[Intel-gfx] [PATCH 5/7] drm/i915: Preallocate stashes for vma page-directories

[Matthew Auld]

On Wed, 8 Jul 2020 at 14:48, Chris Wilson <chris at chris-wilson.co.uk> wrote:
>
> We need to make the DMA allocations used for page directories to be
> performed up front so that we can include those allocations in our
> memory reservation pass. The downside is that we have to assume the
> worst case, even before we know the final layout, and always allocate
> enough page directories for this object, even when there will be overlap.
> This unfortunately can be quite expensive, especially as we have to
> clear/reset the page directories and DMA pages, but it should only be
> required during early phases of a workload when new objects are being
> discovered, or after memory/eviction pressure when we need to rebind.
> Once we reach steady state, the objects should not be moved and we no
> longer need to preallocating the pages tables.
>
> It should be noted that the lifetime for the page directories DMA is
> more or less decoupled from individual fences as they will be shared
> across objects across timelines.
>
> v2: Only allocate enough PD space for the PTE we may use, we do not need
> to allocate PD that will be left as scratch.
>
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Matthew Auld <matthew.auld at intel.com>

<snip>

>
> +static unsigned long pd_count(u64 size, int shift)
> +{
> +       /* Beware later misalignment */
> +       return (size + 2 * (BIT_ULL(shift) - 1)) >> shift;
> +}
> +
> +int i915_vm_alloc_pt_stash(struct i915_address_space *vm,
> +                          struct i915_vm_pt_stash *stash,
> +                          u64 size)
> +{
> +       unsigned long count;
> +       int shift = 21;
> +       int n;

if (gen >= 8)
    shift = 21;
else
    shift = 22;

?

Since pt=4M, pd=2G with the weird legacy ppgtt stuff?

> +
> +       count = pd_count(size, shift);
> +       while (count--) {
> +               struct i915_page_table *pt;
> +
> +               pt = alloc_pt(vm);
> +               if (IS_ERR(pt)) {
> +                       i915_vm_free_pt_stash(vm, stash);
> +                       return PTR_ERR(pt);
> +               }
> +
> +               pt->stash = stash->pt[0];
> +               stash->pt[0] = pt;
> +       }
> +
> +       for (n = 1; n < vm->top; n++) {
> +               shift += 9;
> +               count = pd_count(size, shift);
> +               while (count--) {
> +                       struct i915_page_directory *pd;
> +
> +                       pd = alloc_pd(vm);
> +                       if (IS_ERR(pd)) {
> +                               i915_vm_free_pt_stash(vm, stash);
> +                               return PTR_ERR(pd);
> +                       }
> +
> +                       pd->pt.stash = stash->pt[1];
> +                       stash->pt[1] = &pd->pt;
> +               }
> +       }
> +
> +       return 0;
> +}
> +