Most Qualcomm Atheros PLC chipsets are Ethernet-to-Powerline bridges but the QCA7000 is a Serial-to-Powerline bridge, ... with a big difference. The QCA7000 expects the host serial stream to be segmented into Ethernet frames where each frame is encapsulated by a distinct serial header and trailer. This means the host can format and transmit, or receive and decode, standard Ethernet 802.3 frames over an ordinary SPI or UART interface thereby enabling full Ethernet or Internet protocol communications over powerline at low cost and low speed. The enabling component here is an Ethernet-to-Serial driver that supports the SPI or UART interface connected to the QCA7000. This section covers such a driver. The example driver described here was written for the Freescale iMX28 board support package running a custom Freescale Linux distribution. As such, we believe that this driver is suitable for the iMX28 processor out of the box but it could be adapted to other processors. Also, the QCA7000 support other serial modes but they are not described or explained in this document.

The basic unit of data transfer over MII and powerline is the IEEE 802.3 Ethernet frame. On output, the host must encapsulate each Ethernet frame as shown below before serial transmission to the QCA7000. The QCA7000 strips off the serial header and footer and forwards the frame over powerline. Allocate a 1528-byte buffer to accomodate the largest outgoing serial packet and pad with NUL bytes. Write SOF, compute and write FL, copy the outgoing frame then write EOF. Serially transmit FL + 12 buffer bytes to the QCA7000. Offset Length Symbol Description 0x0000 4 SOF Start Of Frame. Must be 0xAAAAAAAA. 0x0004 2 FL The Ethernet frame length in little endian format. The frame starts at offset 0x0008 here and includes all fields up to but excluding EOF. The minimum is 60. The maximum is 1518 if VLAN is omitted and 1522 if not. 0x0006 2 RSVD Must be 0x0000. Reserved to ensure 4-byte frame alignment. 0x0008 6 DA Destination address. 0x000E 6 SA Source address. This must not be the MAC address of the powerline device. This must be the MAC address of the local host serial interface as assigned by the SP or UART driver. The PLC device and the associated host interface must have different MAC address. 0x0014 4 VLAN Virtual LAN tag. This field may be omitted. 0x0018 2 ET Ethertype. This field starts at offset 0x0014 if VLAN is omitted. 0x001A 46 to 1500 BODY Frame body. This field starts at offset 0x0016 and ranges from 46 to 1500 bytes if VLAN is omitted. 0x004A to 0x05F8 4 EOF End Of Frame. Must be 0x5555. This field starts at offset 0x0008 plus FL. The SPI read frame occurs in response to a read interrupt generated by the QCA7000. The host must acknowledge a read interrupt and service it by reading and acting on QCA7000 SPI register values. For incoming frames, the host reads the 32-bit overall packet length reported by the QCA7000 then read that many bytes, stripping off the serial header and footer and forwarding the Ethernet frame to the host. Allocate a 1532-byte buffer to accomodate the largest incoming serial packet. Read LEN to determine the size of the incoming serial packet. Read LEN bytes into the buffer. Beware that LEN is a multiple of 4-bytes so there may be a few trailing NUL bytes in buffer. Offset Length Symbol Description 0x0000 4 LEN Hardware generated packet length. This field is only generated for SPI packets, not the UART packets. 0x0004 4 SOF Start Of Frame. Must be 0xAAAAAAAA. 0x0008 2 FL Ethernet frame length in little endian format. The frame starts at offset 0x000C here and includes all fields up to but excluding EOF. The minimum length is 60. The maximum is 1518 if VLAN is omitted and 1522 if not. 0x000A 2 RSVD Must be 0x0000. Reserved to ensure 4-byte frame alignment. 0x000C 6 DA Destination address. 0x0012 6 SA Source address. This must not be the MAC address of the powerline device. This must be the MAC address of the local host serial interface as assigned by the SP or UART driver. The PLC device and the associated host interface must have different MAC address. 0x0018 4 VLAN Virtual LAN tag. This field may be omitted. 0x001C 2 ET Ethertype. This field starts offset 0x0018 if VLAN is omitted. 0x001E 46 to 1500 BODY Frame body. This field starts at offset 0x001A and ranges from 46 to 1500 bytes if VLAN is omitted. 0x004A to 0x05F8 4 EOF End Of Frame. Must be 0x5555. This field starts at offset 0x000C plus FL.

This chapter presents pseudo code for the QCA7000 SPI driver. Qualcomm Atheros provides a precoded driver for Linux but customers may want to write their own driver for a non-Linux environment. In either case, the following steps are required for efficient SPI communications. function receive frame allocate an input buffer; if no memory available then return failure; end if; read the read buffer space available register; if read buffer is empty then return an error; end if; write frame length into buffer size register; if mode is legacy then send SPI read command; end if; while read buffer is not empty do read read buffer; extract Ethernet frame from your frame buffer; update any read statistics; end while; return success; end function; function transmit frame do read the write buffer space available register; while transmit queue is not empty and write buffer space is available do encapsulate transmit frame; write transmit frame to write buffer; update transmit statistics; if error then return failure; end if; update transmit statistics; remove frame from transmit queue; end while; return success; end function; The interrupt service routine suspends, waiting for an interrupt from the QCA7000. Use of an interrupt means that the host need not continuously poll the SPI status and space available registers to detect if an operation has completed successfully before starting the next operation. Once an interrupt is detected by the host, this routine disables further interrupts and determines shy the interrupt occured by inspecting the INTR_CAUSE and SPI_STATUS registers. It then handles the interrupt by calling appropriate functions. When done, it clears the interrupt cause and SPI status registers and suspends. function interrupt service routine do while terminate is false do set thread state to interruptable; if no interrupts and synchronization state is synchronized and transmit queue is empty then allow other tasks to run; end if; set thread state to busy; check synchronization state; if syncrhonization state is not synchronized then flush transmit queue; suspend for a while; end if; if interrupt occurred then disable interrupts; read interrupt cause register; if interrupt cause is CPU on then update synchronization state; if synchronization state is synchronized then continue; end if; end if; if interrupt cause is packet available then if synchronization state is synchronized then call receive frame function; end if; end if; if interrupt cause is read buffer error then set synchronization state to unknown; continue; end if; if interrupt cause is write buffer error then set synchronization state to unknown; continue; end if; clear interrupt cause register; clear SPI status register; end if; if transmit queue is not empty then call transmit frame function; end if; end while; set thread state to dormant; return; end function; function synchronize slave do allocate a static reset counter; if synchronization state is CPU on then read QCA7000 signature register; read QCA7000 signature register; if signature is invalid then set synchronization state to unknown; else read write buffer space available register; if write buffer is empty then set qca.SynchState to QCASPI_SYNC_READY; set synchronization state to ready; return; else set synchronization state to unknown; end if; end if; end if; if synchronization state is ready then if mode is legacy then return; end if; read QCA7000 signature register; if signature is invalid then set synchronization state to unknown; return; end if; end if; if synchronization state is unknown then if mode is legacy then use GPIO to reset QCA7000; else read QCA7000 signature register; if signature is invalid then return; end if; set soc_core_reset bit in QCA SPI configuration register; end if; set synchronization state to reset; clear reset counter; return; end if; if synchronization state is reset then increment reset counter; if reset counter exceeds reset limit then set synchronization state to unknown; end if; end if; return; end function; function interrupt handler do increment interrupt count; if thread is available and thread is dormant then wake up thread to service interrupt; end if; return success; end function;

static int qcaspi_spi_thread char void * data Manages synchronization with the exteranl QCA7000. Handles interrupts fomr the external QCA7000. Transmits frames for the transmit queue to the QCA7000.

void qca_qca7k_sync char struct qcaspi * qca int event Keeps track of the current synchonization state.

uint16_t qcaspi_read_register struct qcaspi * qca uint16_t reg Reads a QCA7000 register and returns register content.

void qcaspi_write_register struct qcaspi * qca uint16_t reg uint16_t value Write a value into a QCA7000 register.

int qcaspi_tx_cmd struct qcaspi * qca uint16_t cmd Transmit a 16-bit command to the QCA7000. This is currently used when performing a legacy DMA read or write.

uint32_t disable_spi_interrupts struct qcaspi * qca Disables interrupts by writing 0 to the QCA7000 INTR_ENABLE register.

uint32_t enable_spi_interrupts struct qcaspi * qca uint32_t intr_enable Enables interrupts specified by writing to the QCA7000 INTR_ENABLE register and returns the previous register value.

static irqreturn_t qcaspi_intr_handler int irq void * data Called to service interrupts on rising edge of the QCA7000 interrupt line.

int qcaspi_transmit struct qcaspi * qca Transmit as many frames as possible from the transmit queue.

int qcaspi_tx_frame struct qcaspi * qca struct sk_buff * skb Transmit a single socket buffer over the SPI interface.

void qcaspi_flush_txq struct qcaspi * qca Flush the transmit queue. Typically called when a synchronization issue is detected between the SPI master (host) and SPI slave (QCA7000).

int qcaspi_receive struct qcaspi * qca Reads the QCA7000 read buffer bytes register and reads all available data from the QCA7000. Calls function to parse out the individual Ethernet frames and passes them to the Linux kernel protocol stack.

uint32_t qcaspi_dma_read_burst struct qcaspi * qca uint8_t * buffer uint32_t length Performs QCA7000 DMA burst read.

uint32_t qcaspi_dma_read_legacy struct qcaspi * qca uint8_t * buffer uint32_t length Performs QCA7000 DMA legacy read.

uint32_t qcaspi_dma_write_burst struct qcaspi * qca uint8_t * buffer uint32_t length Called by qcaspi_tx_frame to peform a DMA burst write instead of a legacy write.

uint32_t qcaspi_dma_write_legacy struct qcaspi * qca uint8_t * buffer uint32_t length Called by qcaspi_tx_frame to peform a DMA legacy write instead of a burst write.

uint32_t QcaFrmCreateHeader uint8_t * buffer uint16_t length Encode buffer with the required SPI header and overall frame length.

uint32_t QcaFrmCreateFooter uint8_t * buffer Encode buffer with the required SPI footer.

void QcaFrmFsmInit QcaFrmHdl * frmHdl Initialize the state machine used to decode the incoming QCA7000 byte stream.

uint32_t QcaFrmFsmDecode QcaFrmHdl * frmHdl uint8_t * buffer uint16_t length uint8_t bytevalue Feeds incoming bytes into the state machine and breaks the stream into individual frames. Frames are passed to the Linux kernel.

int qcaspi_netdev_xmit struct sk_buff * skb struct net_device * device Called by the Linux kernel append outgoing frames to the transmit queue.

void qcaspi_netdev_tx_timeout struct net_device * device Called by the Linux kernel after the transmit queue has been stopped for an extended period of time.

static void qcaspi_netdev_uninit struct net_device * device Called when function unregister_netdev is called. For the QCA7000 driver, function qcaspi_mod_exit calls function unregister_netdev.

struct net_device_stats * qcaspi_netdev_get_stats struct net_device * device Returns transmit, receive and error statistics associated with the net device. These are the statistics displayed by ifconfig.

int qcaspi_netdev_change_mtu struct net_device * device int new_mtu Changes the serial interface MTU size.

static int qcaspi_netdev_set_mac_address struct net_device * device void * memory Sets the serial interface MAC address. Called by ifconfig whenever user types ifconfig qca0 hw ether xx:xx:xx:xx:xx:xx.

int qcaspi_netdev_close struct net_device * device Called by ifconfig to disable the network interface.

static void qcaspi_mod_exit void Called by the kernel to shutdown the driver module.