Message block allocation

See ``Messages, STREAMS'' for information about message blocks and how they are allocated.

• STREAMS blocks are allocated with the allocb(D3str), allocb_physreq(D3str), or esballoc(D3str) function. If no STREAMS blocks are available, these calls fail. The driver should silently drop the mesage or data being processed and increment the mac_frame_nosr statistic. STREAMS allocb( ) failures show up in the STREAMS statistics that can be viewed with the netstat command or the crash strstat command.

• When allocating STREAMS buffers greater than 4K in a DDI driver, use the allocb_physreq(D3str) function and specify the buffer size in the phys_dmasize member of the physreq(D4) structure that is pointed to by the last argument of the allocb_physreq( ) function. The pages in these large STREAMS blocks are virtually contiguous but may not be physically contiguous if the buffers are allocated with the allocb(D3str) function. Drivers for adapters that use Bus Master DMA expect that each (address_start, length) scatter element represents a physically contiguous region of memory.

  DDI drivers can use the msgphysreq(D3str) function to check whether a STREAMS message block is physically contiguous.

• DDI drivers that use DMA should allocate a replacement block with the allocb_physreq(D3str) function before attempting to pass the completed receive frame upstream. allocb_physreq( ) points to a physreq(D4) structure that indicates physical address alignment and contiguity constraints on memory.

  Some older STREAMS drivers may include some intricate coding that prevented orphaned descriptors that could occur when STREAMS blocks for DMA were allocated with the allocb(D3str) function. Such code can be removed from DDI drivers when allocb_physreq( ) is used.

• STREAMS drivers should not generate DL_UNITDATA_IND(D7dlpi) messages because of allocb( ) failures for DL_UNITDATA_REQ(D7dlpi) messages.

• For drivers that use memory-mapped I/O, use esballoc(D3str) rather than allocb(D3str).

• Some devices such as the IBM Token-Ring Network 16/4 Busmaster Service Adapter/A are limited to 24 address lines and so cannot perform DMA operations with addresses beyond 16Mb. Because of the dynamic memory allocation scheme, it is not possible to guarantee that all STREAMS memory is in the lowest 16Mb of memory. DDI drivers for DMA devices that cannot perform DMA operations above 16Mb should set the phys_dmasize member of the physreq(D4) structure to the correct size (24 in this case) when running on systems with more than 16Mb of memory configured.