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Writing Device Drivers
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Exporting the Mapping

This section describes how to use the segmap(9E) and devmap(9E) entry points.

The segmap(9E) Entry Point

The segmap(9E) entry point is responsible for setting up a memory mapping requested by an mmap(2) system call. Drivers for many memory-mapped devices use ddi_devmap_segmap(9F) as the entry point rather than defining their own segmap(9E) routine. By providing a segmap() entry point, a driver can take care of general tasks before or after creating the mapping. For example, the driver can check mapping permissions and allocate private mapping resources. The driver can also make adjustments to the mapping to accommodate non-page-aligned device buffers. The segmap() entry point must call the ddi_devmap_segmap(9F) function before returning. The ddi_devmap_segmap() function calls the driver's devmap(9E) entry point to perform the actual mapping.

The segmap() function has the following syntax:

int segmap(dev_t dev, off_t off, struct as *asp, caddr_t *addrp,
     off_t len, unsigned int prot, unsigned int maxprot,
     unsigned int flags, cred_t *credp);

where:

dev

Device whose memory is to be mapped.

off

Offset within device memory at which mapping begins.

asp

Pointer to the address space into which the device memory should be mapped.

Note that this argument can be either a struct as *, as shown in Example 10-1, or a ddi_as_handle_t, as shown in Example 10-2. This is because ddidevmap.h includes the following declaration:

typedef struct as *ddi_as_handle_t
addrp

Pointer to the address in the address space to which the device memory should be mapped.

len

Length (in bytes) of the memory being mapped.

prot

A bit field that specifies the protections. Possible settings are PROT_READ, PROT_WRITE, PROT_EXEC, PROT_USER, and PROT_ALL. See the man page for details.

maxprot

Maximum protection flag possible for attempted mapping. The PROT_WRITE bit can be masked out if the user opened the special file read-only.

flags

Flags that indicate the type of mapping. Possible values include MAP_SHARED and MAP_PRIVATE.

credp

Pointer to the user credentials structure.

In the following example, the driver controls a frame buffer that allows write-only mappings. The driver returns EINVAL if the application tries to gain read access and then calls ddi_devmap_segmap(9F) to set up the user mapping.

Example 10-1 segmap(9E) Routine
static int
xxsegmap(dev_t dev, off_t off, struct as *asp, caddr_t *addrp,
    off_t len, unsigned int prot, unsigned int maxprot,
    unsigned int flags, cred_t *credp)
{
    if (prot & PROT_READ)
        return (EINVAL);
    return (ddi_devmap_segmap(dev, off, as, addrp,
        len, prot, maxprot, flags, cred));
}

The following example shows how to handle a device that has a buffer that is not page-aligned in its register space. This example maps a buffer that starts at offset 0x800, so that mmap(2) returns an address that corresponds to the start of the buffer. The devmap_devmem_setup(9F) function maps entire pages, requires the mapping to be page aligned, and returns an address to the start of a page. If this address is passed through segmap(9E), or if no segmap() entry point is defined, mmap() returns the address that corresponds to the start of the page, not the address that corresponds to the start of the buffer. In this example, the buffer offset is added to the page-aligned address that was returned by devmap_devmem_setup so that the resulting address returned is the desired start of the buffer.

Example 10-2 Using the segmap() Function to Change the Address Returned by the mmap() Call
#define    BUFFER_OFFSET 0x800

int
xx_segmap(dev_t dev, off_t off, ddi_as_handle_t as, caddr_t *addrp, off_t len,
    uint_t prot, uint_t maxprot, uint_t flags, cred_t *credp)
{
        int rval;
        unsigned long pagemask = ptob(1L) - 1L;

        if ((rval = ddi_devmap_segmap(dev, off, as, addrp, len, prot, maxprot,
            flags, credp)) == DDI_SUCCESS) {
                /*
                 * The address returned by ddi_devmap_segmap is the start of the page
                 * that contains the buffer.  Add the offset of the buffer to get the
                 * final address.
                 */
                *addrp += BUFFER_OFFSET & pagemask);
        }
        return (rval);
}

The devmap(9E) Entry Point

The devmap(9E) entry point is called from the ddi_devmap_segmap(9F) function inside the segmap(9E) entry point.

The devmap(9E) entry point is called as a result of the mmap(2) system call. The devmap(9E) function is called to export device memory or kernel memory to user applications. The devmap() function is used for the following operations:

  • Validate the user mapping to the device or kernel memory

  • Translate the logical offset within the application mapping to the corresponding offset within the device or kernel memory

  • Pass the mapping information to the system for setting up the mapping

The devmap() function has the following syntax:

int devmap(dev_t dev, devmap_cookie_t handle, offset_t off,
     size_t len, size_t *maplen, uint_t model);

where:

dev

Device whose memory is to be mapped.

handle

Device-mapping handle that the system creates and uses to describe a mapping to contiguous memory in the device or kernel.

off

Logical offset within the application mapping that has to be translated by the driver to the corresponding offset within the device or kernel memory.

len

Length (in bytes) of the memory being mapped.

maplen

Enables driver to associate different kernel memory regions or multiple physically discontiguous memory regions with one contiguous user application mapping.

model

Data model type of the current thread.

The system creates multiple mapping handles in one mmap(2) system call. For example, the mapping might contain multiple physically discontiguous memory regions.

Initially, devmap(9E) is called with the parameters off and len. These parameters are passed by the application to mmap(2). devmap(9E) sets *maplen to the length from off to the end of a contiguous memory region. The *maplen value must be rounded up to a multiple of a page size. The *maplen value can be set to less than the original mapping length len. If so, the system uses a new mapping handle with adjusted off and len parameters to call devmap(9E) repeatedly until the initial mapping length is satisfied.

If a driver supports multiple application data models, model must be passed to ddi_model_convert_from(9F). The ddi_model_convert_from() function determines whether a data model mismatch exists between the current thread and the device driver. The device driver might have to adjust the shape of data structures before exporting the structures to a user thread that supports a different data model. See Appendix C, Making a Device Driver 64-Bit Ready page for more details.

The devmap(9E) entry point must return -1 if the logical offset, off, is out of the range of memory exported by the driver.

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  Published under the terms fo the Public Documentation License Version 1.01. Design by Interspire