mirror of
https://github.com/AuxXxilium/linux_dsm_epyc7002.git
synced 2024-11-30 07:26:46 +07:00
fb21c2f428
This API will be used to support YUV frame buffer formats in a standard way. Last but not least, create a much needed fbdev API documentation and document the format setting APIs. Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
307 lines
12 KiB
Plaintext
307 lines
12 KiB
Plaintext
The Frame Buffer Device API
|
|
---------------------------
|
|
|
|
Last revised: June 21, 2011
|
|
|
|
|
|
0. Introduction
|
|
---------------
|
|
|
|
This document describes the frame buffer API used by applications to interact
|
|
with frame buffer devices. In-kernel APIs between device drivers and the frame
|
|
buffer core are not described.
|
|
|
|
Due to a lack of documentation in the original frame buffer API, drivers
|
|
behaviours differ in subtle (and not so subtle) ways. This document describes
|
|
the recommended API implementation, but applications should be prepared to
|
|
deal with different behaviours.
|
|
|
|
|
|
1. Capabilities
|
|
---------------
|
|
|
|
Device and driver capabilities are reported in the fixed screen information
|
|
capabilities field.
|
|
|
|
struct fb_fix_screeninfo {
|
|
...
|
|
__u16 capabilities; /* see FB_CAP_* */
|
|
...
|
|
};
|
|
|
|
Application should use those capabilities to find out what features they can
|
|
expect from the device and driver.
|
|
|
|
- FB_CAP_FOURCC
|
|
|
|
The driver supports the four character code (FOURCC) based format setting API.
|
|
When supported, formats are configured using a FOURCC instead of manually
|
|
specifying color components layout.
|
|
|
|
|
|
2. Types and visuals
|
|
--------------------
|
|
|
|
Pixels are stored in memory in hardware-dependent formats. Applications need
|
|
to be aware of the pixel storage format in order to write image data to the
|
|
frame buffer memory in the format expected by the hardware.
|
|
|
|
Formats are described by frame buffer types and visuals. Some visuals require
|
|
additional information, which are stored in the variable screen information
|
|
bits_per_pixel, grayscale, red, green, blue and transp fields.
|
|
|
|
Visuals describe how color information is encoded and assembled to create
|
|
macropixels. Types describe how macropixels are stored in memory. The following
|
|
types and visuals are supported.
|
|
|
|
- FB_TYPE_PACKED_PIXELS
|
|
|
|
Macropixels are stored contiguously in a single plane. If the number of bits
|
|
per macropixel is not a multiple of 8, whether macropixels are padded to the
|
|
next multiple of 8 bits or packed together into bytes depends on the visual.
|
|
|
|
Padding at end of lines may be present and is then reported through the fixed
|
|
screen information line_length field.
|
|
|
|
- FB_TYPE_PLANES
|
|
|
|
Macropixels are split across multiple planes. The number of planes is equal to
|
|
the number of bits per macropixel, with plane i'th storing i'th bit from all
|
|
macropixels.
|
|
|
|
Planes are located contiguously in memory.
|
|
|
|
- FB_TYPE_INTERLEAVED_PLANES
|
|
|
|
Macropixels are split across multiple planes. The number of planes is equal to
|
|
the number of bits per macropixel, with plane i'th storing i'th bit from all
|
|
macropixels.
|
|
|
|
Planes are interleaved in memory. The interleave factor, defined as the
|
|
distance in bytes between the beginning of two consecutive interleaved blocks
|
|
belonging to different planes, is stored in the fixed screen information
|
|
type_aux field.
|
|
|
|
- FB_TYPE_FOURCC
|
|
|
|
Macropixels are stored in memory as described by the format FOURCC identifier
|
|
stored in the variable screen information grayscale field.
|
|
|
|
- FB_VISUAL_MONO01
|
|
|
|
Pixels are black or white and stored on a number of bits (typically one)
|
|
specified by the variable screen information bpp field.
|
|
|
|
Black pixels are represented by all bits set to 1 and white pixels by all bits
|
|
set to 0. When the number of bits per pixel is smaller than 8, several pixels
|
|
are packed together in a byte.
|
|
|
|
FB_VISUAL_MONO01 is currently used with FB_TYPE_PACKED_PIXELS only.
|
|
|
|
- FB_VISUAL_MONO10
|
|
|
|
Pixels are black or white and stored on a number of bits (typically one)
|
|
specified by the variable screen information bpp field.
|
|
|
|
Black pixels are represented by all bits set to 0 and white pixels by all bits
|
|
set to 1. When the number of bits per pixel is smaller than 8, several pixels
|
|
are packed together in a byte.
|
|
|
|
FB_VISUAL_MONO01 is currently used with FB_TYPE_PACKED_PIXELS only.
|
|
|
|
- FB_VISUAL_TRUECOLOR
|
|
|
|
Pixels are broken into red, green and blue components, and each component
|
|
indexes a read-only lookup table for the corresponding value. Lookup tables
|
|
are device-dependent, and provide linear or non-linear ramps.
|
|
|
|
Each component is stored in a macropixel according to the variable screen
|
|
information red, green, blue and transp fields.
|
|
|
|
- FB_VISUAL_PSEUDOCOLOR and FB_VISUAL_STATIC_PSEUDOCOLOR
|
|
|
|
Pixel values are encoded as indices into a colormap that stores red, green and
|
|
blue components. The colormap is read-only for FB_VISUAL_STATIC_PSEUDOCOLOR
|
|
and read-write for FB_VISUAL_PSEUDOCOLOR.
|
|
|
|
Each pixel value is stored in the number of bits reported by the variable
|
|
screen information bits_per_pixel field.
|
|
|
|
- FB_VISUAL_DIRECTCOLOR
|
|
|
|
Pixels are broken into red, green and blue components, and each component
|
|
indexes a programmable lookup table for the corresponding value.
|
|
|
|
Each component is stored in a macropixel according to the variable screen
|
|
information red, green, blue and transp fields.
|
|
|
|
- FB_VISUAL_FOURCC
|
|
|
|
Pixels are encoded and interpreted as described by the format FOURCC
|
|
identifier stored in the variable screen information grayscale field.
|
|
|
|
|
|
3. Screen information
|
|
---------------------
|
|
|
|
Screen information are queried by applications using the FBIOGET_FSCREENINFO
|
|
and FBIOGET_VSCREENINFO ioctls. Those ioctls take a pointer to a
|
|
fb_fix_screeninfo and fb_var_screeninfo structure respectively.
|
|
|
|
struct fb_fix_screeninfo stores device independent unchangeable information
|
|
about the frame buffer device and the current format. Those information can't
|
|
be directly modified by applications, but can be changed by the driver when an
|
|
application modifies the format.
|
|
|
|
struct fb_fix_screeninfo {
|
|
char id[16]; /* identification string eg "TT Builtin" */
|
|
unsigned long smem_start; /* Start of frame buffer mem */
|
|
/* (physical address) */
|
|
__u32 smem_len; /* Length of frame buffer mem */
|
|
__u32 type; /* see FB_TYPE_* */
|
|
__u32 type_aux; /* Interleave for interleaved Planes */
|
|
__u32 visual; /* see FB_VISUAL_* */
|
|
__u16 xpanstep; /* zero if no hardware panning */
|
|
__u16 ypanstep; /* zero if no hardware panning */
|
|
__u16 ywrapstep; /* zero if no hardware ywrap */
|
|
__u32 line_length; /* length of a line in bytes */
|
|
unsigned long mmio_start; /* Start of Memory Mapped I/O */
|
|
/* (physical address) */
|
|
__u32 mmio_len; /* Length of Memory Mapped I/O */
|
|
__u32 accel; /* Indicate to driver which */
|
|
/* specific chip/card we have */
|
|
__u16 capabilities; /* see FB_CAP_* */
|
|
__u16 reserved[2]; /* Reserved for future compatibility */
|
|
};
|
|
|
|
struct fb_var_screeninfo stores device independent changeable information
|
|
about a frame buffer device, its current format and video mode, as well as
|
|
other miscellaneous parameters.
|
|
|
|
struct fb_var_screeninfo {
|
|
__u32 xres; /* visible resolution */
|
|
__u32 yres;
|
|
__u32 xres_virtual; /* virtual resolution */
|
|
__u32 yres_virtual;
|
|
__u32 xoffset; /* offset from virtual to visible */
|
|
__u32 yoffset; /* resolution */
|
|
|
|
__u32 bits_per_pixel; /* guess what */
|
|
__u32 grayscale; /* 0 = color, 1 = grayscale, */
|
|
/* >1 = FOURCC */
|
|
struct fb_bitfield red; /* bitfield in fb mem if true color, */
|
|
struct fb_bitfield green; /* else only length is significant */
|
|
struct fb_bitfield blue;
|
|
struct fb_bitfield transp; /* transparency */
|
|
|
|
__u32 nonstd; /* != 0 Non standard pixel format */
|
|
|
|
__u32 activate; /* see FB_ACTIVATE_* */
|
|
|
|
__u32 height; /* height of picture in mm */
|
|
__u32 width; /* width of picture in mm */
|
|
|
|
__u32 accel_flags; /* (OBSOLETE) see fb_info.flags */
|
|
|
|
/* Timing: All values in pixclocks, except pixclock (of course) */
|
|
__u32 pixclock; /* pixel clock in ps (pico seconds) */
|
|
__u32 left_margin; /* time from sync to picture */
|
|
__u32 right_margin; /* time from picture to sync */
|
|
__u32 upper_margin; /* time from sync to picture */
|
|
__u32 lower_margin;
|
|
__u32 hsync_len; /* length of horizontal sync */
|
|
__u32 vsync_len; /* length of vertical sync */
|
|
__u32 sync; /* see FB_SYNC_* */
|
|
__u32 vmode; /* see FB_VMODE_* */
|
|
__u32 rotate; /* angle we rotate counter clockwise */
|
|
__u32 colorspace; /* colorspace for FOURCC-based modes */
|
|
__u32 reserved[4]; /* Reserved for future compatibility */
|
|
};
|
|
|
|
To modify variable information, applications call the FBIOPUT_VSCREENINFO
|
|
ioctl with a pointer to a fb_var_screeninfo structure. If the call is
|
|
successful, the driver will update the fixed screen information accordingly.
|
|
|
|
Instead of filling the complete fb_var_screeninfo structure manually,
|
|
applications should call the FBIOGET_VSCREENINFO ioctl and modify only the
|
|
fields they care about.
|
|
|
|
|
|
4. Format configuration
|
|
-----------------------
|
|
|
|
Frame buffer devices offer two ways to configure the frame buffer format: the
|
|
legacy API and the FOURCC-based API.
|
|
|
|
|
|
The legacy API has been the only frame buffer format configuration API for a
|
|
long time and is thus widely used by application. It is the recommended API
|
|
for applications when using RGB and grayscale formats, as well as legacy
|
|
non-standard formats.
|
|
|
|
To select a format, applications set the fb_var_screeninfo bits_per_pixel field
|
|
to the desired frame buffer depth. Values up to 8 will usually map to
|
|
monochrome, grayscale or pseudocolor visuals, although this is not required.
|
|
|
|
- For grayscale formats, applications set the grayscale field to one. The red,
|
|
blue, green and transp fields must be set to 0 by applications and ignored by
|
|
drivers. Drivers must fill the red, blue and green offsets to 0 and lengths
|
|
to the bits_per_pixel value.
|
|
|
|
- For pseudocolor formats, applications set the grayscale field to zero. The
|
|
red, blue, green and transp fields must be set to 0 by applications and
|
|
ignored by drivers. Drivers must fill the red, blue and green offsets to 0
|
|
and lengths to the bits_per_pixel value.
|
|
|
|
- For truecolor and directcolor formats, applications set the grayscale field
|
|
to zero, and the red, blue, green and transp fields to describe the layout of
|
|
color components in memory.
|
|
|
|
struct fb_bitfield {
|
|
__u32 offset; /* beginning of bitfield */
|
|
__u32 length; /* length of bitfield */
|
|
__u32 msb_right; /* != 0 : Most significant bit is */
|
|
/* right */
|
|
};
|
|
|
|
Pixel values are bits_per_pixel wide and are split in non-overlapping red,
|
|
green, blue and alpha (transparency) components. Location and size of each
|
|
component in the pixel value are described by the fb_bitfield offset and
|
|
length fields. Offset are computed from the right.
|
|
|
|
Pixels are always stored in an integer number of bytes. If the number of
|
|
bits per pixel is not a multiple of 8, pixel values are padded to the next
|
|
multiple of 8 bits.
|
|
|
|
Upon successful format configuration, drivers update the fb_fix_screeninfo
|
|
type, visual and line_length fields depending on the selected format.
|
|
|
|
|
|
The FOURCC-based API replaces format descriptions by four character codes
|
|
(FOURCC). FOURCCs are abstract identifiers that uniquely define a format
|
|
without explicitly describing it. This is the only API that supports YUV
|
|
formats. Drivers are also encouraged to implement the FOURCC-based API for RGB
|
|
and grayscale formats.
|
|
|
|
Drivers that support the FOURCC-based API report this capability by setting
|
|
the FB_CAP_FOURCC bit in the fb_fix_screeninfo capabilities field.
|
|
|
|
FOURCC definitions are located in the linux/videodev2.h header. However, and
|
|
despite starting with the V4L2_PIX_FMT_prefix, they are not restricted to V4L2
|
|
and don't require usage of the V4L2 subsystem. FOURCC documentation is
|
|
available in Documentation/DocBook/v4l/pixfmt.xml.
|
|
|
|
To select a format, applications set the grayscale field to the desired FOURCC.
|
|
For YUV formats, they should also select the appropriate colorspace by setting
|
|
the colorspace field to one of the colorspaces listed in linux/videodev2.h and
|
|
documented in Documentation/DocBook/v4l/colorspaces.xml.
|
|
|
|
The red, green, blue and transp fields are not used with the FOURCC-based API.
|
|
For forward compatibility reasons applications must zero those fields, and
|
|
drivers must ignore them. Values other than 0 may get a meaning in future
|
|
extensions.
|
|
|
|
Upon successful format configuration, drivers update the fb_fix_screeninfo
|
|
type, visual and line_length fields depending on the selected format. The type
|
|
and visual fields are set to FB_TYPE_FOURCC and FB_VISUAL_FOURCC respectively.
|