sync code with last improvements from OpenBSD

xserver/hw/xfree86/ddc/DDC.HOWTO

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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 continuously 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 activity 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
+   activity 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 preferable. 
+	
+   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 increase 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 retrieved 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 retrieved 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. However, 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: xc/programs/Xserver/hw/xfree86/ddc/DDC.HOWTO,v 1.2 1998/12/06 13:30:39 dawes Exp $