98 lines
3.7 KiB
Plaintext
98 lines
3.7 KiB
Plaintext
DDC.HOWTO
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This file describes how to add DDC support to a chipset driver.
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1) DDC INITIALIZATION
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When implementing DDC in the driver one has the choice between
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DDC1 and DDC2.
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DDC1 data is contiuously transmitted by a DDC1 capable display
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device. The data is send serially over a data line; the Vsync
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signal serves as clock. Only one EDID 1.x data block can be
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transmitted using DDC1. Since transmission of an EDID1 block
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using a regular Vsync frequency would take up several seconds
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the driver can increase the Vsync frequency to up to 25 kHz as
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soon as it detects DDC1 activety on the data line.
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DDC2 data is transmitted using the I2C protocol. This requires
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an additional clock line. DDC2 is capable of transmitting EDID1
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and EDID2 block as well as a VDIF block on display devices that
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support these.
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Display devices switch into the DDC2 mode as soon as they detect
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activety on the DDC clock line. Once the are in DDC2 mode they
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stop transmitting DDC1 signals until the next power cycle.
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Some graphics chipset configurations which are not capable of
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DDC2 might still be able to read DDC1 data. Where available
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DDC2 it is preferrable.
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All relevant prototypes and defines are in xf86DDC.h.
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DDC2 additionally requires I2C support. The I2C prototypes
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are in xf86i2c.h.
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DDC1 Support:
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The driver has to provide a read function which waits for the
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end of the next Vsync signal and reads in and returns the status
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of the DDC line:
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unsigned int XXX_ddc1Read(ScrnInfoPtr pScrn)
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Additionally a function is required to inclrease the Vsync
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frequency to max. 25 kHz.
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void XXX_ddc1SetSpeed(ScrnInfoPtr pScrn, xf86ddcSpeed speed)
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If the speed argument is DDC_FAST the function should increase
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the Vsync frequency on DDC_SLOW it should restore the original
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value. For convenience a generic ddc1SetSpeed() function is provided
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in the vga module for VGA-like chipsets.
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void vgaHWddc1SetSpeed(ScrnInfoPtr pScrn, sf86ddcSpeed speed).
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To read out the DDC1 data the driver should call
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xf86MonPtr xf86DoEDID_DDC1(int scrnIndex,
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void (*DDC1SetSpeed)(ScrnInfoPtr, xf86ddcSpeed),
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unsigned int (*DDC1Read)(ScrnInfoPtr))
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in PreInit(). DDC1SetSpeed is a pointer to the SetSpeed()
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function, DDC1Read has to point to the DDC1 read function.
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The function will return a pointer to the xf86Monitor structure
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which contains all information retreived by DDC.
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NULL will be returned on failure.
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DDC2 Support
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To read out DDC2 information I2C has to be initialized first.
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(See documentation for the i2c module).
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The function
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xf86MonPtr xf86DoEDID_DDC2(int scrnIndex, I2CBusPtr pBus)
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is provided to read out and process DDC2 data. A pointer
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to the I2CBusRec of the appropriate I2C Bus has to be passed
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as the second argument.
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The function will return a pointer to the xf86Monitor structure
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which contains all information retreived by DDC.
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NULL will be returned on failure.
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Printing monitor parameters
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To print out the information contained in the xf86Monitor
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structure the function
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xf86MonPtr xf86PrintEDID(xf86MonPtr monitor)
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is provided.
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Further processing of the xf86Monitor structure is not yet
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implemented. Howerver it is planned to use the information
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about video modes, gamma values etc.
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Therefore it is strongly recommended to read out DDC data
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before any video mode processing is done.
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$XFree86: xc/programs/Xserver/hw/xfree86/ddc/DDC.HOWTO,v 1.2 1998/12/06 13:30:39 dawes Exp $
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