On Thu, Aug 28, 2014 at 9:36 PM, Alexandre Courbot [off-list ref] wrote:

On Thu, Aug 28, 2014 at 12:19 PM, jonsmirl@gmail.com [off-list ref] wrote:

quoted

Is there some existing way to do early printk type output to a
framebuffer that has been set up by the bootloader?  early printk is
before any device drivers are loaded.

If not, what would it take to create a way to do this? Something along
the lines of build in the fbdev library and give it an address plus
x/y layout of the buffer. Assume that everything else is set up and
anything written to the buffer will appear on the display. Then hook
into where the kernel does early printk on uarts and add in support
for this buffer. The core fbdev library implements scrolling and
graphical characters.

simplefb does something like this (implement a console on top of a
framebuffer set up by the bootloader), but I don't think you can use
it for earlyprintk. It would be a very interesting option though (and

simplefb is a device driver so it doesn't process the early output.
This would need to be some custom code that gets the framebuffer
address  and x/y setup very early in the boot process. I'm fairly sure
nothing like it current exists.

even better if it could be used by the kernel decompression code), but
I suspect this is more involved than good old UART as you need to
manage fonts, pixel format, screen geometry, etc.

OTOH it would be very useful for some retail devices that come without
an accessible serial line, but have a framebuffer set up by the
bootloader.

Alex.

-- 
Jon Smirl
jonsmirl@gmail.com