Re: [Linux-fbdev-devel] drm_fb_helper: Impossible to change video

[James Simmons <jsimmons@infradead.org>]

> > > > The fb_ops can only be called from fbcon or the fbdev userland interface. 
> > > > The fbcon calls should only happen when the VC is in KD_TEXT mode. Now 
> > > > with the DRM backend we have the advantage of creating a mapping seperate 
> > > > from the console mapping. A fb_open/fb_close could be used to cleaning up 
> > > > the userland mmap as well as handle the console pinning. We can supply 
> > > > your own fb_mmap hook.
> > > 
> > > Again, the issue is not userspace but that fb_ops hooks can be called
> > > from interrupt context etc.
> > 
> > This should not happen. The VT layer is protected from calling in the 
> > interrupt context. TNor does fbcon call any fb_ops methods from a irq.
> 
> E.g. printk?

Sorry I have been sick. This is true when you are in KD_TEXT mode and using fbcon. 
Now while working on a graphical console yes you can have other hardware devices do
a printk which can alter the state of of your graphics card. As for debugging a 
graphics driver you have to be very careful with your printks. Usually in that case
I use two graphics cards in my system and map one to fbcon and test the other
graphcis cards driver.

P.S
	The fbdev api is such that it doesn't always require a permanent framebuffer
mapping. This was done for the case of graphical hardware that lacks a directly 
accessable framebuffer. If you have accelerated fillrect,copyarea,and imageblit then 
you don't even the mapping except in the case of someone calling mmap on /dev/fbX.
I noticed the nouveau drivers has a accelerated fbcon :-)