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[Qemu-devel] Re: [[RfC PATCH]] linux fbdev display driver prototype.

From: Gerd Hoffmann
Subject: [Qemu-devel] Re: [[RfC PATCH]] linux fbdev display driver prototype.
Date: Tue, 25 May 2010 09:11:16 +0200
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100430 Fedora/3.0.4-2.fc12 Thunderbird/3.0.4

This looks very promissing.

I just got a couple of observations:

- Your patch does not work on my machine with the vesafb driver. It
reports "can't handle 8 bpp frame buffers". It turns out that the
vesafb driver seems to initialize the framebuffer in PSEUDOCOLOR

Depends on the video mode you ask for via vga=$nr, there are also 32bpp modes.

I think we should add a piece of code which tries reinitialize
the framebuffer with the suitable parametters (32bpp/TRUECOLOR).

With vesafb it wouldn't work anyway, you can't switch these parameters at runtime. I think the *drmfb fbdev interface is quite limited too in what it allows to change.

- You should register a Display Allocator and override the
create_displaysurface() method like I did in the DirectFB driver.
This way you save qemu a data copy. fbdev_render_32() should only be
used when the guest framebuffer is not compatible with the physical
framebuffer (guest_bpp != physical_bbp || guest_linesize !=

Isn't a trivial move though. If the console is switched you must stop drawing on the framebuffer.

Right now this is easy: just stop copying. Likewise restoring the screen when switching back is easy: just copy everything.

If we give out pointers to the framebuffer to other qemu code which doesn't know anything about console switching we have to be quite careful get things right ...

- A cool feature would be to be able to stretch the guest display in
fullscreen. My DirectFB driver implements a fullscreen toggle command
by pressing the Ctrl-Alt-Return keys. I think Stefano added a SDL
zoom feature a while ago which we could reuse for this.

The actual stretching is done by SDL I think. For that kind of stuff a rendering library is actually helpful ...

- I'm not very familiar with the scancode stuff, but I think that if
you set your VT fd in the K_RAW keyboard mode, you'll be able to get
true keyboard scancodes that you can directly give to the guest using
the kbd_put_keycode() function.

I'm not sure this is really portable. What do you get in K_RAW mode on !x86 platforms? K_MEDIUMRAW gives you linux input layer key codes no matter what. Also the translation to keysyms (for text consoles) is easier with mediumraw.


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