Intel PCI-X manual Receive Descriptor Control, Rxdctl 02828h R/W, Rxdctl Register Bit Description

Register Descriptions

13.4.46Receive Descriptor Control

RXDCTL (02828h; R/W)

This register controls the fetching and write-back of receive descriptors. The three threshold values are used to determine when descriptors are read from and written to host memory. The values can be in units of cache lines or descriptors (each descriptor is 16 bytes) based on the GRAN flag. If GRAN = 0b (specifications are in cache-line granularity), the thresholds specified (based on the cacheline size specified in the PCI header CLS field) must not represent greater than 31 descriptors.

Table 13-87. RXDCTL Register Bit Description

31 25 24 23 22 21 16 15 14 13 8 7 6 5 0

Reserved GRAN RSV

WTHRESH

RSV

HTHRESH

RSV

PTHRESH

Field	Bit(s)	Initial	Description
		Value
			Prefetch Threshold
			Used to control when a prefetch of descriptors is considered. This
			threshold refers to the number of valid, unprocessed receive
			descriptors the Ethernet controller has in its on-chip buffer. If this
PTHRESH	5:0	0b	number drops below PTHRESH, the algorithm considers
			prefetching descriptors from host memory. This fetch does not
			happen unless there are at least RXDCTL.HTHRESH valid
			descriptors in host memory to fetch. Value of PTHRESH can be in
			either cache line units, or based on number of descriptors based
			on RXDCTL.GRAN.
			Reserved
RSV	7:6	0b	Reads as 0b.
			Should be written as 0b for future compatibility.
			Host Threshold
			Provides the threshold of the valid descriptors in host memory.
			A descriptors prefetch is performed each time enough valid
			descriptors (TXDCTL.HTHRESH) are available in host memory,
HTHRESH	13:8	0b	no other DMA activity of greater priority is pending (descriptor
			fetches and write backs or packet data transfers) and the number
			of receive descriptors the Ethernet controller has on its on-chip
			buffers drops below RXDCTL.PTHRESH. Value of HTHRESH
			can be in either cache line units, or based on number of
			descriptors based on RXDCTL.GRAN.
			Reserved
RSV	15:14	0b	Reads as 0b.
			Should be written as 0b for future compatibility.

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