svatophys - General: Converts a system virtual address to
a physical address
vm_offset_t *phys_addr );
Specifies the kernel virtual address. Specifies a pointer
to the physical address to be filled in.
The svatophys routine converts a system virtual address to
the corresponding physical address. All address and data
structure manipulation done within the kernel is performed
using system virtual addresses. Typically, system virtual
addresses are a means of mapping physical memory and I/O
space, which often consists of device registers and DMA
buffers. In contrast to this, devices are usually unaware
of any virtual addressing and for this reason use physical
addresses. You use the svatophys routine to perform this
As an example of where you can use this address translation,
a disk device driver can use DMA buffers to transfer
blocks of data to the disk (for the case of a write operation).
The data to be written to disk is present in system
memory at a system virtual address known to the driver. To
initiate the DMA operation, the disk driver can set up a
command packet to specify a write operation to the underlying
disk controller hardware. This write command packet
contains (among other things) the location of the DMA
buffer as a physical address and the length of the buffer.
Here, the driver calls the svatophys routine to translate
the system virtual address of the DMA buffer to a physical
address in the command packet issued to the disk driver.
The svatophys routine returns the following: The address
translation has been completed successfully. Unable to
perform address translation. This value indicates that the
address specified by the kern_addr argument is not a valid
kernel or system virtual address.
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