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Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/fb/pxafb.txt | 84 |
1 files changed, 84 insertions, 0 deletions
diff --git a/Documentation/fb/pxafb.txt b/Documentation/fb/pxafb.txt index ad94b5ca009..d143a0a749f 100644 --- a/Documentation/fb/pxafb.txt +++ b/Documentation/fb/pxafb.txt @@ -56,3 +56,87 @@ outputen:POLARITY pixclockpol:POLARITY pixel clock polarity 0 => falling edge, 1 => rising edge + + +Overlay Support for PXA27x and later LCD controllers +==================================================== + + PXA27x and later processors support overlay1 and overlay2 on-top of the + base framebuffer (although under-neath the base is also possible). They + support palette and no-palette RGB formats, as well as YUV formats (only + available on overlay2). These overlays have dedicated DMA channels and + behave in a similar way as a framebuffer. + + However, there are some differences between these overlay framebuffers + and normal framebuffers, as listed below: + + 1. overlay can start at a 32-bit word aligned position within the base + framebuffer, which means they have a start (x, y). This information + is encoded into var->nonstd (no, var->xoffset and var->yoffset are + not for such purpose). + + 2. overlay framebuffer is allocated dynamically according to specified + 'struct fb_var_screeninfo', the amount is decided by: + + var->xres_virtual * var->yres_virtual * bpp + + bpp = 16 -- for RGB565 or RGBT555 + = 24 -- for YUV444 packed + = 24 -- for YUV444 planar + = 16 -- for YUV422 planar (1 pixel = 1 Y + 1/2 Cb + 1/2 Cr) + = 12 -- for YUV420 planar (1 pixel = 1 Y + 1/4 Cb + 1/4 Cr) + + NOTE: + + a. overlay does not support panning in x-direction, thus + var->xres_virtual will always be equal to var->xres + + b. line length of overlay(s) must be on a 32-bit word boundary, + for YUV planar modes, it is a requirement for the component + with minimum bits per pixel, e.g. for YUV420, Cr component + for one pixel is actually 2-bits, it means the line length + should be a multiple of 16-pixels + + c. starting horizontal position (XPOS) should start on a 32-bit + word boundary, otherwise the fb_check_var() will just fail. + + d. the rectangle of the overlay should be within the base plane, + otherwise fail + + Applications should follow the sequence below to operate an overlay + framebuffer: + + a. open("/dev/fb[1-2]", ...) + b. ioctl(fd, FBIOGET_VSCREENINFO, ...) + c. modify 'var' with desired parameters: + 1) var->xres and var->yres + 2) larger var->yres_virtual if more memory is required, + usually for double-buffering + 3) var->nonstd for starting (x, y) and color format + 4) var->{red, green, blue, transp} if RGB mode is to be used + d. ioctl(fd, FBIOPUT_VSCREENINFO, ...) + e. ioctl(fd, FBIOGET_FSCREENINFO, ...) + f. mmap + g. ... + + 3. for YUV planar formats, these are actually not supported within the + framebuffer framework, application has to take care of the offsets + and lengths of each component within the framebuffer. + + 4. var->nonstd is used to pass starting (x, y) position and color format, + the detailed bit fields are shown below: + + 31 23 20 10 0 + +-----------------+---+----------+----------+ + | ... unused ... |FOR| XPOS | YPOS | + +-----------------+---+----------+----------+ + + FOR - color format, as defined by OVERLAY_FORMAT_* in pxafb.h + 0 - RGB + 1 - YUV444 PACKED + 2 - YUV444 PLANAR + 3 - YUV422 PLANAR + 4 - YUR420 PLANAR + + XPOS - starting horizontal position + YPOS - starting vertical position |