Jumbo Frames
BCM5718 Programmer’s Guide
Broadcom
®
January 29, 2016 • 5718-PG108-R
Page 121
There is also a jumbo ring replenishment threshold register 0x2C1C. The controller DMAs BDs from the jumbo
ring in advance and caches them locally in controller memory. The controller initiates DMA of more BDs anytime
the local number of cached BDs falls below the threshold programmed in this register.
The maximum number of BDs that can be held in the jumbo ring is programmed in that ring's ring control block
RCB.
Ring Control Blocks
Each host based ring has an RCB associated with it. The same structure applies to all types of host rings, (i.e.,
receive producer ring, receive return ring, and also send rings). In the case of the BCM5718 family, the RCB
structure is enhanced to accommodate jumbo frames. However, the enhancement applies only to the Receive
Standard Producer ring.
The RCB essentially points to the physical address of the host memory where a producer ring is placed. The
BCM5718 family uses two RCBs, one for the Standard Ring and one for the jumbo ring. The jumbo ring RCB is
implemented over a set of four registers at the address range 0x2440–0x244F. The standard ring RCB register
addresses remain the same at 0x2450–0x245F. The send ring RCB and return ring RCBs are memory-mapped.
Figure 24: Ring Control Block
The host ring address is the host address of the first ring element. The host ring address is in host address
format. In controller-based send rings, the host address is ignored.
The NIC ring address is the address where a portion of a ring is cached in the controller's miscellaneous BD
memory. The driver need not program anything to these fields in most NetXtreme controllers. However, the
BCM5718 family does require the driver to program non-hardware-default values here.
The Max Len field is interpreted differently for different types of rings. In the case of receive producer rings and
receive return rings, this field indicates the maximum number of entries the ring can hold. The BCM5718 family
imposes constraints for different rings as shown below:
• Receive standard producer ring and send ring: Max Len should be programmed by the host to indicate the
maximum number of entries the ring will hold. The allowable values for the Standard Producer Ring Max
Len are 32, 64, 128, 256, 512, 1024, and 2048.
• Receive jumbo producer ring: Max Len should be programmed by the host to indicate the maximum
number of entries the ring will hold. The allowable values for the jumbo producer ring Max Len are 32, 64,
128, 256, 512, and 1024.
Host Ring Address
NIC Ring Address
Max Len
Max Frame Size
31
15
0
2
Flags
16
1