Re: Migrating ports from XFree86 to Xorg (Was: Changing default X11_TYPE from native to modular for some platforms)

[Michael <macallan1888%gmail.com@localhost>]

Hello,

> I don't see how we can use wsfb, though. The genfb driver does not support
> the graphics card's text mode, which we need for acceptable performance
> on such slow hardware. Or am I missing something? So we still have the
> problem to make new Xorg drivers.

You can switch to graphics on ioctl(WSDISPLAYIO_SMODE, WSDISPLAYIO_MAPPED) and 
back on WSDISPLAYIO_EMUL. Some drivers use it to switch the hardware to 24bit 
for X while running the console in 8 bit. See cgfourteen, crmfb and ag10e for 
example.
Also, vcons has a few tricks to speed up dumb framebuffers ( seriously, is the 
blitter /that/ slow? ) - use VCONS_DONT_READ ( see for example the genfb, 
cgfourteen and cgthree drivers ) which eliminates framebuffer reads and instead 
just redraws whatever needs to be redrawn from the relevant text buffer which 
gives a nice speedup on slow, dumb framebuffers. 
If you run something VGA-ish in text mode you may also want to think about 
implementing blitter support - Cirrus and S3 docs shouldn't be that hard to 
find.
You will want to implement at least an accelerated putchar() using whatever 
colour expansion scheme your hardware supports so you don't have to map the 
framebuffer at all ( and VCONS_DONT_READ with an accelerated putchar() may 
scroll faster than the blitter on some old hardware )
I don't know what caching modes the various 68k CPUs support, many more modern 
ones support something that's still uncached but allows for write combining, 
prefetching, relaxed ordering or any combination thereof, which is useful for 
accessing device memory ( like framebuffers ). This can be enabled using the 
BUS_SPACE_MAP_PREFETCHABLE flag in bus_space_(m)map() if your underlying pmap 
supports it. I added support for this on PowerPC ( don't set the G bit for 
relaxed ordering, buffering etc. ), ARM ( allow write buffering ), MIPS ( use 
the 'accelerated' CCA if the CPU supports it ) - if your CPUs have anything 
like this you may want to support it, any little speedup helps and on slow 
hardware with dumb framebuffers it's often quite noticeable. Even if you have 
hardware acceleration the Xserver will want to access video memory at some 
point.

have fun
Michael