[PATCH 1/6] DownloadFromScreenCS: download via a scratch BO if pixmap domain is unknown

[Karl Tomlinson]

Michel Dänzer writes:

> On Mon, 2010-08-23 at 18:27 +1200, Karl Tomlinson wrote: 
>> 
>> radeon_bo_is_busy() may return without setting the domain out-parameter.
>> If this happens, then download via a scratch GTT BO to avoid CPU VRAM read.
>
> I'm on the fence about this one. I can see how it could help *if*
> radeon_bo_is_busy() doesn't set the domain[0] and the BO happens to be
> in VRAM, but OTOH it might also hurt if the BO is not in VRAM? Can you
> elaborate a bit on how often you're encountering this condition and what
> if any measurements you've made about the impact on performance of this
> patch and possibly how often the BO is actually in VRAM or not in those
> cases.
>
> [0] AFAICT that can only happen if messages like
>
>         radeon 0000:00:10.0: ee7e7400 reserve failed for wait
>
> appear in dmesg, which only happens very rarely here and might be
> fixable in the kernel anyway.

I guess ERESTARTSYS is not likely with no wait, so yes, I guess I
would have seen this message, should this have happened.
I haven't seen this message, so maybe it hasn't happened.

I changed this because I knew the disaster that resulted from CPU
VRAM read and didn't want that to happen.

Is it not possible for another process to have reserved the BO
(through DRI perhaps)?

Even if this is unlikely/uncommon, I think it is still worth
choosing the right behavior here.  Even if it only happens rarely,
a 2 second glitch gets noticed.

Again, the consequences of a CPU read from VRAM are much worse, I
expect/hope, than the consequences of GPU blit from GTT.  Maybe a
GPU blit when the BO is in CPU domain would be bad, but is it
likely to be busy in that situation?

I expect that the BO is usually in VRAM.  Pixmaps are created in
VRAM, UTS moves them to VRAM often enough, and most (or all) write
operations move them to VRAM.

I suspect the most likely time that the BO would not be in VRAM is
if it is has been evicted because it is not in use, and so the
choice doesn't matter (because it is not busy).