SCO OpenServer

dma_breakup(D3oddi)

dma_breakup -- size DMA request into 512-byte blocks

Synopsis

   #include <sys/buf.h>
   
   void dma_breakup(int *xxstrategy ( struct buf *bp), struct buf *bp);

Description

The dma_breakup( ) function breaks up Direct Memory Access (DMA) I/O requests into 512-byte units of contiguous memory to avoid limitations imposed by DMA controllers.

Arguments

xxstrategy: name of the strategy(D2oddi) entry point routine
bp: pointer to the buf structure.

Return values

None. However, the following values may change as side effects:

b_flags: if insufficient memory is available for allocation, b_flags is ORed with B_ERROR and B_DONE, and b_error is set to EAGAIN
b_error: has ENXIO set if end of media occurs in the first block transfer
b_flags: has B_ERROR set and b_error is set to 0 (zero) if end of media occurs in transfer of second and subsequent blocks
b_un.b_addr: set to the virtual kernel address
b_blkno: changed
b_bcount: changed
u.u_segflg: set to zero
u.u_base: is incremented by the number of characters transferred
u.u_count: is decremented by the number of characters transferred
u.u_offset: is incremented by the number of characters transferred

Usage

dma_breakup is called indirectly by the physio(D3oddi) function. Place the dma_breakup call in a subroutine and then call the name of that routine from physio.

dma_breakup first determines the correct block number of the data being passed.

If a read is being requested, the strategy(D2oddi) entry point is called to get a buffer header. If a buffer header is not available, dma_breakup( ) blocks sleep until one is free. While blocked, the request for a buffer is protected from signals and from interrupts occurring at or below spl6(D3oddi). When a buffer is free, data is read from user space.

A write request is similar, except that the data is copied to a kernel page from user space before the strategy( ) routine is called. Again, the driver blocks while waiting for a free buffer header.

After ensuring that a buffer header is free, DMA transfer starts. If an error is caused by reaching the end of the media during the first block transfer, ENXIO is set in bp->b_error when dma_breakup returns. An end of media fault, on second and subsequent blocks, causes ENXIO to be cleared from b_error, the number of blocks remaining to be transferred to be put in b_resid, and B_ERROR to be set in bp->b_flags. During DMA transfer, strategy( ) is called to put the current buffer on the buffering mechanism. Each time xxstrategy( ) is called, sleep is also called to wait until buffering occurs.

dma_breakup depends on the following fields of the user structure that are set up by the kernel when the I/O request is passed to the driver:

u.u_base: the virtual base address for the calling program in user space
u.u_count: the number of bytes to be transferred
u.u_offset: offset into the file from/to which data is transferred

In addition, the driver should set b_flags to indicate the type of transfer. Possible values are B_READ or B_WRITE.

Use splx(D3oddi) to save your spl setting before calling dma_breakup because dma_breakup calls spl0(D3oddi) and cancels all previously set spl levels.

Context and synchronization

User context.

Hardware applicability

All

Version applicability

oddi: 1, 2, 2mp

SVR5 DDI compatibility

This function is not supported for DDI drivers. buf_breakup(D3) provides similar functionality.

References

physio(D3oddi), strategy(D2oddi)