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[Qemu-devel] Cirrus issues

From: Mark
Subject: [Qemu-devel] Cirrus issues
Date: Mon, 1 Jul 2013 16:16:41 +0100
User-agent: SquirrelMail/1.4.21


The developer of WinUAE has been adding support for Cirrus-based graphics
cards to the program, based on the Qemu Cirrus emulation. Apparently he
has discovered some bugs, which I thought I'd post about here in case
someone who knows about that code wants to take a look.

[begin quote]

Graphics glitches are actually caused by bug in cirrus logic blitter
emulation, it has incorrect "blit is outside of vram" tests, it assumes
all blits are normal copy blits, for example it can incorrectly reject
color expansion blits (used by RectFill).

Because I am too lazy to create logins to qemu bugtracker or mailing list,
at least not yet, here are bugs I have found:

cirrus_vga.c: BLTUNSAFE() macro is broken. (Graphics glitches are gone if
I uncomment them all, of course it can introduce crashes if there are
actual invalid blits..)

cirrus_vga.c: cirrus_do_copy() "extra x, y", s->cirrus_blt_srcpitch or
s->cirrus_blt_dstpitch can be zeros: division by zero.

CIRRUS_BLT_STATUS bit was missing. PicassoII driver polls this to check if
blit has finished.

And one thing that may not be a bug:

vga.c: vga_draw_graphic() seems to assume shift_control =
(s->gr[VGA_GFX_MODE] >> 5) & 3; must be >=2 (bit 6 of Mode Register is
"256 color mode") or it uses standard VGA graphics mode, even if cirrus
extended registers are used to enabled extended (SVGA) modes. My
workaround was to set bit 6 when any extended mode is enabled.

[end quote]

Original forum posting URL: http://eab.abime.net/896935-post30.html

-- Mark

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