[Xorg] DRI merging

[Alan Cox]

On Sul, 2004-06-13 at 22:41, Jon Smirl wrote:
> On modern processors the user/system transition is minor compared to the time
> needed for a bitblt over the PCI bus. 

As a percentage of system time the user/system transition cost has been
rising not falling on x86 processors. Its especially bad when you want
large memory support on 32bit x86 (x86-64 isnt so bad here)

[If you want to verify my claim just try Larry McVoy's lmbench latency
tests on different setups]

Really however it isnt an important debate. The DDX and XAA driver
interface means the driver module is responsible for

	- Obtain access to resources
	- Drive resources directly or indirectly
	- Arbitrate resources with OS according to OS policy

So you can write an XOrg driver that does everything kernel side if you
wish. For some DMA cards such as the radeon the existing code already
does sometimes use kernel side support for DMA rings when DRI modules
are present.

The core X server really doesn't care what "obtain access to resources"
and "drive resources" come down to. The Linux fb driver uses the kernel
for resources, the Glide driver uses an external drawing library and
many other drivers bang on the hardware.

As you move up the layers you can take more control of the rendering
decisions too. XAA figures out how to use the existing 2D drawing
subsystems and not much else. You can hook above XAA to achieve other
things. Thus it may ultimately be a mistake to see Xorg and Keith's work
ultimately as seperate things. I see no reason an Xorg server cannot
exist that uses Mesa as its base drawing library on one screen and XAA
on another.

> These arguments are different for an embedded device where everything runs as
> root. Go ahead and program everything from user space to get the performance
> gain. Worms and trojans aren't as big of problem for embedded devices.

Tell that to the Japanese mobile phone manufacturers. 

PS: and before telling me to get a $35 new card, please tell me where it
fits in my PA-RISC box, on my iPaq, in my old thinkpad laptop, and so
on. Xorg is a bit bigger than PCdom.

Alan