On 2021-06-03 10:17, Tvrtko Ursulin wrote:
Hi,
On 03/06/2021 09:40, Joonas Lahtinen wrote:
- Tvrtko to take a look
Quoting Konrad Rzeszutek Wilk (2021-05-20 18:12:58)
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.
Can't say I am 100% at home here but what I remember is that the limiting factor was maximum size of a sg segment and not total size of the mapping.
Looking at the code today, if we would replace usage swiotlb_max_segment() with dma_max_mapping_size(), I don't see that would work when we call dma_map_sg_attrs().
Because AFAICT code can end up in dma_direct_max_mapping_size() (not sure when the ops->map_sg path is active and where to trace that) where we have:
size_t dma_direct_max_mapping_size(struct device *dev) { /* If SWIOTLB is active, use its maximum mapping size */ if (is_swiotlb_active() && (dma_addressing_limited(dev) || swiotlb_force == SWIOTLB_FORCE)) return swiotlb_max_mapping_size(dev); return SIZE_MAX; }
So for all swiotlb cases, including force, we get:
size_t swiotlb_max_mapping_size(struct device *dev) { return ((size_t)IO_TLB_SIZE) * IO_TLB_SEGSIZE; }
Which is fixed and doesn't align with swiotlb_max_segment(). But you guys are the experts here so please feel to correct me.
But swiotlb_max_segment is also effectively fixed for a given system coinfigration, at either a page (under certain circumstances), or a value considerably larger than what the longest mappable SG segment actually is. Neither seems particularly useful, and to be honest I suspect the forced-bounce cases only set it to a page as a sledgehammer to make things work *because* the "normal" value is nonsense.
Robin.