This file contains some talk about what is involved with a good
register-level graphics driver interface. This is an old file,
you might also consider reading 'man 7 svgalib.faq'

Section 1: Mode Setting

The following describes what happens during a mode set.

A request is made for a mode with given width, height, and color
resolution. Optionally the request can specify a specific pixel size,
scanline offset (line width), and pixel size (e.g. 3 vs. 4 pixels for
24-bit color modes), otherwise the driver chooses defaults for these
properties. It is the intention that requests for resolutions that don't
match an availabe mode timing can still be honoured by choosing a higher
resolution (for example, 700x500 is requested and 800x600 timing is
programmed). The scanline offset should ensure correct drawing of
graphics. 

The user-level driver tries to match the requested mode properties with
either fixed modes defined by the selected kernel module driver, or with a
flexible mode timing if the selected kernel module driver supports mode
timing. In the case of fixed modes, the user-level driver must ask the
kernel driver about the supported fixed modes, and in the case of mode
timings, the user-level driver must ask the kernel driver about physical
limits such as the amount of video memory, maximum pixel clocks for each
pixel size, range and mapping of horizontal timing parameters, and the
available pixel clock frequencies. In both cases, modes/timings that fall
outside of the configured monitor specs (probably stored in a file in
/etc) will not be selected. In the case of flexible mode timings, the
highest refresh timing that is possible will be selected. 

Once a mode timing or fixed mode is selected, the kernel module driver is
requested to set the mode on the hardware. A mode state is defined which
is a collection of VGA and driver-specific extended register values (byte
values) that describes the mode that the video card is set to. During the
mode set proper, the following actions are performed: 

1. The registers values that make up the state are read from the video
   card and stored in a state in system memory (this is the state of
   the video card before the mode set).

2. An initialization function modifies the values in the state according
   to the requested mode. Nothing is written to the card; only the state
   in system memory is changed.

3. The actual mode set on the hardware is performed by writing the values
   in the state to the corresponding registers on the video card.

A special case is VGA-compatible textmode state, which would normally be
active at the time of a graphics mode set. This mode cannot be initialized
in the way of (2) above. Instead, the mode is saved using (1), and the
resulting state is restored using (3) when textmode needs to be restored. 

Another special case is the setting of a mode for which (1) and (2) have
already been performed, and the resulting state has been saved. In this
case the mode can be set accomplished by just (3), provided that certain
hardware-specific settings on the card have not been changed in the time
between (1) and (3) (this would normally be the case). 

When a VT switch away from a graphics modes happens, the current hardware
state should be saved, instead of the initialized mode state, since an
application can have changed it (banking, displaystart, accelerator state
etc.). This is subject to all the extended registers being readable, which
may be a problem with some cards (note that for simple mode setting
followed by textmode restoration, this is not a problem since the mode
initialization overwrites most of the delicate extended registers, and the
saved textmode state is largely VGA registers, and doesn't deal with
delicate extended registers). A possible solution would be to have
functions that save and restore or re-initialize the 'drawing' state,
which would include banking, displaystart and accelerator registers. Upon
VT switching back, the mode initialization function would be used followed
by the restoring of the saved drawing state. 

Section 2: What should be in the kernel driver.

A decision that has to be made is whether all chipset specific settings
should be handled within the kernel module driver. There's a fair amount
of information required to select a suitable timing, which the user-level
driver will have to request, and it might have to do additional queries
during its evaluation. The abstractions used for this communication will
include a growing number of properties to accomodate new card/chipset
drivers. It is however desirable that the simple loading of a new module
driver is enough to make full use it, without the requirement of having an
updated user-level driver. 

Harm Hanemaayer (hhanemaa@cs.ruu.nl)