[PATCH] drm/ttm: remove fence_lock
maarten.lankhorst at canonical.com
Thu Oct 18 04:38:23 PDT 2012
Op 18-10-12 13:02, Thomas Hellstrom schreef:
> On 10/18/2012 10:37 AM, Maarten Lankhorst wrote:
>> 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
Well either block on reserve or return -EBUSY if reserved, presumably the
ttm_bo_vm_fault does the latter already, anyway.
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 next time someone wants to do a wait it will go through the fastpath and
unset the sync_obj member, since it's already signaled, or it's removed when
ttm_execbuffer_util is used.
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