[Bug 55500] [sna gen4] corrupt rendering (including wrong rendering of characters and flickering on redraw)

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=55500

--- Comment #118 from sergio.callegari at gmail.com ---
Incorrect rendering of some glyphs is still there as of yesterday's git
snapshot (24/7/13). I'm reporting it as I read somewhere that newer released
versions of the driver implemented a lower limit on MAX_FLUSH_VERTICES in order
to reduce the impact of the bug, but I really see no difference. Yet, it may be
the case that the reduction is only applied to the released driver and not in
the devel version (haven't checked the actual code).

A couple of notes:

- The way in which the glyphs corruption appears is weird. Without any
scrolling, just typing in characters at some place causes some random character
to get corrupted somewhere else (e.g. maybe 1 line above, maybe 10 cm to the
right, etc). Some times typing some more character is enough to have the
corruption disappear, something it does not.  I have not been able to determine
if the glyph that gets corrupted is the same one that is being typed (i.e, I
type in an 'e' and somewhere and 'e' gets corrupted), but I suspect that this
is not the case. 

- The issue seems to be much less frequent if I type slowly.

- Once a glyph is corrupted, just 'selecting' some random character around it,
but not necessarily very close to it (e.g., 15 chars to the left or to the
right), seems to be enough to cause a redraw of the glyph that fixes its
rendering.  This seems somehow similar to how pressing 'PrtScrn' to try to get
a snapshot causes a redraw that fixes the wrong glyphs, so that it is difficult
to get a screenshot of the issue, unless one relies on a camera.

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