problem with exaBufferGlyph()

[Michael <macallan%netbsd.org@localhost>]

Hello,

I've sent this to xorg-devel@ as well.

First some context - I've been writing EXA support for Permedia 2 and
3, mostly because these cards are still kinda useful on sparc and alpha
hardware. For pm2 there's actually documentation, and the chip can be
used to accelerate at least some xrender operations.
The problem - this chip can't deal with A8 masks for rendering glyphs.
It's perfectly happy to render ARGB though, and that's a problem with
current EXA.
As it is right now, exaGlyphs() will call CheckComposite() with an A8
Picture as destination to see if the driver supports that, and fall
back to ARGB if it doesn't. That's fine, although it may be better to
do that test once on startup instead of every time a glyph needs to be
drawn.
The problem is, that exaBufferGlyph() will always cache glyphs in the
format returned by GetGlyphPicture(), not the one requested in the
destination Picture handed to it. To drivers that can't support A8 this
will render the cache unusable to the accelerator, which results in
glyphs constantly being copied back & forth between video and main
memory, which kills performance to the point that software rendering is
faster.

So, what I'm proposing is something like this:
diff -u -r1.2 exa_glyphs.c
--- exa_glyphs.c        22 Dec 2016 21:31:08 -0000      1.2
+++ exa_glyphs.c        23 Dec 2016 05:42:08 -0000
@@ -544,7 +544,20 @@
                INT16 ySrc, INT16 xMask, INT16 yMask, INT16 xDst, INT16 yDst)
 {
     ExaScreenPriv(pScreen);
+    /*
+     * XXX
+     * Request the new glyph in the format we need to draw in, not whatever
+     * GetGlyphPicture() hands us, which will (almost?) always be A8.
+     * That way drivers that can't handle A8 but can do Xrender ops in ARGB
+     * will be able to do hardware rendering in and out of the glyph cache.
+     * This results in a major performance boost on such hardware.
+     * Drivers that can handle A8 shouldn't see any difference.
+     */
+#if 1
+    unsigned int format = pDst->format;
+#else
     unsigned int format = (GetGlyphPicture(pGlyph, pScreen))->format;
+#endif
     int width = pGlyph->info.width;
     int height = pGlyph->info.height;
     ExaCompositeRectPtr rect;

Without this, I get about 9000/sec with x11perf -aaftext on an Ultra 60
- software rendering yields 15000/s. With this it's 75000/s. Not
earth-shatteringly fast but still more than what I expected from such
an old chip that wasn't exactly known for its speed even back in its
day.

Any thoughts? Am I missing something?

have fun
Michael