_max_mapping_size "should" be just fine.

> 
> Once that is fixed I'd like to kill off swiotlb_max_segment as it is
> a horribly confusing API.