DDC.HOWTO (3742B)
1 DDC.HOWTO 2 3 This file describes how to add DDC support to a chipset driver. 4 5 1) DDC INITIALIZATION 6 7 When implementing DDC in the driver one has the choice between 8 DDC1 and DDC2. 9 DDC1 data is continuously transmitted by a DDC1 capable display 10 device. The data is send serially over a data line; the Vsync 11 signal serves as clock. Only one EDID 1.x data block can be 12 transmitted using DDC1. Since transmission of an EDID1 block 13 using a regular Vsync frequency would take up several seconds 14 the driver can increase the Vsync frequency to up to 25 kHz as 15 soon as it detects DDC1 activity on the data line. 16 DDC2 data is transmitted using the I2C protocol. This requires 17 an additional clock line. DDC2 is capable of transmitting EDID1 18 and EDID2 block as well as a VDIF block on display devices that 19 support these. 20 Display devices switch into the DDC2 mode as soon as they detect 21 activity on the DDC clock line. Once the are in DDC2 mode they 22 stop transmitting DDC1 signals until the next power cycle. 23 24 Some graphics chipset configurations which are not capable of 25 DDC2 might still be able to read DDC1 data. Where available 26 DDC2 it is preferable. 27 28 All relevant prototypes and defines are in xf86DDC.h. 29 DDC2 additionally requires I2C support. The I2C prototypes 30 are in xf86i2c.h. 31 32 DDC1 Support: 33 34 The driver has to provide a read function which waits for the 35 end of the next Vsync signal and reads in and returns the status 36 of the DDC line: 37 38 unsigned int XXX_ddc1Read(ScrnInfoPtr pScrn) 39 40 Additionally a function is required to increase the Vsync 41 frequency to max. 25 kHz. 42 43 void XXX_ddc1SetSpeed(ScrnInfoPtr pScrn, xf86ddcSpeed speed) 44 45 If the speed argument is DDC_FAST the function should increase 46 the Vsync frequency on DDC_SLOW it should restore the original 47 value. For convenience a generic ddc1SetSpeed() function is provided 48 in the vga module for VGA-like chipsets. 49 50 void vgaHWddc1SetSpeed(ScrnInfoPtr pScrn, sf86ddcSpeed speed). 51 52 To read out the DDC1 data the driver should call 53 54 xf86MonPtr xf86DoEDID_DDC1(int scrnIndex, 55 void (*DDC1SetSpeed)(ScrnInfoPtr, xf86ddcSpeed), 56 unsigned int (*DDC1Read)(ScrnInfoPtr)) 57 58 in PreInit(). DDC1SetSpeed is a pointer to the SetSpeed() 59 function, DDC1Read has to point to the DDC1 read function. 60 The function will return a pointer to the xf86Monitor structure 61 which contains all information retrieved by DDC. 62 NULL will be returned on failure. 63 64 DDC2 Support 65 66 To read out DDC2 information I2C has to be initialized first. 67 (See documentation for the i2c module). 68 The function 69 70 xf86MonPtr xf86DoEDID_DDC2(int scrnIndex, I2CBusPtr pBus) 71 72 is provided to read out and process DDC2 data. A pointer 73 to the I2CBusRec of the appropriate I2C Bus has to be passed 74 as the second argument. 75 The function will return a pointer to the xf86Monitor structure 76 which contains all information retrieved by DDC. 77 NULL will be returned on failure. 78 79 Printing monitor parameters 80 81 To print out the information contained in the xf86Monitor 82 structure the function 83 84 xf86MonPtr xf86PrintEDID(xf86MonPtr monitor) 85 86 is provided. 87 88 Further processing of the xf86Monitor structure is not yet 89 implemented. However, it is planned to use the information 90 about video modes, gamma values etc. 91 Therefore it is strongly recommended to read out DDC data 92 before any video mode processing is done. 93 94 95 96 97 $XFree86: xc/programs/Xserver/hw/xfree86/ddc/DDC.HOWTO,v 1.2 1998/12/06 13:30:39 dawes Exp $