Created attachment 96626 [details]
Image of font corruption in Emacs

After upgrading from xf86-video-intel-2.20.13 to xf86-video-intel-2.21.15 (both with sna enabled), I see font corruption appear.  This font corruption did not occur with xf86-video-intel-2.20.13 and does not appear when UXA is used instead.  

The font corruption occurs across several applications (Firefox, LibreOffice, Emacs).  The corruption does not occur reliably (i.e, not in the same place every time), but happens often.  The font corruption disappears when I highlight the corrupted text (I assume causing it to be redrawn).  


I have found other reports of apparently the same bug at:
https://forums.gentoo.org/viewtopic-t-972582.html (my post),
https://bbs.archlinux.org/viewtopic.php?id=171128, and
http://forum.manjaro.org/index.php?topic=7269.0


Bug 37946 appears similar, but the font corruption appears different; in my case, the corrupted character does not resemble the actual character.  

Some examples shown in bug 55500 also appear similar, but I do NOT experience any of the other problems mentioned: flicker, image corruption, and parts of the screen blanked.  Due to the differences, I believe I am experiencing a different bug.  


Up until recently, my workaround was to use the older xf86-video-intel-2.20.13, but a recent ABI change in my upgrade from xorg-server-1.14.3-r2 to xorg-server-1.15.0 required using the newer xf86-video-intel-2.21.15.  

I am using Gentoo Linux on this computer (if it makes a difference).  I'll be happy to do what I can to help, but I'm not very familiar with the code & build environment yet.  Please let me know if I should provide any additional information or examples to help debug this problem.  


lspci -vnn:
00:02.0 VGA compatible controller [0300]: Intel Corporation Mobile 4 Series Chipset Integrated Graphics Controller [8086:2a42] (rev 07) (prog-if 00 [VGA controller])
        Subsystem: Lenovo Device [17aa:20e4]
        Flags: bus master, fast devsel, latency 0, IRQ 42
        Memory at f0000000 (64-bit, non-prefetchable) [size=4M]
        Memory at d0000000 (64-bit, prefetchable) [size=256M]
        I/O ports at 1800 [size=8]
        Expansion ROM at <unassigned> [disabled]
        Capabilities: [90] MSI: Enable+ Count=1/1 Maskable- 64bit-
        Capabilities: [d0] Power Management version 3
        Kernel driver in use: i915

00:02.1 Display controller [0380]: Intel Corporation Mobile 4 Series Chipset Integrated Graphics Controller [8086:2a43] (rev 07)
        Subsystem: Lenovo Device [17aa:20e4]
        Flags: bus master, fast devsel, latency 0
        Memory at f0400000 (64-bit, non-prefetchable) [size=1M]
        Capabilities: [d0] Power Management version 3