[PATCH] drm/ttm: remove fence_lock

Thomas Hellstrom thomas at shipmail.org
Thu Oct 18 04:55:39 PDT 2012

On 10/18/2012 01:38 PM, Maarten Lankhorst wrote:
> Op 18-10-12 13:02, Thomas Hellstrom schreef:
>> On 10/18/2012 10:37 AM, Maarten Lankhorst wrote:
>>> Hey,
>>> Op 18-10-12 09:59, Thomas Hellstrom schreef:
>>>> On 10/18/2012 09:28 AM, Thomas Hellstrom wrote:
>>>>> Hi, Maarten,
>>>>> As you know I have been having my doubts about this change.
>>>>> To me it seems insane to be forced to read the fence pointer under a
>>>>> reserved lock, simply because when you take the reserve lock, another
>>>>> process may have it and there is a substantial chance that that process
>>>>> will also be waiting for idle while holding the reserve lock.
>>> I think it makes perfect sense, the only times you want to read the fence
>>> is when you want to change the members protected by the reservation.
>> No, that's not true. A typical case (or the only case)
>> is where you want to do a map with no_wait semantics. You will want
>> to be able to access a buffer's results even if the eviction code
>> is about to schedule an unbind from the GPU, and have the buffer
>> reserved?
> Well either block on reserve or return -EBUSY if reserved, presumably the
> former..
> ttm_bo_vm_fault does the latter already, anyway

ttm_bo_vm_fault only trylocks to avoid a deadlock with mmap_sem, it's really
a waiting reserve but for different reasons. Typically a user-space app 
will keep
asynchronous maps to TTM during a buffer lifetime, and the buffer 
lifetime may
be long as user-space apps keep caches.
That's not the same as accessing a buffer after the GPU is done with it.

> You don't need to hold the reservation while performing the wait itself though,
> you could check if ttm_bo_wait(no_wait_gpu = true) returns -EBUSY, and if so
> take a ref to the sync_obj member and then wait after unreserving. You won't
> reset sync_obj member to NULL in that case, but that should be harmless.
> This will allow you to keep the reservations fast and short. Maybe a helper
> would be appropriate for this since radeon and nouveau both seem to do this.

The problem is that as long as other users are waiting for idle with 
held, for example to switch GPU engine or to delete a GPU bind, waiting
for reserve will in many case mean wait for GPU.


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