How to add an (S)VGA driver to XFree86 : The Driver Itself
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6. The Driver Itself

Now it's time to get down to the real work - writing the major driver functions in the files sdc_driver.c. First, an overview of what the responsibilities of the driver are:

  1. Provide a chipset-descriptor data structure to the server. This data structure contains pointers to the driver functions and some data-structure initialization as well.
  2. Provide a driver-local data structure to hold the contents of the chipset registers. This data structure will contain a generic part and a driver-specific part. It is used to save the initial chipset state, and is initialized by the driver to put the chipset into different modes.
  3. Provide an identification function that the server will call to list the chipsets that the driver is capable of supporting.
  4. Provide a probe function that will identify this chipset as different from all others, and return a positive response if the chipset this driver supports is installed, and a negative response otherwise.
  5. Provide a function to select dot-clocks available on the board.
  6. Provide functions to save, restore, and initialize the driver- local data structure.
  7. Provide a function to set the starting address for display in the video memory. This implements the virtual-screen for the server.
  8. Perhaps provide a function for use during VT-switching.
  9. Perhaps provide a function to check if each mode is suitable for the chipset being used.

Before stepping through the driver file in detail, here are some important issues:

  1. If your driver supports both the color and monochrome servers, you should take care of both cases in the same file. Most things are the same - you can differentiate between the two with the MONOVGA #define. If the 16 color server is supported, code specific to it can be enabled with the XF86VGA16 #define. In most cases it is sufficient to put the following near the top of the stub_driver.c file:
       #ifdef XF86VGA16
       #define MONOVGA
  2. The color server uses the SVGA's 8-bit packed-pixel mode. The monochrome and vga16 servers uses the VGA's 16-color mode (4 bit-planes). Only one plane is enabled for the monochrome server.
  3. It is possible for you to define your monochrome driver so that no bank-switching is done. This is not particularly desirable, as it yields only 64k of viewing area.
Keeping these things in mind, you need to find the registers from your SVGA chipset that control the desired features. In particular, registers that control:
  1. Clock select bits. The two low-order bits are part of the standard Miscellaneous Output Register; most SVGA chipsets will include 1 or 2 more bits, allowing the use of 8 or 16 discrete clocks.
  2. Bank selection. The SVGA chipset will have one or two registers that control read/write bank selection.
  3. CRTC extensions. The standard VGA registers don't have enough bits to address large displays. So the SVGA chipsets have extension bits.
  4. Interlaced mode. Standard VGA does not support interlaced displays. So the SVGA chipset will have a bit somewhere to control interlaced mode. Some chipsets require additional registers to be set up to control interlaced mode
  5. Starting address. The standard VGA only has 16 bits in which to specify the starting address for the display. This restricts the screen size usable by the virtual screen feature. The SVGA chipset will usually provide one or more extension bits.
  6. Lock registers. Many SVGA chipset prevent modification of extended registers unless the registers are first ``unlocked''. You will need to disable protection of any registers you will need for other purposes.
  7. Any other facilities. Some chipset may, for example, require that certain bits be set before you can access extended VGA memory (beyond the IBM-standard 256k). Or other facilities; read through all of the extended register descriptions and see if anything important leaps out at you.

If you are fortunate, the chipset vendor will include in the databook some tables of register settings for various BIOS modes. You can learn a lot about what manipulations you must do by looking at the various BIOS modes.

6.1. Multiple Chipsets And Options

It is possible, and in fact desirable, to have a single driver support multiple chipsets from the same vendor. If there are multiple supported chipsets, then you would have a series of #define's for them, and a variable `SDCchipset', which would be used throughout the driver when distinctions must be made. See the Trident and PVGA1/WD drivers for examples (the Tseng ET3000 and ET4000 are counter-examples - these were implemented before the driver interface allowed for multiple chipsets, so this example should NOT be followed). Note that you should only distinguish versions when your driver needs to do things differently for them. For example, suppose the SDC driver supports the SDC-1a, SDC-1b, and SDC-2 chipsets. The -1a and -1b are essentially the same, but different from the -2 chipset. Your driver should support the -1 and -2 chipsets, and not distinguish between the -1a and -1b. This will simplify things for the end user.

In cases where you want to give the user control of driver behavior, or there are things that cannot be determined without user intervention, you should use ``option'' flags. Say that board vendors that use the SDC chipsets have the option of providing 8 or 16 clocks. There's no way you can determine this from the chipset probe, so you provide an option flag to let the user select the behavior from the XF86Config file. The option flags are defined in the file ``xf86_option.h''. You should look to see if there is already a flag that can be reused. If so, use it in your driver. If not, add a new #define, and define the string->symbol mapping in the table in that file. To see how option flags are used, look at the ET4000, PVGA1/WD, and Trident drivers.

6.2. Data Structures

Once you have an understanding of what is needed from the above description, it is time to fill in the driver data structures. First we will deal with the `vgaSDCRec' structure. This data structure is the driver-local structure that holds the SVGA state information. The first entry in this data structure is ALWAYS `vgaHWRec std'. This piece holds the generic VGA portion of the information. After that, you will have one `unsigned char' field for each register that will be manipulated by your driver. That's all there is to this data structure.

Next you must initialize the `SDC' structure (type `vgaVideoChipRec'). This is the global structure that identifies your driver to the server. Its name MUST be `SDC', in all caps - i.e. it must match the directory name for your driver. This is required so that the Link Kit reconfiguration can identify all of the requisite directories and global data structures.

The first section of this structure simply holds pointers to the driver functions.

Next, you must initialize the information about how your chipset does bank switching. The following fields must be filled in:

  1. ChipMapSize - the amount of memory that must be mapped into the server's address space. This is almost always 64k (from 0xA0000 to 0xAFFFF). Some chipsets use a 128k map (from 0xA0000 to 0xBFFFF). If your chipset gives an option, use the 64k window, as a 128k window rules out using a Hercules or Monochrome Display Adapter card with the SVGA.
  2. ChipSegmentSize - the size of each bank within the ChipMapSize window. This is usually also 64k, however, some chipsets split the mapped window into a read portion and a write portion (for example the PVGA1/Western Digital chipsets).
  3. ChipSegmentShift - the number of bits by which an address will be shifted right to mask of the bank number. This is log-base-2 of ChipSegmentSize.
  4. ChipSegmentMask - a bitmask used to mask off the address within a given bank. This is (ChipSegmentSize-1).
  5. ChipReadBottom,ChipReadTop - the addresses within the mapped window in which read operations can be done. Usually 0, and 64k, respectively, except for those chipset that have separate read and write windows.
  6. ChipWriteBottom,ChipWriteTop - same as above, for write operations.
  7. ChipUse2Banks - a boolean value for whether this chipset has one or two bank registers. This is used to set up the screen-to-screen operations properly.
There are three more fields that must be filled in:
  1. ChipInterlaceType - this is either VGA_NO_DIVIDE_VERT or VGA_DIVIDE_VERT. Some chipsets require that the vertical timing numbers be divided in half for interlaced modes. Setting this flag will take care of that.
  2. ChipOptionFlags - this should always be `{0,}' in the data structure initialization. This is a bitfield that contains the Option flags that are valid for this driver. The appropriate bits are initialized at the end of the Probe function.
  3. ChipRounding - this gets set to the multiple by which the virtual width of the display must be rounded for the 256-color server. This value is usually 8, but may be 4 or 16 for some chipsets.

6.3. The Ident() function

The Ident() function is a very simple function. The server will call this function repeatedly, until a NULL is returned, when printing out the list of configured drivers. The Ident() function should return a chipset name for a supported chipset. The function is passed a number which increments from 0 on each iteration.

6.4. The ClockSelect() function

The ClockSelect() function is used during clock probing (i.e. when no `Clocks' line is specified in the XF86Config file) to select the dot-clock indicated by the number passed in the parameter. The function should set the chipset's clock-select bits according to the passed-in number. Two dummy values will be passed in as well (CLK_REG_SAVE, CLK_SAVE_RESTORE). When CLK_REG_SAVE is passed, the function should save away copies of any registers that will be modified during clock selection. When CLK_REG_RESTORE is passed, the function should restore these registers. This ensure that the clock-probing cannot corrupt registers.

This function should return FALSE if the passed-in index value is invalid or if the clock can't be set for some reason.

6.5. The Probe() function

The Probe() function is perhaps the most important, and perhaps the least intuitive function in the driver. The Probe function is required to identify the chipset independent of all other chipsets. If the user has specified a `Chipset' line in the XF86Config file, this is a simple string comparison check. Otherwise, you must use some other technique to figure out what chipset is installed. If you are lucky, the chipset will have an identification mechanism (ident/version registers, etc), and this will be documented in the databook. Otherwise, you will have to determine some scheme, using the reference materials listed below.

The identification is often done by looking for particular patterns in register, or for the existence of certain extended registers. Or with some boards/chipsets, the requisite information can be obtained by reading the BIOS for certain signature strings. The best advise is to study the existing probe functions, and use the reference documentation. You must be certain that your probe is non-destructive - if you modify a register, it must be saved before, and restored after.

Once the chipset is successfully identified, the Probe() function must do some other initializations:

  1. If the user has not specified the `VideoRam' parameter in the XF86Config file, the amount of installed memory must be determined.
  2. If the user has not specified the `Clocks' parameter in the XF86Config file, the values for the available dot-clocks must be determined. This is done by calling the vgaGetClocks() function, and passing it the number of clocks available and a pointer to the ClockSelect() function.
  3. It is recommended that the `maxClock' field of the server's `vga256InfoRec' structure be filled in with the maximum dot-clock rate allowed for this chipset (specified in KHz). If this is not filled in a probe time, a default (currently 90MHz) will be used.
  4. The `chipset' field of the server's `vga256InfoRec' structure must be initialized to the name of the installed chipset.
  5. If the driver will be used with the monochrome server, the `bankedMono' field of the server's `vga256InfoRec' structure must be set to indicate whether the monochrome driver supports banking.
  6. If any option flags are used by this driver, the `ChipOptionFlags' structure in the `vgaVideoChipRec' must be initialized with the allowed option flags using the OFLG_SET() macro.

6.6. The EnterLeave() function

The EnterLeave() function is called whenever the virtual console on which the server runs is entered or left (for OSs without virtual consoles, the function is called when the server starts and again when it exits). The purpose of this function is to enable and disable I/O permissions (for OSs where such is required), and to unlock and relock access to ``protected'' registers that the driver must manipulate. It is a fairly trivial function, and can be implemented by following the comments in the stub driver.

6.7. The Restore() function

The Restore() function is used for restoring a saved video state. Note that `restore' is a bit of a misnomer - this function is used to both restore a saved state and to install a new one created by the server. The Restore() function must complete the following actions:

  1. Ensure that Bank 0 is selected, and that any other state information required prior to writing out a new state has been set up.
  2. Call vgaHWRestore() to restore the generic VGA portion of the state information. This function is in the vgaHW.c file.
  3. Restore the chipset-specific portion of the state information. This may be done by simply writing out the register, or by doing a read/modify/write cycle if only certain bits are to be modified. Be sure to note the comment in the sample driver about how to handle clock-select bits.

6.8. The Save() function

The Save() function is used to extract the initial video state information when the server starts. The Save() function must complete the following actions:

  1. Ensure that Bank 0 is selected.
  2. Call vgaHWSave() to extract the generic VGA portion of the state information. This function is in the vgaHW.c file.
  3. Extract the chipset-specific portion of the state information.

6.9. The Init() function

The Init() function is the second most important function in the driver (after the Probe() function). It is used to initialize a data structure for each of the defined display modes in the server. This function is required to initialize the entire `vgaSDCRec' data structure with the information needed to put the SVGA chipset into the required state. The generic VGA portion of the structure is initialized with a call to vgaHWInit() (also located in vgaHW.c).

Once the generic portion is initialized, the Init() function can override any of the generic register initialization, if necessary. All of the other fields are filled in with the correct initialization. The information about the particular mode being initialized is passed in the `mode' parameter, a pointer to a `DisplayModeRec' structure. This can be dereferenced to determine the needed parameters.

If you only know how to initialize certain bits of the register, do that here, and make sure that the Restore() function does a read/modify/write to only manipulate those bits. Again, refer to the existing drivers for examples of what happens in this function.

6.10. The Adjust() function

The Adjust() function is another fairly basic function. It is called whenever the server needs to adjust the start of the displayed part of the video memory, due to scrolling of the virtual screen or when changing the displayed resolution. All it does is set the starting address on the chipset to match the specified coordinate. Follow the comments in the stub driver for details on how to implement it.

6.11. The ValidMode() function

The ValidMode() function is required. It is used to check for any chipset-dependent reasons why a graphics mode might not be valid. It gets called by higher levels of the code after the Probe() stage. In many cases no special checking will be required and this function will simply return TRUE always.

6.12. The SaveScreen() function

The SaveScreen() function is not needed by most chipsets. This function would only be required if the extended registers that your driver needs will be modified when a synchronous reset is performed on the SVGA chipset (your databook should tell you this). If you do NOT need this function, simply don't define it, and put `NoopDDA' in its place in the vgaVideoChipRec structure initialization (NoopDDA is a generic-use empty function).

If you DO need this function, it is fairly simple to do. It will be called twice - once before the reset, and again after. It will be passed a parameter of SS_START in the former case, and SS_FINISH in the latter. All that needs to be done is to save any registers that will be affected by the reset into static variables on the SS_START call, and then restore them on the SS_FINISH call.

6.13. The GetMode() function

The GetMode() function is not used as of XFree86 1.3; its place in the vgaVideoChipRec should be initialized to `NoopDDA'.

At some point in the future, this function will be used to enable the server and/or a standalone program using the server's driver libraries to do interactive video mode adjustments. This function will read the SVGA registers and fill in a DisplayModeRec structure with the current video mode.

6.14. The FbInit() function

The FbInit() function is required for drivers with accelerated graphics support. It is used to replace default cfb.banked functions with accelerated chip-specific versions. vga256LowlevFuncs is a struct containing a list of functions which can be replaced. This struct defined in vga256.h. Examples of FbInit() functions can be found in the et4000, pvga1 and cirrus drivers.

If you do NOT need this function, simply don't define it, and put `NoopDDA' in its place in the vgaVideoChipRec structure initialization.

How to add an (S)VGA driver to XFree86 : The Driver Itself
Previous: The Bank-Switching Functions
Next: Building The New Server