82583V Datasheet, PDF (251/374 Pages) Intel Corporation – Intel® 82583V GbE Controller

Driver Programing Interface—82583V GbE Controller
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PTHRESH is used to control when a prefetch of descriptors are considered. This
threshold refers to the number of valid, unprocessed transmit descriptors the chip has
in its on-chip buffer. If this number drops below PTHRESH, the algorithm considers pre-
fetching descriptors from host memory. However, this fetch does not happen unless
there are at least HTHRESH valid descriptors in host memory to fetch.
HTHRESH should be given a non-zero value when ever PTHRESH is used.
WTHRESH controls the write-back of processed transmit descriptors. This threshold
refers to the number of transmit descriptors in the on-chip buffer that are ready to be
written back to host memory. In the absence of external events (explicit flushes), the
write back occurs only after at least WTHRESH descriptors are available for write back.
• Possible values:
— GRAN = 1b (descriptor granularity):
— PTHRESH = 0..47
— WTHRESH = 0..63
— HTHRESH = 0..63
— GRAN = 0 (cacheline granularity):
— PTHRESH = 0..3 (for 16 descriptors cacheline - 256 bytes)
— WTHRESH = 0..3
— HTHRESH = 0..4
For any WTHRESH value other than zero - packet and absolute timers must get a non-
zero value for the WTHRESH feature to take affect.
Since the default value for write-back threshold is zero, descriptors are normally
written back as soon as they are processed. WTHRESH must be a non-zero value to
take advantage of the write-back bursting capabilities of the 82583V.
Since write-back of transmit descriptors is optional (under the control of RS bit in the
descriptor), not all processed descriptors are counted with respect to WTHRESH.
Descriptors start accumulating after a descriptor with RS is set. Furthermore, with
transmit descriptor bursting enabled, some descriptors are written back that did not
have RS set in their respective descriptors.
Leaving this value at its default causes descriptor processing to be similar to previous
devices.
As descriptors are transmitted the number of descriptors waiting in the transmit
descriptor queue decreases as noted by the transmit descriptor head and tail positions
in the circular queue. When the number of waiting descriptors drops to LWTHRESH (the
head and tail positions are sufficiently close to one another) an interrupt is asserted.
LWTHRESH controls the number of descriptors in transmit ring, at which a transmit
descriptor-low interrupt (ICR.TXD_LOW) is reported. This might enable software to
operate more efficiently by maintaining a continuous addition of transmit work,
interrupting only when the hardware nears completion of all submitted work.
LWTHRESH specifies a multiple of eight descriptors. An interrupt is asserted when the
number of descriptors available transitions from (threshold level=8*LWTHRESH)+1 ‡
(threshold level=8*LWTHRESH). Setting this value to zero disables this feature.
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