On Mon, Oct 10, 2016 at 02:30:40PM +0100, Tvrtko Ursulin wrote:
>
> On 10/10/2016 12:49, Chris Wilson wrote:
> >commit 1625e7e549c5 ("drm/i915: make compact dma scatter lists creation
> >work with SWIOTLB backend") took a heavy handed approach to undo the
> >scatterlist compaction in the face of SWIOTLB. (The compaction hit a bug
> >whereby we tried to pass a segment larger than SWIOTLB could handle.) We
> >can be a little more intelligent and try compacting the scatterlist up
> >to the maximum SWIOTLB segment size (when using SWIOTLB).
> >
> >Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> >CC: Imre Deak <imre.deak@xxxxxxxxx>
> >CC: Daniel Vetter <daniel.vetter@xxxxxxxx>
> >Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> >---
> > drivers/gpu/drm/i915/i915_gem.c | 23 +++++++++++------------
> > 1 file changed, 11 insertions(+), 12 deletions(-)
> >
> >diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> >index ca1a5a5c6f19..8b3474d215a5 100644
> >--- a/drivers/gpu/drm/i915/i915_gem.c
> >+++ b/drivers/gpu/drm/i915/i915_gem.c
> >@@ -2201,6 +2201,7 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
> > struct sgt_iter sgt_iter;
> > struct page *page;
> > unsigned long last_pfn = 0; /* suppress gcc warning */
> >+ unsigned long max_segment;
>
> unsigned int would be enough.
>
Current maximum object size >> PAGE_SHIFT is 36 bits. We don't impose
any other restriction that would limit a sg chunk.
> > int ret;
> > gfp_t gfp;
> >@@ -2211,6 +2212,12 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
> > GEM_BUG_ON(obj->base.read_domains & I915_GEM_GPU_DOMAINS);
> > GEM_BUG_ON(obj->base.write_domain & I915_GEM_GPU_DOMAINS);
> >+ max_segment = obj->base.size;
> >+#ifdef CONFIG_SWIOTLB
> >+ if (swiotlb_nr_tbl())
> >+ max_segment = IO_TLB_SEGSIZE << PAGE_SHIFT;
> >+#endif
> >+
>
> Do you want to use IS_ENABLED here?
The symbol swiotlb_nr_tbl() is absent unless SWIOTLB is enabled at compile
time. So we need the cpp guard, or do you mean switch to
#if IS_ENABLED(CONFIG_SWIOTLB)
which we probably should indeed.
> > st = kmalloc(sizeof(*st), GFP_KERNEL);
> > if (st == NULL)
> > return ERR_PTR(-ENOMEM);
> >@@ -2252,15 +2259,9 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
> > goto err_pages;
> > }
> > }
> >-#ifdef CONFIG_SWIOTLB
> >- if (swiotlb_nr_tbl()) {
> >- st->nents++;
> >- sg_set_page(sg, page, PAGE_SIZE, 0);
> >- sg = sg_next(sg);
> >- continue;
> >- }
> >-#endif
> >- if (!i || page_to_pfn(page) != last_pfn + 1) {
> >+ if (!i ||
> >+ sg->length >= max_segment ||
>
> I think this can overflow by a page, should be "sg->length >=
> (max_segment - PAGE_SIZE)", or alternatively substract one page at
> the max_segment assignment.
We are looking at the previous sg, right? (and we only ever increment by
PAGE_SIZE).
So: when the previous sg reaches the maximum length, start a new sg
element. Otherwise we extend the previous sg element by a PAGE, so on
the else branch the maximum of sg->length after the increment is
max_segment.
> >+ page_to_pfn(page) != last_pfn + 1) {
> > if (i)
> > sg = sg_next(sg);
> > st->nents++;
> >@@ -2273,9 +2274,7 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
> > /* Check that the i965g/gm workaround works. */
> > WARN_ON((gfp & __GFP_DMA32) && (last_pfn >= 0x00100000UL));
> > }
> >-#ifdef CONFIG_SWIOTLB
> >- if (!swiotlb_nr_tbl())
> >-#endif
> >+ if (st->nents < st->orig_nents)
> > sg_mark_end(sg);
>
> I wondered a few times that we could just terminate the table
> unconditionally.
The caveat being that if we do insert orig_nents, then sg at this point
is NULL. Which is clearer:
if (st->nents < st->orig_nents) sg_mark_end(sg);
or
if (sg) sg_mark_end(sg); /* coalesced sg table */
?
-Chris
--
Chris Wilson, Intel Open Source Technology Centre
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