325 lines
12 KiB
Plaintext
325 lines
12 KiB
Plaintext
The Framebuffer Console
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=======================
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The framebuffer console (fbcon), as its name implies, is a text
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console running on top of the framebuffer device. It has the functionality of
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any standard text console driver, such as the VGA console, with the added
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features that can be attributed to the graphical nature of the framebuffer.
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In the x86 architecture, the framebuffer console is optional, and
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some even treat it as a toy. For other architectures, it is the only available
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display device, text or graphical.
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What are the features of fbcon? The framebuffer console supports
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high resolutions, varying font types, display rotation, primitive multihead,
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etc. Theoretically, multi-colored fonts, blending, aliasing, and any feature
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made available by the underlying graphics card are also possible.
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A. Configuration
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The framebuffer console can be enabled by using your favorite kernel
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configuration tool. It is under Device Drivers->Graphics Support->Support for
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framebuffer devices->Framebuffer Console Support. Select 'y' to compile
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support statically, or 'm' for module support. The module will be fbcon.
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In order for fbcon to activate, at least one framebuffer driver is
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required, so choose from any of the numerous drivers available. For x86
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systems, they almost universally have VGA cards, so vga16fb and vesafb will
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always be available. However, using a chipset-specific driver will give you
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more speed and features, such as the ability to change the video mode
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dynamically.
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To display the penguin logo, choose any logo available in Logo
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Configuration->Boot up logo.
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Also, you will need to select at least one compiled-in fonts, but if
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you don't do anything, the kernel configuration tool will select one for you,
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usually an 8x16 font.
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GOTCHA: A common bug report is enabling the framebuffer without enabling the
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framebuffer console. Depending on the driver, you may get a blanked or
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garbled display, but the system still boots to completion. If you are
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fortunate to have a driver that does not alter the graphics chip, then you
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will still get a VGA console.
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B. Loading
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Possible scenarios:
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1. Driver and fbcon are compiled statically
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Usually, fbcon will automatically take over your console. The notable
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exception is vesafb. It needs to be explicitly activated with the
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vga= boot option parameter.
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2. Driver is compiled statically, fbcon is compiled as a module
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Depending on the driver, you either get a standard console, or a
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garbled display, as mentioned above. To get a framebuffer console,
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do a 'modprobe fbcon'.
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3. Driver is compiled as a module, fbcon is compiled statically
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You get your standard console. Once the driver is loaded with
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'modprobe xxxfb', fbcon automatically takes over the console with
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the possible exception of using the fbcon=map:n option. See below.
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4. Driver and fbcon are compiled as a module.
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You can load them in any order. Once both are loaded, fbcon will take
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over the console.
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C. Boot options
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The framebuffer console has several, largely unknown, boot options
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that can change its behavior.
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1. fbcon=font:<name>
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Select the initial font to use. The value 'name' can be any of the
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compiled-in fonts: VGA8x16, 7x14, 10x18, VGA8x8, MINI4x6, RomanLarge,
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SUN8x16, SUN12x22, ProFont6x11, Acorn8x8, PEARL8x8.
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Note, not all drivers can handle font with widths not divisible by 8,
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such as vga16fb.
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2. fbcon=scrollback:<value>[k]
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The scrollback buffer is memory that is used to preserve display
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contents that has already scrolled past your view. This is accessed
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by using the Shift-PageUp key combination. The value 'value' is any
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integer. It defaults to 32KB. The 'k' suffix is optional, and will
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multiply the 'value' by 1024.
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3. fbcon=map:<0123>
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This is an interesting option. It tells which driver gets mapped to
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which console. The value '0123' is a sequence that gets repeated until
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the total length is 64 which is the number of consoles available. In
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the above example, it is expanded to 012301230123... and the mapping
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will be:
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tty | 1 2 3 4 5 6 7 8 9 ...
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fb | 0 1 2 3 0 1 2 3 0 ...
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('cat /proc/fb' should tell you what the fb numbers are)
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One side effect that may be useful is using a map value that exceeds
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the number of loaded fb drivers. For example, if only one driver is
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available, fb0, adding fbcon=map:1 tells fbcon not to take over the
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console.
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Later on, when you want to map the console the to the framebuffer
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device, you can use the con2fbmap utility.
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4. fbcon=vc:<n1>-<n2>
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This option tells fbcon to take over only a range of consoles as
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specified by the values 'n1' and 'n2'. The rest of the consoles
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outside the given range will still be controlled by the standard
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console driver.
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NOTE: For x86 machines, the standard console is the VGA console which
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is typically located on the same video card. Thus, the consoles that
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are controlled by the VGA console will be garbled.
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4. fbcon=rotate:<n>
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This option changes the orientation angle of the console display. The
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value 'n' accepts the following:
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0 - normal orientation (0 degree)
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1 - clockwise orientation (90 degrees)
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2 - upside down orientation (180 degrees)
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3 - counterclockwise orientation (270 degrees)
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The angle can be changed anytime afterwards by 'echoing' the same
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numbers to any one of the 2 attributes found in
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/sys/class/graphics/fbcon
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rotate - rotate the display of the active console
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rotate_all - rotate the display of all consoles
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Console rotation will only become available if Console Rotation
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Support is compiled in your kernel.
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NOTE: This is purely console rotation. Any other applications that
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use the framebuffer will remain at their 'normal'orientation.
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Actually, the underlying fb driver is totally ignorant of console
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rotation.
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C. Attaching, Detaching and Unloading
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Before going on on how to attach, detach and unload the framebuffer console, an
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illustration of the dependencies may help.
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The console layer, as with most subsystems, needs a driver that interfaces with
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the hardware. Thus, in a VGA console:
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console ---> VGA driver ---> hardware.
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Assuming the VGA driver can be unloaded, one must first unbind the VGA driver
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from the console layer before unloading the driver. The VGA driver cannot be
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unloaded if it is still bound to the console layer. (See
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Documentation/console/console.txt for more information).
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This is more complicated in the case of the framebuffer console (fbcon),
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because fbcon is an intermediate layer between the console and the drivers:
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console ---> fbcon ---> fbdev drivers ---> hardware
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The fbdev drivers cannot be unloaded if it's bound to fbcon, and fbcon cannot
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be unloaded if it's bound to the console layer.
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So to unload the fbdev drivers, one must first unbind fbcon from the console,
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then unbind the fbdev drivers from fbcon. Fortunately, unbinding fbcon from
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the console layer will automatically unbind framebuffer drivers from
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fbcon. Thus, there is no need to explicitly unbind the fbdev drivers from
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fbcon.
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So, how do we unbind fbcon from the console? Part of the answer is in
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Documentation/console/console.txt. To summarize:
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Echo a value to the bind file that represents the framebuffer console
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driver. So assuming vtcon1 represents fbcon, then:
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echo 1 > sys/class/vtconsole/vtcon1/bind - attach framebuffer console to
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console layer
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echo 0 > sys/class/vtconsole/vtcon1/bind - detach framebuffer console from
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console layer
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If fbcon is detached from the console layer, your boot console driver (which is
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usually VGA text mode) will take over. A few drivers (rivafb and i810fb) will
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restore VGA text mode for you. With the rest, before detaching fbcon, you
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must take a few additional steps to make sure that your VGA text mode is
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restored properly. The following is one of the several methods that you can do:
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1. Download or install vbetool. This utility is included with most
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distributions nowadays, and is usually part of the suspend/resume tool.
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2. In your kernel configuration, ensure that CONFIG_FRAMEBUFFER_CONSOLE is set
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to 'y' or 'm'. Enable one or more of your favorite framebuffer drivers.
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3. Boot into text mode and as root run:
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vbetool vbestate save > <vga state file>
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The above command saves the register contents of your graphics
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hardware to <vga state file>. You need to do this step only once as
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the state file can be reused.
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4. If fbcon is compiled as a module, load fbcon by doing:
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modprobe fbcon
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5. Now to detach fbcon:
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vbetool vbestate restore < <vga state file> && \
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echo 0 > /sys/class/vtconsole/vtcon1/bind
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6. That's it, you're back to VGA mode. And if you compiled fbcon as a module,
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you can unload it by 'rmmod fbcon'
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7. To reattach fbcon:
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echo 1 > /sys/class/vtconsole/vtcon1/bind
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8. Once fbcon is unbound, all drivers registered to the system will also
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become unbound. This means that fbcon and individual framebuffer drivers
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can be unloaded or reloaded at will. Reloading the drivers or fbcon will
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automatically bind the console, fbcon and the drivers together. Unloading
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all the drivers without unloading fbcon will make it impossible for the
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console to bind fbcon.
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Notes for vesafb users:
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=======================
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Unfortunately, if your bootline includes a vga=xxx parameter that sets the
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hardware in graphics mode, such as when loading vesafb, vgacon will not load.
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Instead, vgacon will replace the default boot console with dummycon, and you
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won't get any display after detaching fbcon. Your machine is still alive, so
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you can reattach vesafb. However, to reattach vesafb, you need to do one of
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the following:
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Variation 1:
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a. Before detaching fbcon, do
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vbetool vbemode save > <vesa state file> # do once for each vesafb mode,
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# the file can be reused
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b. Detach fbcon as in step 5.
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c. Attach fbcon
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vbetool vbestate restore < <vesa state file> && \
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echo 1 > /sys/class/vtconsole/vtcon1/bind
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Variation 2:
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a. Before detaching fbcon, do:
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echo <ID> > /sys/class/tty/console/bind
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vbetool vbemode get
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b. Take note of the mode number
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b. Detach fbcon as in step 5.
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c. Attach fbcon:
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vbetool vbemode set <mode number> && \
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echo 1 > /sys/class/vtconsole/vtcon1/bind
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Samples:
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========
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Here are 2 sample bash scripts that you can use to bind or unbind the
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framebuffer console driver if you are in an X86 box:
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---------------------------------------------------------------------------
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#!/bin/bash
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# Unbind fbcon
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# Change this to where your actual vgastate file is located
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# Or Use VGASTATE=$1 to indicate the state file at runtime
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VGASTATE=/tmp/vgastate
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# path to vbetool
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VBETOOL=/usr/local/bin
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for (( i = 0; i < 16; i++))
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do
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if test -x /sys/class/vtconsole/vtcon$i; then
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if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \
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= 1 ]; then
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if test -x $VBETOOL/vbetool; then
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echo Unbinding vtcon$i
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$VBETOOL/vbetool vbestate restore < $VGASTATE
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echo 0 > /sys/class/vtconsole/vtcon$i/bind
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fi
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fi
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fi
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done
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---------------------------------------------------------------------------
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#!/bin/bash
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# Bind fbcon
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for (( i = 0; i < 16; i++))
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do
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if test -x /sys/class/vtconsole/vtcon$i; then
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if [ `cat /sys/class/vtconsole/vtcon$i/name | grep -c "frame buffer"` \
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= 1 ]; then
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echo Unbinding vtcon$i
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echo 1 > /sys/class/vtconsole/vtcon$i/bind
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fi
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fi
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done
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---------------------------------------------------------------------------
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--
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Antonino Daplas <adaplas@pol.net>
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