Re: [Gumstix-users] "Nokia" Displays from Spark Fun...

[Dave H. <dhy...@gm...>]

Hi Holly,

> I haven't gotten around to trying this out yet. Do you remember if there
> is a simple way to set up a shadow framebuffer? Is is just as easy as
> doing some kind of mknod in /dev (but then how do you get the fb memory
> allocated and linked to /dev/fb1 or whatever)? We are using the linux
> command line to start pxafb with parameters; do you know of some way to
> start two fb devices in that manner?

The actual shadow buffer itself is just a hunk of allocated memory
that can then be mmap'd into userspace.

Then there's a whole wack of infrastructure that you need if you want
to make this look like a framebuffer to X-Windows. If you don't care
about X-Windows compatability, then it's pretty simple. The very first
framebuffer I wrote for microwindows didn't have any X-Windows
compatability.

Doing the mknod basically just provides a mapping from a symbolic name
(like fb1) to a major number. There isn't any particular reason why
fb1 and fb0 have to be mapped to the same major number or the same
driver.

Drivers can be loaded whether a /dev entry exists or not, and /dev
entries can exist even when there is no driver. The /dev entries are
really just a fancy lookup table. Using devfs allows a driver to
advertise itself and you don't need to do a mknod, but that's really
more of a convenience/management thing.

If you made the second framebuffer a loadable module, then you could
provide command line options at load time (through a startup script),
or you could configure it to use kernel command line parameters, or
you can use /proc entries (or maybe you don't even have any command
line options). It's pretty arbitrary if you're writing the driver :)

--
Dave Hylands
Vancouver, BC, Canada
http://www.DaveHylands.com/