On 10/08/2011 12:03 AM, Marek Olšák wrote:
On Fri, Oct 7, 2011 at 10:00 AM, Thomas Hellstromthomas@shipmail.org wrote:
OK. First I think we need to make a distinction: bo sync objects vs driver fences. The bo sync obj api is there to strictly provide functionality that the ttm bo subsystem is using, and that follows a simple set of rules:
- the bo subsystem does never assume sync objects are ordered. That means
the bo subsystem needs to wait on a sync object before removing it from a buffer. Any other assumption is buggy and must be fixed. BUT, if that assumption takes place in the driver unknowingly from the ttm bo subsystem (which is usually the case), it's OK.
- When the sync object(s) attached to the bo are signaled the ttm bo
subsystem is free to copy the bo contents and to unbind the bo.
- The ttm bo system allows sync objects to be signaled in different ways
opaque to the subsystem using sync_obj_arg. The driver is responsible for setting up that argument.
- Driver fences may be used for or expose other functionality or adaptions
to APIs as long as the sync obj api exported to the bo sybsystem follows the above rules.
This means the following w r t the patch.
A) it violates 1). This is a bug that must be fixed. Assumptions that if one sync object is singnaled, another sync object is also signaled must be done in the driver and not in the bo subsystem. Hence we need to explicitly wait for a fence to remove it from the bo.
B) the sync_obj_arg carries *per-sync-obj* information on how it should be signaled. If we need to attach multiple sync objects to a buffer object, we also need multiple sync_obj_args. This is a bug and needs to be fixed.
C) There is really only one reason that the ttm bo subsystem should care about multiple sync objects, and that is because the driver can't order them efficiently. A such example would be hardware with multiple pipes reading simultaneously from the same texture buffer. Currently we don't support this so only the *last* sync object needs to be know by the bo subsystem. Keeping track of multiple fences generates a lot of completely unnecessary code in the ttm_bo_util file, the ttm_bo_vm file, and will be a nightmare if / when we truly support pipelined moves.
As I understand it from your patches, you want to keep multiple fences around only to track rendering history. If we want to do that generically, i suggest doing it in the execbuf util code in the following way:
struct ttm_eu_rendering_history { void *last_read_sync_obj; void *last_read_sync_obj_arg; void *last_write_sync_obj; void *last_write_sync_obj_arg; }
Embed this structure in the radeon_bo, and build a small api around it, including *optionally* passing it to the existing execbuf utilities, and you should be done. The bo_util code and bo_vm code doesn't care about the rendering history. Only that the bo is completely idle.
Note also that when an accelerated bo move is scheduled, the driver needs to update this struct
OK, sounds good. I'll fix what should be fixed and send a patch when it's ready. I am now not so sure whether doing this generically is a good idea. :)
Marek
Marek, Any progress on this. The merge window is about to open soon I guess and we need a fix by then.
/Thomas