Xinerama infrastructure

[Mike A. Harris]

On Mon, 16 Feb 2004, Benjamin Herrenschmidt wrote:

>> (to be fair, on certain cards, in my specific case the matrix g400, 
>> xinerama is indeed as slow as molasses, for as yet unknown reasons -
>> although it's nice to know that it's tied only to certain cards, and
>> not the entire universe :)
>
>I suspect the biggest reason is that by having 2 outputs, you
>double the load on the card's bus, thus slowing down everything

Yes, but this isn't any weakness in X, or in the video drivers 
per se.  The person's email implies that Xinerama itself needs to 
be somehow accelerated, which makes no sense.  2D graphics are 
what is accelerated or not.  If Xinerama is used, then 2 or more 
screens are being used.  Yes the bandwidth over the bus may 
double, but it's _still_ being 2D accelerated.

I don't understand the concept of "accelerated Xinerama", unless 
perhaps merged framebuffer pseudo-xinerama is faster somehow for 
2D.

What exactly is "accelerated Xinerama" supposed to mean?  Unless 
we have a clear technical explanation of what _specifically_ is 
meant by that in technical terms, it is just guesswork as to what 
the original poster means really.

For example, it is not like Windows drivers are going to do 
accelerated graphics to two displays any faster than X can 
becaues of some magic equivalent of whatever "Xinerama 
acceleration" is supposed to mean.  Speaking purely in terms of 
2D acceleration - Xinerama is already accelerated unless a given 
driver disables it, or is broken in Xinerama mode.

Perhaps I just don't grok the non-technical aspect of the 
discussion however.  ;o)

-- 
Mike A. Harris