Re: [PATCH] drm/ttm: pass buffer object for bind/unbind callback

[Thomas Hellstrom <thellstrom@xxxxxxxxxx>]

On 11/19/2011 11:54 PM, Jerome Glisse wrote:

As mentioned previously, and in the discussion with Ben, the page tables
would not need to be rebuilt on each CS. They would be rebuilt only on the
first CS following a move_notify that caused a page table invalidation.

move_notify:
if (is_incompatible(new_mem_type)) {
 bo->page_tables_invalid = true;
 invalidate_page_tables(bo);
}

command_submission:
if (bo->page_tables_invalid) {
  set_up_page_tables(bo);
  bo->page_tables_invalid = false;
}
    

Why is it different from updating page table in move notify ? I don't
see any bonus here, all the information we need are already available
in move_notify.

  

I've iterated the pros of this approach at least two times before, but for completeness let's do it again:

8<----------------------------------------------------------------------------------------------------

1) TTM doesn't need to care about the driver re-populating its GPU page
tables.
Since swapin is handled from the tt layer not the bo layer, this makes it a
bit easier on us.
2) Transition to page-faulted GPU virtual maps is straightforward and
consistent. A non-page-faulting driver sets up the maps at CS time, A
pagefaulting driver can set them up directly from an irq handler without
reserving, since the bo is properly fenced or pinned when the pagefault
happens.
3) A non-page-faulting driver knows at CS time exactly which
page-table-entries really do need populating, and can do this more
efficiently.

8<-----------------------------------------------------------------------------------------------------

And some extra items like partially populated TTMs that were mentioned elsewhere.

Memory types in TTM are completely orthogonal to allowed GPU usage. The GPU
may access a bo if it's reserved, fenced or pinned, regardless of its
placement.

A TT memory type is a *single* GPU aperture that may be mapped from the
aperture side by the CPU (AGP). It may also be used by a single unmappable
aperture that wants to use TTM's range management and eviction (vmwgfx GMR).
The driver can define more than one such memory type (psb), but a bo can
only be placed in one of those at a time, so this approach is unsuitable for
multiple apertures pointing to the same pages.
    

radeon virtual memory have a special address space, the system address
space, it's managed by ttm through a TTM_TT (exact same code as
current one). All the other address space are not managed by ttm but
we require a bo to be bound to ttm_tt to be use, thought we can relax
that. That's the reason why i consider system placement as different.

  

Yes for Radeon system memory may be different, and that's fine. But as also previously mentioned we're trying to design a generic interface here, in which we need to consider GPU- mappable system memory.

I think the pros of this interface design compared to populating in bo_move are pretty well established, so can you please explain why you keep arguing against it? What is it that I have missed?

/Thomas

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