Bug 59085

Summary:	Buffer not flushed when drawing R plots
Product:	cairo	Reporter:	Milan Bouchet-Valat <nalimilan>
Component:	xlib backend	Assignee:	Chris Wilson <chris>
Status:	RESOLVED NOTOURBUG	QA Contact:	cairo-bugs mailing list <cairo-bugs>
Severity:	normal
Priority:	medium
Version:	1.12.8
Hardware:	x86-64 (AMD64)
OS:	All
Whiteboard:
i915 platform:		i915 features:

Description Milan Bouchet-Valat 2013-01-06 17:50:22 UTC

This bug appeared somewhere between Cairo 1.10.2 and 1.12.4, as I could check by downgrading my packages. It has been experienced on Fedora [1] and Arch [2], at least.

Using the R statistical framework, plots are not drawn until one resizes the plot window. Disabling buffered rendering using X11.options(type="nbcairo") fixes the problem. If you are interested in reproducing the bug, calling plot(1:10) is enough.

The problem has been reported by people with Intel GPUs (i915 driver) and with ATI GPUs (Catalyst driver). So it seems to be a Cairo bug, or at least a change in Cairo unveiled a bug in these drivers.

If you have any suspicion of the commit that may have introduced this, I can test that version.



1: https://bugzilla.redhat.com/show_bug.cgi?id=891983
2: https://bugs.archlinux.org/task/32597

Comment 1 Uli Schlachter 2013-01-06 18:26:27 UTC

Broad guess: R is not calling cairo_surface_flush() when it should. Before this function is called, the content of the drawable used is undefined.
Cairo 1.12 makes more use of this requirement (which has been there since forever). I would be very surprised if this were actually a bug in cairo.

Comment 2 Uli Schlachter 2013-01-06 19:48:34 UTC

Ok, I went and checked R's source code:

R-2.15.2/src/modules/X11/devX11.c, function handleEvent(), line 750. This is the handling of Expose events and it just does a cairo_paint(xd->xcc). This also needs cairo_surface_flush(xd->xcs).

Comment 3 Milan Bouchet-Valat 2013-01-07 10:17:24 UTC

Thanks for looking at the code, but your suggestion does not make any difference in my tests. The problem is, the Expose event is never triggered by X so that code is not run.

I also tried adding cairo_surface_flush() calls in many other places, with no result. So I sent a mail to R-devel, let's see what they say:
https://mailman.stat.ethz.ch/pipermail/r-devel/2013-January/065567.html

Thanks!

Comment 4 Milan Bouchet-Valat 2013-01-16 14:41:08 UTC

Sorry for reopening, but somebody downstream told me the bug does not happen using the Nouveau driver - so it would be an Intel specific bug. Does that help you identifying where the bug might be? Is it still possible that it is an R bug? I guess you're in the best position to tell me who I should complain to. ;-)

Thanks!

Comment 5 Milan Bouchet-Valat 2013-01-16 17:36:31 UTC

Actually, the bug happens with ATI's Catalyst proprietary driver, but not with NVidia's. To me, this sounds like the NVidia driver forces a draw/flush/something, while the others do not. Is that of any help?

Comment 6 Milan Bouchet-Valat 2013-01-16 19:18:33 UTC

Damn, I was wrong again. Let me restate the situation: Intel and ATI Catalyst drivers suffer from the bug; Nouveau (*not* NVidia proprietary) does not suffer from it.

Comment 7 Milan Bouchet-Valat 2013-01-19 11:06:35 UTC

OK, R developers have found how to use cairo_surface_flush() to fix the bug (where I failed), so you were right.

But they complained that cairo_surface_flush() documentation "does not support this usage". Do you think this could be improved?

Thanks for your help so far!

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