some half-baked ttm ideas

[Thomas Hellström (Intel)]

On 9/16/20 6:28 AM, Dave Airlie wrote:
> On Wed, 16 Sep 2020 at 14:19, Dave Airlie <airlied at gmail.com> wrote:
>> On Wed, 16 Sep 2020 at 00:12, Christian König
>> <ckoenig.leichtzumerken at gmail.com> wrote:
>>> Hi Dave,
>>>
>>> I think we should just completely nuke ttm_tt_bind() and ttm_tt_unbind()
>>> and all of that.
>>>
>>> Drivers can to this from their move_notify() callback now instead.
>> Good plan, I've put a bunch of rework into the same branch,
>>
>> https://github.com/airlied/linux/commits/ttm-half-baked-ideas
>>
>> but I've fried my brain a bit, I'm having trouble reconciling move
>> notify and unbinding in the right places, I feel like I'm circling
>> around the answer but haven't hit it yet.
> drm/ttm: add unbind to move notify paths.
>
> In that tree is incorrect and I think where things fall apart, since
> if we are moving TTM to VRAM that will unbind the TTM object from the
> GTT at move notify time before the move has executed.
>
> I'm feeling a move_complete_notify might be an idea, but I'm wondering
> if it's a bad idea.
>
> Dave.

I don't know if this complicates things more, but move_notify was 
originally only thought to be an invalidation callback, and was never 
intended to drive any other actions in the driver than to invalidate 
various GPU bindings.

The idea was that TTM should really never set up any GPU bindings, but 
just provide memory where it was gpu-bindable and make sure it was 
CPU-mappable where needed. The "exception" was mappable AGP-type 
gpu-bindings, for the simple reason that they were needed to provide 
CPU-mappings on systems where you couldn't map the pages directly. But 
since we set up a GPU map on these systems anyway, many (most) drivers 
just made use of that, but others took the step further insisting on 
using move_notify() to set up GPU bindings, which was never intended and 
adds error paths in the TTM move code that are pretty hard to follow.

So if we're changing things here,  I'd vote for the following:

* Driver calls ttm_bo_validate to put memory where it is cpu-mappable 
and gpu-bindable
* On successful validate, driver sets up GPU bindings itself.

* move_notify only invalidates GPU bindings and should really return a void.

So that bind() and unbind() stuff is really only needed for cpu-map 
through aperture. If we ditch that, then we need to re-define the task 
of TTM to provide memory in a cpu-mappable location and figure how 
drivers that require cpu-map-through-aperture should handle this, since 
they can't use the TTM fault handler for that memory anymore. The same 
holds for drivers that want to manage their translation table 
themselves, and needs some cpu-mapping operations to go through the 
aperture rather than to the pages directly.

If the driver has no special cpu-mapping requirements, it should be 
perfectly legal for it to not provide any bind() or unbind() functionality.

/Thomas


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