Subversion Repositories or1k_soc_on_altera_embedded_dev_kit

                        modedb default video mode support

Currently all frame buffer device drivers have their own video mode databases,

which is a mess and a waste of resources. The main idea of modedb is to have

  - one routine to probe for video modes, which can be used by all frame buffer

    devices

  - one generic video mode database with a fair amount of standard videomodes

    (taken from XFree86)

  - the possibility to supply your own mode database for graphics hardware that

    needs non-standard modes, like amifb and Mac frame buffer drivers (which

    use macmodes.c)

When a frame buffer device receives a video= option it doesn't know, it should

consider that to be a video mode option. If no frame buffer device is specified

in a video= option, fbmem considers that to be a global video mode option.

Valid mode specifiers (mode_option argument):

    x[M][R][-][@][i][m]

    [-][@]

with , ,  and  decimal numbers and  a string.

Things between square brackets are optional.

If 'M' is specified in the mode_option argument (after  and before

 and , if specified) the timings will be calculated using

VESA(TM) Coordinated Video Timings instead of looking up the mode from a table.

If 'R' is specified, do a 'reduced blanking' calculation for digital displays.

If 'i' is specified, calculate for an interlaced mode.  And if 'm' is

specified, add margins to the calculation (1.8% of xres rounded down to 8

pixels and 1.8% of yres).

       Sample usage: 1024x768M@60m - CVT timing with margins

***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo *****

What is the VESA(TM) Coordinated Video Timings (CVT)?

From the VESA(TM) Website:

     "The purpose of CVT is to provide a method for generating a consistent

      and coordinated set of standard formats, display refresh rates, and

      timing specifications for computer display products, both those

      employing CRTs, and those using other display technologies. The

      intention of CVT is to give both source and display manufacturers a

      common set of tools to enable new timings to be developed in a

      consistent manner that ensures greater compatibility."

This is the third standard approved by VESA(TM) concerning video timings.  The

first was the Discrete Video Timings (DVT) which is  a collection of

pre-defined modes approved by VESA(TM).  The second is the Generalized Timing

Formula (GTF) which is an algorithm to calculate the timings, given the

pixelclock, the horizontal sync frequency, or the vertical refresh rate.

The GTF is limited by the fact that it is designed mainly for CRT displays.

It artificially increases the pixelclock because of its high blanking

requirement. This is inappropriate for digital display interface with its high

data rate which requires that it conserves the pixelclock as much as possible.

Also, GTF does not take into account the aspect ratio of the display.

The CVT addresses these limitations.  If used with CRT's, the formula used

is a derivation of GTF with a few modifications.  If used with digital

displays, the "reduced blanking" calculation can be used.

From the framebuffer subsystem perspective, new formats need not be added

to the global mode database whenever a new mode is released by display

manufacturers. Specifying for CVT will work for most, if not all, relatively

new CRT displays and probably with most flatpanels, if 'reduced blanking'

calculation is specified.  (The CVT compatibility of the display can be

determined from its EDID. The version 1.3 of the EDID has extra 128-byte

blocks where additional timing information is placed.  As of this time, there

is no support yet in the layer to parse this additional blocks.)

CVT also introduced a new naming convention (should be seen from dmesg output):

    M[-R]

    where: pix = total amount of pixels in MB (xres x yres)

           M   = always present

           a   = aspect ratio (3 - 4:3; 4 - 5:4; 9 - 15:9, 16:9; A - 16:10)

          -R   = reduced blanking

          example:  .48M3-R - 800x600 with reduced blanking

Note: VESA(TM) has restrictions on what is a standard CVT timing:

      - aspect ratio can only be one of the above values

      - acceptable refresh rates are 50, 60, 70 or 85 Hz only

      - if reduced blanking, the refresh rate must be at 60Hz

If one of the above are not satisfied, the kernel will print a warning but the

timings will still be calculated.

***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo ***** oOo *****

To find a suitable video mode, you just call

int __init fb_find_mode(struct fb_var_screeninfo *var,

                        struct fb_info *info, const char *mode_option,

                        const struct fb_videomode *db, unsigned int dbsize,

                        const struct fb_videomode *default_mode,

                        unsigned int default_bpp)

with db/dbsize your non-standard video mode database, or NULL to use the

standard video mode database.

fb_find_mode() first tries the specified video mode (or any mode that matches,

e.g. there can be multiple 640x480 modes, each of them is tried). If that

fails, the default mode is tried. If that fails, it walks over all modes.

To specify a video mode at bootup, use the following boot options:

    video=:x[-][@refresh]

where  is a name from the table below.  Valid default modes can be

found in linux/drivers/video/modedb.c.  Check your driver's documentation.

There may be more modes.

    Drivers that support modedb boot options

    Boot Name     Cards Supported

    amifb       - Amiga chipset frame buffer

    aty128fb    - ATI Rage128 / Pro frame buffer

    atyfb       - ATI Mach64 frame buffer

    tdfxfb      - 3D Fx frame buffer

    tridentfb   - Trident (Cyber)blade chipset frame buffer

BTW, only a few drivers use this at the moment. Others are to follow

(feel free to send patches).