On Friday 18 February 2005 16:03, Keith Whitwell wrote:
Ben Skeggs wrote:
I still have a 100% reproducable bug which I need to find the cause of,
but time is once again a problem for me. If I move a window over the
top
of a glxgears window my machine locks up immediately, but sysrq still works
fine.
I just discovered (and should've checked before), that I can ssh in and successfuly
kill glxgears, then X returns to normal. I can have a partially covered glxgears
window and everything is fine, but as soon as the entire window (not incl. window
decorations) is covered, it seems that the 2d driver is unable to update the screen.
I think some of the other drivers do a 'sched_yeild()' or 'usleep(0)' in the zero cliprect case to get away from this sort of behaviour.
Well, I can reproduce this bug and I tracked it down. There are a number of problems here, and they all have to do with DMA buffer accounting.
The first (trivial) problem is that nr_released_bufs was never reset to 0. I've already fixed that in CVS.
The real problem is that the following situation can occur when we have zero cliprects:
1. The command buffer contains a DISCARD command for a DMA buffer.
2. We simply drop that command buffer because there are no cliprects, i.e. nothing can be drawn.
3. As a consequence, DMA buffers aren't freed.
4. The rendering loop continues even though DMA buffers have been leaked, which eventually causes all DMA buffers to be exhausted, and this causes an infinite loop in r300RefillCurrentDmaRegion.
The root cause is that we drop the command buffers with the DISCARD. I can see two possible solutions to this problem:
1. Wait until we have a cliprect again before submitting command buffers.
2. Submit command buffers even when we have no cliprects. The kernel module would basically ignore everything but the DISCARD commands.
3. Something else?
I don't like option (1) because it somehow assumes that the user program only cares about OpenGL (and that's quite selfish). There are many use cases where it is plainly the incorrect thing to do:
- A user running something like Quake in listenserver mode; if they switch away from Quake for some reason (incoming messages, whatever), the server will stop and eventuall all clients will timeout.
- Imagine a chat application that uses some fancy 3D graphics for whatever reason (glitz, for example). Now this application may just be in the middle of drawing something when the user moves some other application above it. The end result will be that the applications essentially becomes locked up until it becomes visible again; in the mean time, the chat might time out and disconnect the user.
So (1) clearly isn't a good solution.
Option (2) is more correct, but it does seem a little bit hackish.
Note that well written applications will notice when there are no cliprects and stop rendering. Probably gears will do this too, but it can't guarentee that you'll never see rendering in the zero-cliprect case.
Option 2 plus a yeild/usleep/whatever is pretty much standard.
Keith
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