Nouveau fences?

Thomas Hellstrom thomas at
Sun Nov 28 23:28:27 PST 2010

On 11/28/2010 10:55 PM, Francisco Jerez wrote:
> Thomas Hellstrom<thomas at>  writes:
>> On 11/28/2010 05:11 PM, Francisco Jerez wrote:
>>> Francisco Jerez<currojerez at>   writes:
>>>> Thomas Hellstrom<thomas at>   writes:
>>>>> Ben,
>>>>> I'm looking at a way to make TTM memory management asynchronous with
>>>>> the CPU. The idea is that you should basically be able to DMA data to
>>>>> and from memory regions without waiting for idle, as long as the GPU
>>>>> has a means to provide operation ordering.
>>>> Sounds good. I guess you're mainly dealing with BO eviction
>>>> synchronization? The only problem I see on our side is that calls to our
>>>> move() hook aren't guaranteed to be carried out in order (because of the
>>>> multiple hardware channels). I'm thinking that move() could be extended
>>>> with an optional sync_obj argument, that way move() would be able to
>>>> make sure that evictions are strictly ordered with respect to the fence
>>>> specified.
>> The way evictions will work is that they appear to take place
>> "instantly", but are scheduled on a channel, and there will be a data
>> structure that keeps track about what fences need to be signaled
>> before a managed area can be reused.
>> The driver will need to provide a function that, given a list of
>> fences, returns a fence that when it signals, guarantees that all
>> other fences in the list have signaled.
> Ah, so, evictions made in response to ttm_bo_mem_force_space() are still
> going to be synchronous after the changes you have in mind (because in
> that case you need to reuse the freed memory immediately), right?
No and yes. Evictions will be asynchronous, but the new user of the 
memory area needs to
take appropriate action to make sure it doesn't overwrite old contents. 
If it's a CPU upload, it needs to wait on a fence. Single-channel GPU 
with dma uploads needs to do nothing.
Multi-channel GPU needs to insert a barrier before uploading, that waits 
on the eviction DMA.

So you're right in that we need to give the new move function 
information on what to wait on / insert barriers for. I was initially 
thinking of a single fence object (and that's why the order function is 

> In other cases (e.g. evictions triggered by BO validation), what exactly
> would we gain from this function? I mean, why can't we just push waiting
> down to ttm_bo_move_ttm/memcpy?

That's essentially what's going to happen, but those functions also need 
to know what exactly to wait on.

>> Single-channel hardware will just return the fence with the highest
>> sequence. Multi-channel hardware may need to insert command stream
>> barriers if available and create a new sync object to return or resort
>> to simply waiting to determine which fence signals last.
>> I guess Nouveau can do command stream barriers, (waiting for other
>> channels to reach a certain command before progressing?)
> Yep, that's what nouveau_fence_sync() does.

OK, thanks.


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