[Nouveau] Synchronization mostly missing?

[Francisco Jerez]

Hi,

Luca Barbieri <luca at luca-barbieri.com> writes:

> It seems that Noveau is assuming that once the FIFO pointer is past a
> command, that command has finished executing, and all the buffers it
> used are no longer needed.
>
> However, this seems to be false at least on G71.
> In particular, the card may not have even finished reading the input
> vertex buffers when the pushbuffer "fence" triggers.
> While Mesa does not reuse the buffer object itself, the current
> allocator tends to return memory that has just been freed, resulting
> in the buffer actually been reused.
> Thus Mesa will overwrite the vertices before the GPU has used them.
>
> This results in all kinds of artifacts, such as vertices going to
> infinity, and random polygons appearing.
> This can be seen in progs/demos/engine, progs/demos/dinoshade,
> Blender, Extreme Tux Racer and probably any non-trivial OpenGL
> software.
>

Can you reproduce this with your vertex buffers in VRAM instead of GART?
(to rule out that it's a fencing issue).

> The problem can be significantly reduced by just adding a waiting loop
> at the end of draw_arrays and draw_elements, or by synchronizing
> drawing by adding and calling the following function instead of
> pipe->flush in nv40_vbo.c:
> I think the remaining artifacts may be due to missing 2D engine
> synchronization, but I'm not sure how that works.
> Note that this causes the CPU to wait for rendering, which is not the
> correct solution
>
> static void nv40_sync(struct nv40_context *nv40)
> {
> 	nouveau_notifier_reset(nv40->screen->sync, 0);
>
> //	BEGIN_RING(curie, 0x1d6c, 1);
> //	OUT_RING(0x5c0);
>
> //	static int value = 0x23;
> //	BEGIN_RING(curie, 0x1d70, 1);
> //	OUT_RING(value++);
>
> 	BEGIN_RING(curie, NV40TCL_NOTIFY, 1);
> 	OUT_RING(0);
> 	
> 	BEGIN_RING(curie, NV40TCL_NOP, 1);
> 	OUT_RING(0);
> 	
> 	FIRE_RING(NULL);
>
> 	nouveau_notifier_wait_status(nv40->screen->sync, 0, 0, 0);
> }
>
> It seems that NV40TCL_NOTIFY (which must be followed by a nop for some
> reason) triggers a notification of rendering completion.
> Furthermore, the card will probably put the value set with 0x1d70
> somewhere, where 0x1d6c has an unknown use
> The 1d70/1d6c is frequently used by the nVidia driver, with 0x1d70
> being a sequence number, while 0x1d6c is always set to 0x5c0, while
> NV40TCL_NOTIFY seems to be inserted on demand.
> On my machine, setting 0x1d6c/0x1d70 like the nVidia driver does
> causes a GPU lockup. That is probably because the location where the
> GPU is supposed to put the value has not been setup correctly.
>
> So it seems that the current model is wrong, and the current fence
> should only be used to determine whether the pushbuffer itself can be
> reused.
> It seems that, after figuring out where the GPU writes the value and
> how to use the mechanism properly, this should be used by the kernel
> driver as the bo->sync_obj implementation.
> This will delay destruction of the buffers, and thus prevent
> reallocation of them, and artifacts, without synchronizing rendering.
>
> I'm not sure why this hasn't been noticed before though.
> Is everyone getting randomly misrendered OpenGL or is my machine
> somehow more prone to reusing buffers?
>
> What do you think? Is the analysis correct?
> _______________________________________________
> Nouveau mailing list
> Nouveau at lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/nouveau
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