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DDC.HOWTO
This file describes how to add DDC support to a chipset driver.
1) DDC INITIALIZATION
When implementing DDC in the driver one has the choice between
DDC1 and DDC2.
DDC1 data is contiuously transmitted by a DDC1 capable display
device. The data is send serially over a data line; the Vsync
signal serves as clock. Only one EDID 1.x data block can be
transmitted using DDC1. Since transmission of an EDID1 block
using a regular Vsync frequency would take up several seconds
the driver can increase the Vsync frequency to up to 25 kHz as
soon as it detects DDC1 activety on the data line.
DDC2 data is transmitted using the I2C protocol. This requires
an additional clock line. DDC2 is capable of transmitting EDID1
and EDID2 block as well as a VDIF block on display devices that
support these.
Display devices switch into the DDC2 mode as soon as they detect
activety on the DDC clock line. Once the are in DDC2 mode they
stop transmitting DDC1 signals until the next power cycle.
Some graphics chipset configurations which are not capable of
DDC2 might still be able to read DDC1 data. Where available
DDC2 it is preferrable.
All relevant prototypes and defines are in xf86DDC.h.
DDC2 additionally requires I2C support. The I2C prototypes
are in xf86i2c.h.
DDC1 Support:
The driver has to provide a read function which waits for the
end of the next Vsync signal and reads in and returns the status
of the DDC line:
unsigned int XXX_ddc1Read(ScrnInfoPtr pScrn)
Additionally a function is required to inclrease the Vsync
frequency to max. 25 kHz.
void XXX_ddc1SetSpeed(ScrnInfoPtr pScrn, xf86ddcSpeed speed)
If the speed argument is DDC_FAST the function should increase
the Vsync frequency on DDC_SLOW it should restore the original
value. For convenience a generic ddc1SetSpeed() function is provided
in the vga module for VGA-like chipsets.
void vgaHWddc1SetSpeed(ScrnInfoPtr pScrn, sf86ddcSpeed speed).
To read out the DDC1 data the driver should call
xf86MonPtr xf86DoEDID_DDC1(int scrnIndex,
void (*DDC1SetSpeed)(ScrnInfoPtr, xf86ddcSpeed),
unsigned int (*DDC1Read)(ScrnInfoPtr))
in PreInit(). DDC1SetSpeed is a pointer to the SetSpeed()
function, DDC1Read has to point to the DDC1 read function.
The function will return a pointer to the xf86Monitor structure
which contains all information retreived by DDC.
NULL will be returned on failure.
DDC2 Support
To read out DDC2 information I2C has to be initialized first.
(See documentation for the i2c module).
The function
xf86MonPtr xf86DoEDID_DDC2(int scrnIndex, I2CBusPtr pBus)
is provided to read out and process DDC2 data. A pointer
to the I2CBusRec of the appropriate I2C Bus has to be passed
as the second argument.
The function will return a pointer to the xf86Monitor structure
which contains all information retreived by DDC.
NULL will be returned on failure.
Printing monitor parameters
To print out the information contained in the xf86Monitor
structure the function
xf86MonPtr xf86PrintEDID(xf86MonPtr monitor)
is provided.
Further processing of the xf86Monitor structure is not yet
implemented. Howerver it is planned to use the information
about video modes, gamma values etc.
Therefore it is strongly recommended to read out DDC data
before any video mode processing is done.
$XFree86$
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