Selecting default framebuffer through kernel command-line ?
yashi at atmark-techno.com
Wed Feb 15 06:14:28 UTC 2012
I wasn't sure what the "default framebuffer" you meant. so, would you
clarify a bit?
At Wed, 15 Feb 2012 10:56:06 +0530,
Tushar Behera wrote:
> Currently the LCD FB is registered as /dev/fb0 and the HDMI FB is
> registered as /dev/fb1. While booting up, by default /dev/fb0 is used as
> the primary display device and the output is routed accordingly. To use
> the HDMI display, we need to update /etc/X11/xorg.conf (and this needs
> to be updated every time we update the file-system.)
can you clarify what you mean by the primary display device, and
who's output is routed to there? is it the kernel log, some boot
splash, android or X?
> If we don't enable support for LCD FB, the display comes on HDMI during
> boot-time, but again that is not desirable considering we want to use
> both the LCD and HDMI with the same kernel binary.
> Is there any way where we can force the default frame-buffer (whether to
> choose /dev/fb0 or /dev/fb1) during boot-time, might be through some
> kernel command-line options?
it seems like fbcon has a kernel param for tty mapping. does it help?
taken from: Documentation/fb/fbcon.txt
> 3. fbcon=map:<0123>
> This is an interesting option. It tells which driver gets mapped to
> which console. The value '0123' is a sequence that gets repeated until
> the total length is 64 which is the number of consoles available. In
> the above example, it is expanded to 012301230123... and the mapping
> will be:
> tty | 1 2 3 4 5 6 7 8 9 ...
> fb | 0 1 2 3 0 1 2 3 0 ...
> ('cat /proc/fb' should tell you what the fb numbers are)
> One side effect that may be useful is using a map value that exceeds
> the number of loaded fb drivers. For example, if only one driver is
> available, fb0, adding fbcon=map:1 tells fbcon not to take over the
> Later on, when you want to map the console the to the framebuffer
> device, you can use the con2fbmap utility.
BTW, last I checked, android uses /dev/fb0 directly; meaning framework
source code has the string "/dev/fb0" planted deep inside. so under
the android framework, it always displays to the device (LCD or HDMI)
the kernel detected first.
my two cents,
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