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 reservation held, for example to switch GPU engine or to delete a GPU bind, waiting for reserve will in many case mean wait for GPU. /Thomas
