SPRUI30H November 2015 – May 2024 DRA745 , DRA746 , DRA750 , DRA756
After reset, the host must write zeroes to all TX DMA State head descriptor pointers. The TX port may then be enabled. To initiate packet transmission the host constructs transmit queues in memory (one or more packets for transmission) and then writes the appropriate TX DMA state head descriptor pointers. For each buffer added to a transmit queue, the host must initialize the TX buffer descriptor values as follows:
The port begins TX packet transmission on a given channel when the host writes the channel's TX queue head descriptor pointer with the address of the first buffer descriptor in the queue (nonzero value). Each channel may have one or more queues, so each channel may have one or more head descriptor pointers. The first buffer descriptor for each TX packet must have the Start of Packet (SOP) bit and the Ownership bit set to one by the host. The last buffer descriptor for each TX packet must have the End of Packet (EOP) bit set to one by the host. The port will transmit packets until all queued packets have been transmitted and the queue(s) are empty. When each packet transmission is complete, the port will clear the Ownership bit in the packet's SOP buffer descriptor and issue an interrupt to the host by writing the packet's last buffer descriptor address to the queue's TX DMA State Completion Pointer. The interrupt is generated by the write, regardless of the value written. When the last packet in a queue has been transmitted, the port sets the End Of Queue bit in the EOP buffer descriptor, clears the Ownership bit in the SOP Descriptor, zeroes the appropriate DMA state head descriptor pointer, and then issues a TX interrupt to the host by writing to the queue's associated TX completion pointer (address of the last buffer descriptor processed by the port). The port issues a maskable level interrupt (which may then be routed through external interrupt control logic to the host).
On interrupt from the port, the host processes the buffer queue, detecting transmitted packets by the status of the Ownership bit in the SOP buffer descriptor. If the Ownership bit is cleared to zero, then the packet has been transmitted and the host may reclaim the buffers associated with the packet. The host continues queue processing until the end of the queue or until a SOP buffer descriptor is read that contains a set Ownership bit indicating that the packet transmission is not complete. The host determines that all packets in the queue have been transmitted when the last packet in the queue has a cleared Ownership bit in the SOP buffer descriptor, the End of Queue bit is set in the last packet EOP buffer descriptor, and the Next Descriptor Pointer of the last packet EOP buffer descriptor is zero. The host acknowledges an interrupt by writing the address of the last buffer descriptor to the queue's associated TX Completion Pointer in the TX DMA State. If the host written buffer address value is different from the buffer address written by the port, then the level interrupt remains asserted. If the host written buffer address value is equal to the port written value, then the level interrupt is de-asserted. The port write to the completion pointer actually stores the value in the state register (RAM). The host written value is actually not written to the register location. The host written value is compared to the register contents (which was written by the port) and if the two values are equal, the interrupt is removed, otherwise the interrupt remains asserted. The host may process multiple packets previous to acknowledging an interrupt, or the host may acknowledge interrupts for every packet.
A mis-queued packet condition may occur when the host adds a packet to a queue for transmission as the port finishes transmitting the previous last packet in the queue. The mis-queued packet is detected by the host when queue processing detects a cleared Ownership bit in the SOP buffer descriptor, a set End of Queue bit in the EOP buffer descriptor, and a nonzero Next Descriptor Pointer in the EOP buffer descriptor. A mis-queued packet means that the port read the last EOP buffer descriptor before the host added the new last packet to the queue, so the port determined queue empty just before the last packet was added. The host corrects the mis-queued packet condition by initiating a new packet transfer for the mis-queued packet by writing the mis-queued packet's SOP buffer descriptor address to the appropriate DMA State TX Queue head Descriptor Pointer.
The host may add packets to the tail end of an active TX queue at any time by writing the Next Descriptor Pointer to the current last descriptor in the queue. If a TX queue is empty (inactive), the host may initiate packet transmission at any time by writing the appropriate TX DMA State head descriptor pointer. The host software should always check for and reinitiate transmission for mis-queued packets during queue processing on interrupt from the port. In order to preclude software underrun, the host should avoid adding buffers to an active queue for any TX packet that is not complete and ready for transmission.
The port determines that a packet is the last packet in the queue by detecting the End of Packet bit set with a zero Next Descriptor Pointer in the packet buffer descriptor. If the End of Packet bit is set and the Next Descriptor Pointer is nonzero, then the queue still contains one or more packets to be transmitted. If the EOP bit is set with a zero Next Descriptor Pointer, then the port will set the EOQ bit in the packet's EOP buffer descriptor and then zero the appropriate head descriptor pointer previous to interrupting the port (by writing the completion pointer) when the packet transmission is complete.