Re: my opinion about VGA devices

[Richard B. Johnson]

On Wed, 20 Oct 2004, Tomas Carnecky wrote:

> Richard B. Johnson wrote:
>
> > On ix86 machines, regardless of whatever code initialized the
> > hardware, if the screen-card is not put into graphics mode,
> > anybody can write characters and attributes at 0xb8000 directly
> > to the screen. Even user-mode code can mmap() that area and write
> > to it. So, the key seems to be to get out of graphics mode
> > before suspend, and go back later after resume.
>
> Why do you let user-mode programs access the hardware directly?
> You don't do this with network devices (there you have syscalls), you don't 
> do this with sound devices (alsa).

Any root process can mmap() any of the memory-mapped hardware
including network devices. This isn't normally done because
handling interrupts from such hardware isn't very efficient
in user-mode, and redistributing data meant for another
process would be a nightmare. However, it can be done.

> IMO it makes a proper power managment implementation impossible.

Wrong. The 'normal' user can't do such I/O, root can. See iopl(), which
sets the I/O privilege level. This has nothing to do with power-
management.

> You can say that there are two different drivers for screen-cards in the 
> kernel. One is the VGA which enables the card during early boot time to 
> display the first text messages and the other is fb/DRI or even an nvidia/ati 
> kernel module which is enabled later on.

Doesn't require two drivers.

> Last time I've tried a LiveCD distro I've seen a nice boot console with 
> background picture, high resolution (1024x768) and nice small font. That 
> means that the framebuffer driver had to be initialized at that time. I don't 
> have framebuffer drivers compiled into my kernel so I don't know at which 
> point these are initialized, but it must be at a quite early point in the 
> boot process.

Even Fedora, which boots in a 'graphical' mode, really boots standard
text-mode until 'init' gets control. They just hide the console output
by setting the grub command-line parameter, "quiet".

The kernel messages are still available using `dmesg`. If you want
to eliminate any possibility of losing kernel messages because
the kernel failed to get up all the way, just use /dev/ttyS0 as
your console during boot.

> When looking at the output of dmesg I can see that the first thing that is 
> initialized are the CPU's, ACPI, IRQ's and then the PCI bus is scanned. Did 
> anyones machine crash during these steps? I don't think a healthy box will 
> crash here. And at this point you can initialize your graphics card driver 
> like it is done in the LiveCD distro.
>
> tom

Cheers,
Dick Johnson
Penguin : Linux version 2.6.9 on an i686 machine (5537.79 GrumpyMips).
                 98.36% of all statistics are fiction.
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