forked from luck/tmp_suning_uos_patched
418d1ce618
Update Documentation/fb/fbcon.txt to reflect the following changes: 1. Simple illustration of the binding of the console down to individual framebuffer drivers 2. Usage of userspace tools to help with recovery of text console 3. How to use the attributes in /sys/class/tty/console to unload fbcon and the framebuffer drivers Signed-off-by: Antonino Daplas <adaplas@pol.net> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
276 lines
10 KiB
Plaintext
276 lines
10 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 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 the ID number of the 'frame buffer driver' to:
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sys/class/tty/console/bind - attach framebuffer console to console layer
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sys/class/tty/console/unbind - detach framebuffer console from 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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'cat /sys/class/tty/console/backend' and take note of the ID
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The above is probably needed only once. Then:
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vbetool vbestate restore < <vga state file> && \
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echo <ID> > /sys/class/tty/console/unbind
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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 <ID> > /sys/class/tty/console/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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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 <ID> > /sys/class/tty/console/bind
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--
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Antonino Daplas <adaplas@pol.net>
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