From: Torgeir Veimo <Torgeir.Veimo@vertech.no>
  To  : ggi-develop@eskimo.com
  Date: Tue, 15 Sep 1998 21:03:27 +0000

Re: Frame buffer device thinkings... (fwd)

Stefan Mars wrote:
> 
> >From the Linux-Kernel list

Geert, you are reading this (GGI) list right? (Too busy to track
linux-kernel these days..)

> Currently a frame buffer device needs code to change the resolution and/or
> color depth. However, for sane hardware there's no need to put this code in the
> kernel. The question then is: how to tell the kernel that the video mode was
> changed?

A video mode implies a hardware state. Without discussing wether this
should be done in kernel or user space, I suggest you look at the notion
of HW state, and setting / getting / restoring such.

> Since the kernel can't change the video mode anymore, the video mode has to be
> the same for all VCs associated with one video board, so it has to change for
> all VCs at once (can be done by fbmem.c). I think that's acceptable.

>   - where's the MMIO region?

Or better yet; how to write to it from user space, using either FIFO's
or DMA buffers.

> Panning is more difficult: a user space program could pan the screen, but a VC
> switch can no longer change the screen offsets. But since XFree86
> disables/enables VC switching, it could restore the screen offsets.

Or better yet, restore (switch) HW state.

The reason I'm interested in saving and restoring of HW state is because
the kernel can do this fast (if the hardware supports it) to enable
virtualized direct HW access, eg. letting an X server and an OpenGL
rendering process both access the hardware without knowing about each
other. (They only know the window coordinates and graphcis ID's to make
them visible correct wrt. clipping and windows overlapping).

Such a HW state would not include the graphics mode in this context, but
one could separate graphic mode state and accelerator mode state.
Several more advanced graphics accelerators are beginning to support the
notion of different types of graphics states.

-- 
-Torgeir