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

[Ben Skeggs <skeggsb@gmail.com>]

On Fri, 2011-11-18 at 15:30 +0100, Thomas Hellstrom wrote:
> On 11/18/2011 02:15 PM, Ben Skeggs wrote:
> > On Fri, 2011-11-18 at 08:57 +0100, Thomas Hellstrom wrote:
> >    
> >> Jerome,
> >>
> >> I don't like this change for the following reasons
> >>      
> > -snip-
> >
> >    
> >>> One can take advantage of move notify callback but, there are
> >>> corner case where bind/unbind might be call without move notify
> >>> being call (in error path mostly). So to make sure that each
> >>> virtual address space have a consistent view of wether a buffer
> >>> object is backed or not by system page it's better to pass the
> >>> buffer object to bind/unbind.
> >>>        
> > As I discussed previously with Jerome on IRC, I believe the move_notify
> > hook is sufficient.  I fixed a couple of issues back with it back when I
> > implemented support for this in nouveau.
> >
> > Jerome did point out two issues however, which I believe can be solved
> > easily enough.
> >
> > The first was that the error path after move_notify() has been called
> > doesn't reverse the move_notify() too, leaving incorrect page table
> > entries.  This, I think, could easily be solved by swapping bo->mem and
> > mem, and re-calling move_notify() to have the driver reverse whatever it
> > did.
> >
> > The second issue is that apparently move_notify() doesn't get called in
> > the destroy path.  Nouveau's GEM layer takes care of this for our
> > userspace bos, and we don't use any kernel bos that are mapped into a
> > channel's address space so we don't hit it.  This can probably be solved
> > easily enough too I expect.
> >
> > Ben.
> >
> >    
> 
> OK. I understand. Surely if a move_notify is missing somewhere, that can 
> easily be fixed.
> 
> It might be good if we can outline how the virtual tables are set up. In 
> my world, the following would happen:
> 
> 1) Pre command submission:
> 
> a) reserve bo
> b) call ttm_bo_validate to set placement. move_notify will tear down any 
> existing GPU page tables for the bo.
> c) Set up GPU page tables.
Nouveau doesn't do this exactly.  I wanted to avoid having to check if a
bo was bound to a particular address space on every command submission.

We allocate the virtual address when a client first gets a reference to
a bo (ie. creates new buffer, or opens an existing handle). If the
buffer happens to be in VRAM or TT memory at this point, it's also
mapped.

And as buffers move around, we get called by TTM through move_notify()
of course, and do any mapping/unmapping that's necessary as a result.

Ben.

> d) Command submission
> e) unreserve_bo().
> 
> 
> 2) When eviction happens:
> a) reserve bo
> b) move_notify is called to tear down page tables
> c) change placement
> d) Unreserve bo.
> 
> Let's say an error occurs in 1d) Why would you need to undo 1c?
> 
> Similarly if an error occurs in 2c) Why would you  need to undo 2b)?
> 
> Thanks,
> Thomas
> 
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