[PATCH 1/2] drm/ttm: add a way to bo_wait for either the last read or last write

[Thomas Hellstrom]

On 10/07/2011 12:42 AM, Marek Olšák wrote:
> On Wed, Oct 5, 2011 at 7:54 AM, Thomas Hellstrom<thomas at shipmail.org>  wrote:
>    
>> In any case, I'm not saying fences is the best way to flush but since the bo
>> code assumes that signaling a sync object means "make the buffer contents
>> available for CPU read / write", it's usually a good way to do it; there's
>> even a sync_obj_flush() method that gets called when a potential flush needs
>> to happen.
>>      
> I don't think we use it like that. To my knowledge, the purpose of the
> sync obj (to Radeon Gallium drivers at least) is to be able to wait
> for the last use of a buffer. Whether the contents can or cannot be
> available to the CPU is totally irrelevant.
>
> Currently (and it's a very important performance optimization),
> buffers stay mapped and available for CPU read/write during their
> first map_buffer call. Unmap_buffer is a no-op. The unmapping happens
> on buffer destruction. We only call bo_wait when we want to wait for
> the GPU until it's done with the buffer (we don't always want that,
> sometimes we want to use the unsychronized flag). Otherwise the
> contents of buffers are available at *any time*.
>
> We could probably implement bo_wait privately in the kernel driver and
> not use ttm_bo_wait. I preferred code sharing though.
>
> Textures (especially the tiled ones) are never mapped directly and a
> temporary staging resource is used instead, so we don't actually
> pollute address space that much. (in case you would have such a
> remark) We will use staging resources for buffers too, but it's really
> the last resort to avoid waiting when direct access can't be used.
>
>
>    
>>>> 2) Can't we say that a write_sync_obj is simply a sync_obj? What's the
>>>> difference between those two? I think we should remove the write_sync_obj
>>>> bo
>>>> member.
>>>>
>>>>          
>>> Okay, but I think we should remove sync_obj instead, and keep write
>>> and read sync objs. In the case of READWRITE usage, read_sync_obj
>>> would be equal to write_sync_obj.
>>>
>>>
>>>        
>> Sure, I'm fine with that.
>>
>> One other thing, though, that makes me a little puzzled:
>>
>> Let's assume you don't allow readers and writers at the same time, which is
>> my perception of how read- and write fences should work; you either have a
>> list of read fences or a single write fence (in the same way a read-write
>> lock works).
>>
>> Now, if you only allow a single read fence, like in this patch. That implies
>> that you can only have either a single read fence or a single write fence at
>> any one time. We'd only need a single fence pointer on the bo, and
>> sync_obj_arg would tell us whether to signal the fence for read or for write
>> (assuming that sync_obj_arg was set up to indicate read / write at
>> validation time), then the patch really isn't necessary at all, as it only
>> allows a single read fence?
>>
>> Or is it that you want to allow read- and write fences co-existing? In that
>> case, what's the use case?
>>      
> There are lots of read-write use cases which don't need any barriers
> or flushing. The obvious ones are color blending and depth-stencil
> buffering. The OpenGL application is also allowed to use a subrange of
> a buffer as a vertex buffer (read-only) and another disjoint subrange
> of the same buffer for transform feedback (write-only), which kinda
> makes me think about whether we should track subranges instead of
> treating a whole buffer as "busy". It gets even more funky with
> ARB_shader_image_load_store, which supports atomic read-modify-write
> operations on textures, not to mention atomic memory operations in
> compute shaders (wait, isn't that also exposed in GL as
> GL_ARB_shader_atomic_counters?).
>
> I was thinking whether the two sync objs should be "read" and
> "readwrite", or "read" and "write". I chose the latter, because it's
> more fine-grained and we have to keep at least two of them around
> anyway.
>
> So now that you know what we use sync objs for, what are your ideas on
> re-implementing that patch in a way that is okay with you? Besides
> removing the third sync objs of course.
>
> Marek
>    
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:

1) 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.

2) 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.

3) 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.

4) 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

/Thomas