Re: [PATCH v4] drm/i915: stop using swiotlb

[Robert Beckett <bob.beckett@xxxxxxxxxxxxx>]

On 26/07/2022 14:12, Tvrtko Ursulin wrote:
> On 25/07/2022 15:18, Robert Beckett wrote:
> > Calling swiotlb functions directly is nowadays considered harmful. See
> > https://lore.kernel.org/intel-gfx/20220711082614.GA29487@xxxxxx/
> >
> > Replace swiotlb_max_segment() calls with dma_max_mapping_size().
> > In i915_gem_object_get_pages_internal() no longer consider max_segment
> > only if CONFIG_SWIOTLB is enabled. There can be other (iommu related)
> > causes of specific max segment sizes.
> >
> > Cc: Christoph Hellwig <hch@xxxxxx>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>
> > Cc: Thomas Hellstrom <thomas.hellstrom@xxxxxxxxx>
> > Cc: Matthew Auld <matthew.auld@xxxxxxxxx>
> >
> > v2: - restore UINT_MAX clamp in i915_sg_segment_size()
> >      - drop PAGE_SIZE check as it will always be >= PAGE_SIZE
> > v3: - actually clamp to UINT_MAX in i915_sg_segment_size()
> > v4: - round down max segment size to PAGE_SIZE
> >
> > Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> > Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> > Signed-off-by: Robert Beckett <bob.beckett@xxxxxxxxxxxxx>
> > ---
> >   drivers/gpu/drm/i915/gem/i915_gem_internal.c | 19 ++++---------------
> >   drivers/gpu/drm/i915/gem/i915_gem_shmem.c    |  2 +-
> >   drivers/gpu/drm/i915/gem/i915_gem_ttm.c      |  4 ++--
> >   drivers/gpu/drm/i915/gem/i915_gem_userptr.c  |  2 +-
> >   drivers/gpu/drm/i915/i915_scatterlist.h      | 17 ++++-------------
> >   5 files changed, 12 insertions(+), 32 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_internal.c 
> > b/drivers/gpu/drm/i915/gem/i915_gem_internal.c
> > index c698f95af15f..24f37658f1bb 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_internal.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_internal.c
> > @@ -6,7 +6,6 @@
> >   #include <linux/scatterlist.h>
> >   #include <linux/slab.h>
> > -#include <linux/swiotlb.h>
> >   #include "i915_drv.h"
> >   #include "i915_gem.h"
> > @@ -38,22 +37,12 @@ static int 
> > i915_gem_object_get_pages_internal(struct drm_i915_gem_object *obj)
> >       struct scatterlist *sg;
> >       unsigned int sg_page_sizes;
> >       unsigned int npages;
> > -    int max_order;
> > +    int max_order = MAX_ORDER;
> > +    unsigned int max_segment;
> >       gfp_t gfp;
> > -    max_order = MAX_ORDER;
> > -#ifdef CONFIG_SWIOTLB
> > -    if (is_swiotlb_active(obj->base.dev->dev)) {
> > -        unsigned int max_segment;
> > -
> > -        max_segment = swiotlb_max_segment();
> > -        if (max_segment) {
> > -            max_segment = max_t(unsigned int, max_segment,
> > -                        PAGE_SIZE) >> PAGE_SHIFT;
> > -            max_order = min(max_order, ilog2(max_segment));
> > -        }
> > -    }
> > -#endif
> > +    max_segment = i915_sg_segment_size(i915->drm.dev) >> PAGE_SHIFT;
> > +    max_order = min(max_order, ilog2(max_segment));
> >       gfp = GFP_KERNEL | __GFP_HIGHMEM | __GFP_RECLAIMABLE;
> >       if (IS_I965GM(i915) || IS_I965G(i915)) {
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c 
> > b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> > index 4eed3dd90ba8..34b9c76cd8e6 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_shmem.c
> > @@ -194,7 +194,7 @@ static int shmem_get_pages(struct 
> > drm_i915_gem_object *obj)
> >       struct intel_memory_region *mem = obj->mm.region;
> >       struct address_space *mapping = obj->base.filp->f_mapping;
> >       const unsigned long page_count = obj->base.size / PAGE_SIZE;
> > -    unsigned int max_segment = i915_sg_segment_size();
> > +    unsigned int max_segment = i915_sg_segment_size(i915->drm.dev);
> >       struct sg_table *st;
> >       struct sgt_iter sgt_iter;
> >       struct page *page;
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c 
> > b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> > index 5a5cf332d8a5..7a828c9c0f6d 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_ttm.c
> > @@ -189,7 +189,7 @@ static int i915_ttm_tt_shmem_populate(struct 
> > ttm_device *bdev,
> >       struct drm_i915_private *i915 = container_of(bdev, 
> > typeof(*i915), bdev);
> >       struct intel_memory_region *mr = 
> > i915->mm.regions[INTEL_MEMORY_SYSTEM];
> >       struct i915_ttm_tt *i915_tt = container_of(ttm, 
> > typeof(*i915_tt), ttm);
> > -    const unsigned int max_segment = i915_sg_segment_size();
> > +    const unsigned int max_segment = 
> > i915_sg_segment_size(i915->drm.dev);
> >       const size_t size = (size_t)ttm->num_pages << PAGE_SHIFT;
> >       struct file *filp = i915_tt->filp;
> >       struct sgt_iter sgt_iter;
> > @@ -568,7 +568,7 @@ static struct i915_refct_sgt 
> > *i915_ttm_tt_get_st(struct ttm_tt *ttm)
> >       ret = sg_alloc_table_from_pages_segment(st,
> >               ttm->pages, ttm->num_pages,
> >               0, (unsigned long)ttm->num_pages << PAGE_SHIFT,
> > -            i915_sg_segment_size(), GFP_KERNEL);
> > +            i915_sg_segment_size(i915_tt->dev), GFP_KERNEL);
> >       if (ret) {
> >           st->sgl = NULL;
> >           return ERR_PTR(ret);
> > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c 
> > b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
> > index 094f06b4ce33..dfc35905dba2 100644
> > --- a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
> > +++ b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
> > @@ -129,7 +129,7 @@ static void 
> > i915_gem_object_userptr_drop_ref(struct drm_i915_gem_object *obj)
> >   static int i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
> >   {
> >       const unsigned long num_pages = obj->base.size >> PAGE_SHIFT;
> > -    unsigned int max_segment = i915_sg_segment_size();
> > +    unsigned int max_segment = i915_sg_segment_size(obj->base.dev->dev);
> >       struct sg_table *st;
> >       unsigned int sg_page_sizes;
> >       struct page **pvec;
> > diff --git a/drivers/gpu/drm/i915/i915_scatterlist.h 
> > b/drivers/gpu/drm/i915/i915_scatterlist.h
> > index 9ddb3e743a3e..1377a97a20cf 100644
> > --- a/drivers/gpu/drm/i915/i915_scatterlist.h
> > +++ b/drivers/gpu/drm/i915/i915_scatterlist.h
> > @@ -9,7 +9,7 @@
> >   #include <linux/pfn.h>
> >   #include <linux/scatterlist.h>
> > -#include <linux/swiotlb.h>
> > +#include <linux/dma-mapping.h>
> >   #include "i915_gem.h"
> > @@ -127,19 +127,10 @@ static inline unsigned int 
> > i915_sg_dma_sizes(struct scatterlist *sg)
> >       return page_sizes;
> >   }
> > -static inline unsigned int i915_sg_segment_size(void)
> > +static inline unsigned int i915_sg_segment_size(struct device *dev)
> >   {
> > -    unsigned int size = swiotlb_max_segment();
> > -
> > -    if (size == 0)
> > -        size = UINT_MAX;
> > -
> > -    size = rounddown(size, PAGE_SIZE);
> > -    /* swiotlb_max_segment_size can return 1 byte when it means one 
> > page. */
> > -    if (size < PAGE_SIZE)
> > -        size = PAGE_SIZE;
> > -
> > -    return size;
> > +    size_t max = min_t(size_t, UINT_MAX, dma_max_mapping_size(dev));
>
> Hope your patience is not growing thin but blank line here as per 
> checkpatch. I could fudge when applying though.

no worries, I'll issue a new version shortly.
I only recently guided a mentee to "always do your checkpatch, even if 
only doing a quick 1 character change, you never know if it you make a 
small mistake"
I guess do as I say, not as I do applies here :)

> Otherwise it finally passed CI, yay! Now do we dig deeper to a) 
> understand how/where it failed and b) check what are the possible return 
> values from dma_max_mapping_size(), or call it done?

I will have to call it done myself as I am unable to repo it locally.
I could spend a lot of time debugging it via trybot, but I expect it 
will be easier, quicker and cheaper to get someone with local hw with a 
repro case to debug it if you want to root cause it.

> Regards,
>
> Tvrtko
>
> > +    return round_down(max, PAGE_SIZE);
> >   }
> >   bool i915_sg_trim(struct sg_table *orig_st);