Intel PCI-X manual Receive Descriptor Write-Back, Receive Descriptor Queue Structure

Receive and Transmit Description

3.2.5Receive Descriptor Write-Back

Processors have cache line sizes that are larger than the receive descriptor size (16 bytes). Consequently, writing back descriptor information for each received packet would cause expensive partial cache line updates. Two mechanisms minimize the occurrence of partial line write backs:

•Receive descriptor packing

•Null descriptor padding

The following sections explain these mechanisms.

3.2.5.1Receive Descriptor Packing

To maximize memory efficiency, receive descriptors are “packed” together and written as a cache line whenever possible. Descriptors accumulate and are written out in one of three conditions:

•RXDCTL.WTHRESH descriptors have been used (the specified max threshold of unwritten used descriptors has been reached)

•The receive timer expires (RADV or RDTR)

•Explicit software flush (RDTR.FPD)

For the first condition, if the number of descriptors specified by RXDCTL.WTHRESH are used, they are written back, regardless of cacheline alignment. It is therefore recommended that WTHRESH be a multiple of cacheline sizes.

In the second condition, a timer (RDTR or RADV) expiration causes all used descriptors to be written back prior to initiating an interrupt.

In the second condition for the 82544GC/EI, a timer (RDTR) is included to force timely write– back of descriptors. The first packet after timer initialization starts the timer. Timer expiration flushes any accumulated descriptors and sets an interrupt event (receiver timer interrupt). In general, the arrival rate is sufficiently fast enough that packing is the common case under load.

For the final condition, software may explicitly flush accumulated descriptors by writing the timer register with the high order bit set.

3.2.5.2Null Descriptor Padding

Hardware stores no data in descriptors with a null data address. Software can make use of this property to cause the first condition under receive descriptor packing to occur early. Hardware writes back null descriptors with the DD bit set in the status byte and all other bits unchanged.

3.2.6Receive Descriptor Queue Structure

Figure 3-2shows the structure of the receive descriptor ring. Hardware maintains a circular ring of descriptors and writes back used descriptors just prior to advancing the head pointer. Head and tail pointers wrap back to base when “size” descriptors have been processed.

Software adds receive descriptors by writing the tail pointer with the index of the entry beyond the last valid descriptor. As packets arrive, they are stored in memory and the head pointer is incremented by hardware. When the head pointer is equal to the tail pointer, the ring is empty. Hardware stops storing packets in system memory until software advances the tail pointer, making more receive buffers available.

26 Software Developer’s Manual