Re: [PATCH 6/7] drm/radeon: allow asynchronous waiting on foreign fences

["Christian König" <deathsimple@vodafone.de>]

Am 04.09.2014 um 14:08 schrieb Maarten Lankhorst:
> Hey,
>
> Op 04-09-14 om 13:54 schreef Christian König:
>> Am 04.09.2014 um 13:42 schrieb Maarten Lankhorst:
>>> Use the semaphore mechanism to make this happen, this uses signaling
>>> from the cpu instead of signaling by the gpu.
>> I'm not sure if this will work reliable when the semaphores are in system memory. We might need to reserve some VRAM for them instead.
>>
>> Regards,
>> Christian.
> Why would it be unreliable? I mostly kept it in semaphore for simplicity.

The semaphore block tries to avoid memory accesses whenever possible.

For example when a signal for address A arrives the block doesn't 
necessary writes that to memory but instead tries to match it 
immediately with a wait for address A. Similar is true if a wait for 
address A arrives and the semaphore block thinks it knows the memory 
value at address A.

Also I'm not sure if the semaphore block really polls the memory address 
for changes, instead it might just snoop the MC for writes to this 
address. Since CPU writes to system memory aren't seen by the GPU MC the 
semaphore block would never know something changed.

I need to check the docs how to do this correctly,
Christian.

>
> ~Maarten
>