Jumbo FramesBCM5718 Programmer’s Guide
Broadcom®
January 29, 2016 • 5718-PG108-R Page 122
• Receive return rings: Max Len should be programmed by the host to indicate the maximum number of
entries the ring will hold. In case of return rings, the host must program this field to be greater than or equal
to the combined value of Max Len fields of the receive standard producer ring and the receive jumbo
producer ring. For example, if Standard Ring Max Len == 32 and jumbo ring Max Len == 32, then return
ring Max Len must be 64 or higher. This means the allowable values for the return ring Max Len are 32, 64,
128, 256, 512,1024, 2048, and 4096.
The receive standard ring RCB uses the Max Frame Size field to indicate the maximum length of each buffer to
be described by the buffer descriptor placed into the standard ring. In this manner any frame received that is
larger than this value causes the controller to attempt to use a jumbo ring buffer instead of a standard ring buffer.
The Max Frame Size field is unused in the receive jumbo ring RCB, receive return ring RCB and send ring RCB.
The Flags field is described below in Ta b le 33.
Receive Return Ring(s)
There are no structural changes to the return rings in the BCM5718 family. There are no functional changes from
standard size frames or from a Receive Side Scaling (RSS) perspective. The impact of the jumbo frame feature
to the return rings is clarified here:
• The same return ring carries jumbo frames as well as standard size frames. Intermixing can happen
without any limitation.
• The total number of return rings remains 4. However, when RSS is disabled, return ring 0 is the only active
return ring and all RX frames are returned over Ring 0.
• When an extended buffer descriptor is returned to a receive return ring, the extended portion of the
descriptor is truncated by the controller. The offset range 0x00–0x01F is the extended portion (see
Figure 24 on page 121).
• Furthermore, when an extended BD is returned in a return ring, the length of the entire RX frame is
consolidated in the Len 0 field even if any or all of the scatter buffer pieces of the extended BD were used
to place the frame in host memory.
Send Buffer Descriptor
The send buffer descriptor (SBD) has been updated to accommodate LSO over jumbo frames. Figure 25 on
page 123 below illustrates the updated SBD format.
Table 33: Receive BD Flags
Bit # Flag Name Flag Description
0 Reserved Reserved
1 RCB_FLAG_RING_DISABLED Indicates that the Ring is not in use
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