uint_t dlim_addr_lo; /* low range of 32 bit addressing capability */
uint_t dlim_addr_hi; /* inclusive upper bound of addressing */
/* capability */
uint_t dlim_cntr_max; /* inclusive upper bound of dma engine's */
/* address limit * /
uint_t dlim_burstsizes; /* binary encoded dma burst sizes */
uint_t dlim_minxfer; /* minimum effective dma transfer size */
uint_t dlim_dmaspeed; /* average dma data rate (kb/s) */
The dlim_addr_lo and dlim_addr_hi fields specify the address range the device's DMA engine can access. The dlim_addr_lo field describes the lower 32-bit boundary of the device's DMA engine, the dlim_addr_hi describes the inclusive upper 32-bit boundary. The system allocates DMA resources in a way that the address for programming the device's DMA engine (see ddi_dma_cookie(9S) or ddi_dma_htoc(9F)) is within this range. For example, if your
device can access the whole 32-bit address range, you may use [0,0xFFFFFFFF]. If your device has just a 16-bit address register but will access the top of the 32-bit address range, then [0xFFFF0000,0xFFFFFFFF]
is the right limit.
The dlim_cntr_max field describes an inclusive upper bound for the device's DMA engine address register. This handles a fairly common case where a portion of the address register is only a latch rather than a full register. For example, the upper
8 bits of a 32-bit address register can be a latch. This splits the address register into a portion that acts as a true address register (24 bits) for a 16 Mbyte segment and a latch (8 bits) to hold a segment number. To describe these limits, specify 0xFFFFFF in the dlim_cntr_max structure.
The dlim_burstsizes field describes the possible burst sizes the device's DMA engine can accept. At the time of a DMA resource request, this element defines the possible DMA burst
cycle sizes that the requester's DMA engine can handle. The format of the data is binary encoding of burst sizes assumed to be powers of two. That is, if a DMAengine is capable of doing 1-, 2-, 4-, and 16-byte transfers,
the encoding ix 0x17. If the device is an SBus device and can take advantage of a 64-bit SBus, the lower 16 bits are used to specify the burst size for 32-bit transfers and the upper 16 bits are used to specify the burst size for 64-bit transfers. As the resource request is handled by
the system, the burstsizes value can be modified. Prior to enabling DMA for the specific device, the driver that owns the DMA engine should check (using ddi_dma_burstsizes(9F)) what the allowed burstsizes have become and program the DMA engine appropriately.
The dlim_minxfer field describes the minimum effective DMA transfer size (in units of bytes). It must be a power of two. This value specifies the minimum effective granularity of the DMA engine. It is distinct from dlim_burstsizes in that it describes the minimum amount of access a DMA transfer will effect. dlim_burstsizes describes in what electrical fashion the DMA engine might perform its accesses, while dlim_minxfer describes the minimum amount of memory that can be touched by the DMA transfer. As a resource request is handled by the system, the dlim_minxfer value can be modified contingent upon the presence (and use) of I/O caches
and DMA write buffers in between the DMA engine and the object that DMA is being performed on. After DMA resources have been allocated, the resultant minimum transfer value
can be gotten using ddi_dma_devalign(9F).
The field dlim_dmaspeed is the expected average data rate for the DMA engine (in units of kilobytes per second). Note that this should not be the maximum, or peak, burst data rate, but a reasonable guess as to the average throughput. This field
is entirely optional and can be left as zero. Its intended use is to provide some hints about how much of the DMA resource this device might need.