Present - Programming - Radeon Framebuffer Device
Table of Contents
The text of this page is copyright © 2003-2004 C. Daelhousen and licensed under the Creative Commons Attribution/Share-Alike License. It may be freely used, modified, and published, provided it (and derivative works of it) retains attribution and remains under this license.
This page explains configuration of the radeonfb framebuffer driver for the Linux 2.4 series of kernels. It is intended for intermediate to advanced users. For more information on video timings, framebuffers, and video hardware, search for the relevant HOWTOs.
Tired of 80x25 consoles? Have a flatpanel monitor that makes them look ugly? The framebuffer device may be just what you're looking for! Here's how to run the console in a sorta-text, sorta-graphics mode, using the radeonfb driver as an example.
First, the kernel needs to be compiled with framebuffer support, which is as of 2.4.22 still experimental. Therefore, prompting for development and/or incomplete code/drivers must be enabled. Then, under "Console drivers", framebuffer support and at least a driver for the appropriate card is needed. It's fine to configure in multiple drivers; one is selected when booting the kernel. Compile them in; they will not work as desired as modules.
To play with fonts, choose "Select compiled-in fonts" and choose as desired. This is not strictly necessary, as normal VGA font loading still works. (I speak for PC/x86 systems; additional information on other architectures welcomed.)
Other options may or may not need set; YMMV. My system was fine with the defaults. Recompile and reboot the kernel. Keep a backup on hand, just in case.
All of the framebuffers are subdrivers of video, and are configured by
video= option to the kernel at bootup. The exact driver
must come first, followed by a colon, for instance
Some of the drivers have documentation in
"/usr/src/linux/Documentation/fb"; reading them if available is
highly recommended. The radeon driver currently (2.4.22) supports several
options, separated by commas. Options listed below which are not marked with a
version were present in 2.4.20 when I started using radeonfb.
|font:<any_fontname>||Sets the fontname to be <any_fontname>. See below.|
|panel_yres:<number>||Sets the height in pixels of the DFP-based monitor to <number>.|
|dfp||Forces the driver to believe that there is a DFP-based monitor attached to the DVI port.|
|crt||(2.4.22+) Forces the driver to believe that there is a CRT monitor attached to the VGA port.|
|nolcd||(2.4.22+) Forces the driver to believe that there is no LCD monitor. By my reading, this only affects PowerPC.|
|noaccel||Disable acceleration. This is much slower and much more stable.|
|nomtrr||(2.4.22+) Disable use of MTRRs by the driver. This is less drastic than noaccel.|
|mirror||(2.4.22+) Send output to both the DVI and VGA ports.|
|<X_res>x<Y_res>-<depth>@<refresh>||Set video mode. See below.|
If given a panel_yres, the corresponding X resolution is selected off of a table.
Each particular font file under drivers/video, such as font_8x16.c, contains
at the very end of it a
struct fbcon_font_desc containing the
actual name of the font as its second element. The fonts available depend on
which were compiled into the kernel. Following is a list of font names as used
by the font option in 2.4.22, with their descriptions from kernel
configuration. (For brevity, the size of the font is not repeated.)
|PEARL8x8||Pearl (old m68k) console|
* Not supported by all drivers.
The format of the video mode looks like it should be the same for all drivers using modedb. (Documentation/fb/modedb.txt has an outdated list of drivers that support it.) <X_res> and <Y_res> are the size of the screen in pixels. <depth> is the number of bits per pixel to use--8 for 256 colors, 16 for 65536 colors, and 24 or 32 for 16 million colors (dependent upon hardware). Finally, <refresh> is the desired refresh rate in Hz, because most modes support more than one. It must be chosen to be within the specifications of the monitor. Not all fields are required. The mode may also be "off" to disable the driver.
The database containing all timing information for each available mode is
stored in "/usr/src/linux/drivers/video/modedb.c", in an array near
the top. Each mode contains a comment stating what mode the timings translate
into, such as
/* 640x480 @ 85 Hz, 43.27 kHz hsync */. Remember to
keep the hsync and refresh rate within the monitor's capabilities. Select an
appropriate mode, and build the kernel option from it. For instance:
|VGA video: 80x25 characters, text only|
|640x480 pixels, using the first entry of this size in the database|
|640x480 pixels at 85 Hz|
|1280x1024 pixels, 61 Hz refresh rate, with a DFP monitor on the DVI port|
|1280x1024 pixels, 85 Hz refresh rate, and 65,536 colors|
|1280x1024 pixels, 61 Hz refresh rate, with the size of the monitor set to 1280x1024 pixels if autodetection fails|
|1024x768 pixels, with the 8x16 pixel SPARC font|
The number of characters onscreen can be calculated by dividing the width by the font size, which is 8x16 by default. For 640x480 with an 8x16 font, the screen will hold 80x30 characters. At 1280x1024 with the same font, the screen will hold 160x64 characters.
Once the kernel option is created, it needs to be passed to the kernel at bootup. Testing the option before making it default is recommended. The exact procedures vary with bootloader. For the sake of completeness, use "append=video=..." under the appropriate "image" section in lilo.conf for LILO; or append the relevant "video=..." parameter to the appropriate "kernel" line in grub.conf for GRUB. For other bootloaders, consult the bootloader's documentation.
In 2.4.20 and 2.4.21, using the Radeon with a DVI cable will cause the display to lose signal and go blank when the framebuffer is initialized. This is because the timings are copied from the (unconnected and off) VGA/CRT monitor, instead of being calculated from the new mode parameters. Patches are available. Also note that in these versions, the "noaccel" option is forced on.
In 2.4.22, radeonfb tries to get the display size from the Radeon's BIOS, then an EDID block, and examines panel_yres only if those two methods fail. If the card reports the correct size, this is ideal, but mine does not. My card always reports 1024x768, and specifying a larger size causes the display to show a 1024x768-sized chunk in the upper-left of the display. A patch to make panel_yres override the BIOS and EDID is available in the patch list. Furthermore, acceleration is enabled by default. The screen tends to collect stray pixels while editing, but redrawing erases them again.
No changes have been made to radeonfb between 2.4.22 and 2.4.25.
Somewhere in the 2.5 series, all framebuffer device names have been changed
to end in "fb" for consistency. So a proper video line should read
In kernel 2.6.4 and possibly earlier, the new radeonfb driver supports getting the EDID data from the monitor. This should allow it to come up with a sane size and refresh rate without any video parameter. Some garbage is displayed on the screen when switching the console to the framebuffer, but it is harmless.
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