37.6.3.3 Receive AHB Buffers

Received frames, optionally including FCS, are written to receive AHB buffers stored in memory. The receive buffer depth is programmable in the range of 64 Bytes to 16 KBytes through the DMA Configuration register (GMAC_DCFGR), with the default being 128 Bytes.

The start location for each receive AHB buffer is stored in memory in a list of receive buffer descriptors at an address location pointed to by the receive buffer queue pointer. The base address for the receive buffer queue pointer is configured in software using the Receive Buffer Queue Base Address register (GMAC_RBQB).

Each list entry consists of two words. The first is the address of the receive AHB buffer and the second the receive status.

If the length of a receive frame exceeds the AHB buffer length, the status word for the used buffer is written with zeroes except for the “Start of Frame” bit, which is always set for the first buffer in a frame.

Bit zero of the address field is written to 1 to show that the buffer has been used. The receive buffer manager then reads the location of the next receive AHB buffer and fills that with the next part of the received frame data. AHB buffers are filled until the frame is complete and the final buffer descriptor status word contains the complete frame status. See the following table for details of the receive buffer descriptor list.

Table 37-2. Receive Buffer Descriptor Entry
Bit	Function
Word 0
31:2	Address of beginning of buffer
1	Wrap—marks last descriptor in receive buffer descriptor list.
0	Ownership—needs to be zero for the GMAC to write data to the receive buffer. The GMAC sets this to one once it has successfully written a frame to memory. Software has to clear this bit before the buffer can be used again.
Word 1
31	Global all ones broadcast address detected
30	Multicast hash match
29	Unicast hash match
28	–
27	Specific Address Register match found, bit 25 and bit 26 indicate which Specific Address Register causes the match.
26:25	Specific Address Register match. Encoded as follows: 00: Specific Address Register 1 match 01: Specific Address Register 2 match 10: Specific Address Register 3 match 11: Specific Address Register 4 match If more than one specific address is matched only one is indicated with priority 4 down to 1.
24	This bit has a different meaning depending on whether RX checksum offloading is enabled. With RX checksum offloading disabled: (bit 24 clear in Network Configuration Register) Type ID register match found, bit 22 and bit 23 indicate which type ID register causes the match. With RX checksum offloading enabled: (bit 24 set in Network Configuration Register) 0: The frame was not SNAP encoded and/or had a VLAN tag with the Canonical Format Indicator (CFI) bit set. 1: The frame was SNAP encoded and had either no VLAN tag or a VLAN tag with the CFI bit not set.
23:22	This bit has a different meaning depending on whether RX checksum offloading is enabled. With RX checksum offloading disabled: (bit 24 clear in Network Configuration) Type ID register match. Encoded as follows: 00: Type ID register 1 match 01: Type ID register 2 match 10: Type ID register 3 match 11: Type ID register 4 match If more than one Type ID is matched only one is indicated with priority 4 down to 1. With RX checksum offloading enabled: (bit 24 set in Network Configuration Register) 00: Neither the IP header checksum nor the TCP/UDP checksum was checked. 01: The IP header checksum was checked and was correct. Neither the TCP nor UDP checksum was checked. 10: Both the IP header and TCP checksum were checked and were correct. 11: Both the IP header and UDP checksum were checked and were correct.
21	VLAN tag detected—type ID of 0x8100. For packets incorporating the stacked VLAN processing feature, this bit will be set if the second VLAN tag has a type ID of 0x8100
20	Priority tag detected—type ID of 0x8100 and null VLAN identifier. For packets incorporating the stacked VLAN processing feature, this bit will be set if the second VLAN tag has a type ID of 0x8100 and a null VLAN identifier.
19:17	VLAN priority—only valid if bit 21 is set.
16	Canonical format indicator (CFI) bit (only valid if bit 21 is set).
15	End of frame—when set the buffer contains the end of a frame. If end of frame is not set, then the only valid status bit is start of frame (bit 14).
14	Start of frame—when set the buffer contains the start of a frame. If both bits 15 and 14 are set, the buffer contains a whole frame.
13	This bit has a different meaning depending on whether jumbo frames and ignore FCS modes are enabled. If neither mode is enabled this bit will be zero. With jumbo frame mode enabled: (bit 3 set in Network Configuration Register) Additional bit for length of frame (bit[13]), that is concatenated with bits[12:0] With ignore FCS mode enabled and jumbo frames disabled: (bit 26 set in Network Configuration Register and bit 3 clear in Network Configuration Register) This indicates per frame FCS status as follows: 0: Frame had good FCS 1: Frame had bad FCS, but was copied to memory as ignore FCS enabled.
12:0	These bits represent the length of the received frame which may or may not include FCS depending on whether FCS discard mode is enabled. With FCS discard mode disabled: (bit 17 clear in Network Configuration Register) Least significant 12 bits for length of frame including FCS. If jumbo frames are enabled, these 12 bits are concatenated with bit[13] of the descriptor above. With FCS discard mode enabled: (bit 17 set in Network Configuration Register) Least significant 12 bits for length of frame excluding FCS. If jumbo frames are enabled, these 12 bits are concatenated with bit[13] of the descriptor above.

Each receive AHB buffer start location is a word address. The start of the first AHB buffer in a frame can be offset by up to three Bytes, depending on the value written to bits 14 and 15 of the Network Configuration register (GMAC_NCFGR). If the start location of the AHB buffer is offset, the available length of the first AHB buffer is reduced by the corresponding number of Bytes.

To receive frames, the AHB buffer descriptors must be initialized by writing an appropriate address to bits 31:2 in the first word of each list entry. Bit 0 must be written with zero. Bit 1 is the wrap bit and indicates the last entry in the buffer descriptor list.

The start location of the receive buffer descriptor list must be written with the receive buffer queue base address before reception is enabled (receive enable in the Network Control register GMAC_NCR). Once reception is enabled, any writes to the Receive Buffer Queue Base Address register (GMAC_RBQB) are ignored. When read, it will return the current pointer position in the descriptor list, though this is only valid and stable when receive is disabled.

If the filter block indicates that a frame should be copied to memory, the receive data DMA operation starts writing data into the receive buffer. If an error occurs, the buffer is recovered.

An internal counter within the GMAC represents the receive buffer queue pointer and it is not visible through the CPU interface. The receive buffer queue pointer increments by two words after each buffer has been used. It re-initializes to the receive buffer queue base address if any descriptor has its wrap bit set.

As receive AHB buffers are used, the receive AHB buffer manager sets bit zero of the first word of the descriptor to logic one indicating the AHB buffer has been used.

Software should search through the “used” bits in the AHB buffer descriptors to find out how many frames have been received, checking the start of frame and end of frame bits.

When the DMA is configured in the packet buffer Partial Store And Forward mode, received frames are written out to the AHB buffers as soon as enough frame data exists in the packet buffer. For both cases, this may mean several full AHB buffers are used before some error conditions can be detected. If a receive error is detected the receive buffer currently being written will be recovered. Previous buffers will not be recovered. As an example, when receiving frames with cyclic redundancy check (CRC) errors or excessive length, it is possible that a frame fragment might be stored in a sequence of AHB receive buffers. Software can detect this by looking for start of frame bit set in a buffer following a buffer with no end of frame bit set.

To function properly, a 10/100 Ethernet system should have no excessive length frames or frames greater than 128 Bytes with CRC errors. Collision fragments will be less than 128 Bytes long, therefore it will be a rare occurrence to find a frame fragment in a receive AHB buffer, when using the default value of 128 Bytes for the receive buffers size.

When in packet buffer full store and forward mode, only good received frames are written out of the DMA, so no fragments will exist in the AHB buffers due to MAC receiver errors. There is still the possibility of fragments due to DMA errors, for example used bit read on the second buffer of a multi-buffer frame.

If bit zero of the receive buffer descriptor is already set when the receive buffer manager reads the location of the receive AHB buffer, the buffer has been already used and cannot be used again until software has processed the frame and cleared bit zero. In this case, the “buffer not available” bit in the receive status register is set and an interrupt triggered. The receive resource error statistics register is also incremented.

When the DMA is configured in the packet buffer full store and forward mode, the user can optionally select whether received frames should be automatically discarded when no AHB buffer resource is available. This feature is selected via the DMA Discard Receive Packets bit in the DMA Configuration register (GMAC_DCFGR.DDRP). By default, the received frames are not automatically discarded. If this feature is off, then received packets will remain to be stored in the SRAM-based packet buffer until AHB buffer resource next becomes available. This may lead to an eventual packet buffer overflow if packets continue to be received when bit zero (used bit) of the receive buffer descriptor remains set.

Note: After a used bit has been read, the receive buffer manager will re-read the location of the receive buffer descriptor every time a new packet is received. When the DMA is not configured in the packet buffer full store and forward mode and a used bit is read, the frame currently being received will be automatically discarded.

When the DMA is configured in the packet buffer full store and forward mode, a receive overrun condition occurs when the receive SRAM-based packet buffer is full, or because HRESP was not OK. In all other modes, a receive overrun condition occurs when either the AHB bus was not granted quickly enough, or because HRESP was not OK, or because a new frame has been detected by the receive block, but the status update or write back for the previous frame has not yet finished. For a receive overrun condition, the receive overrun interrupt is asserted and the buffer currently being written is recovered. The next frame that is received whose address is recognized reuses the buffer.

In any packet buffer mode, writing a '1' to the Flush Next Package bit in the NCR register (GMAC_NCR.FNP) will force a packet from the external SRAM-based receive packet buffer to be flushed. This feature is only acted upon when the RX DMA is not currently writing packet data out to AHB, i.e., it is in an IDLE state. If the RX DMA is active, GMAC_NCR.FNP=1 is ignored.