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TSEC rx DMA putting in wrong spot?

Question asked by Zachary Wilcox on Feb 6, 2019
Latest reply on Feb 27, 2019 by Zachary Wilcox

I'm writing drivers for the TSEC.  I'm able to transmit.  I'm trying working on a bug while receiving.

While setting up my Receive Buffer descriptors, I set the buffer pointer to their corresponding arrays.

When I receive a packet, the DMA puts it in 4 bytes before where I put my buffer pointer.
IE The DMA places the packet in 0x20d188 if I set my buffer pointer to 0x20d18c.
It does this for every packet I receive.  
I noticed the memory location is 64 bit aligned.  I know the Buffer Descriptor needs to be 64-bit aligned.  Does the buffer that the BD buffer pointer points to need to 64 bit aligned as well?  I receive the packet at the correct location when I aligned the buffer to 64 bits.  

Below is a snippet of creating the BDs, buffers, and setting up the BD's


#define NUM_OF_BD_BUFFER 10

typedef struct {
uint16_t flags; //Flags Fields see Section - table 15-1106 in RM
uint16_t length; //Buffer length
uint32_t bufptr; //Buffer Pointer
} txbd_t;

typedef struct {
uint16_t flags; //Flags Fields see Section - table 15-1108 in RM
uint16_t length; //Buffer Length
uint32_t bufptr; //Buffer Pointer
} rxbd_t;

typedef struct {

//The TxBD and RxBD need to be 64 bit aligned
//this is due to the tbase/rbase register for the DMA needing to be 64 bit aligned
txbd_t txbd[NUM_OF_BD_BUFFER] __attribute__ ((aligned (64)));
rxbd_t rxbd[NUM_OF_BD_BUFFER] __attribute__ ((aligned (64)));

tsec_t *regs;
phy* phy;

uint32_t rx_idx; //index of the current RX buffer
uint32_t tx_idx; //index of the current TX buffer
uint64_t mac_address;
uint32_t ip_address;
} eth_device_t;

#define RX_BUFFER_SIZE 0x00001000 // size per receive buffer
//Why does it work when I align this to 64 bits?
uint8_t rx_buffers[NUM_OF_BD_BUFFER][RX_BUFFER_SIZE]; //__attribute__ ((aligned (64)));

static void setup_rx_buffer_bd(eth_device_t *dev)

for(uint8_t i = 0; i < NUM_OF_BD_BUFFER; i++)
dev->rxbd[i].bufptr = (uint32_t)(&rx_buffers[i][0]);
//prepare the rx bds and mark them as empty
for(uint8_t i = 0; i < NUM_OF_BD_BUFFER; i++)
//SEE Section for flags
dev->rxbd[i].flags = RXBD_EMPTY | RXBD_INTERRUPT;

dev->rxbd[NUM_OF_BD_BUFFER - 1].flags |= RXBD_WRAP;

* Set the RBASE register. RBASE points to the start of
* the receive BD ring. Note that at least two BDs should
* be present in a transmit or receive BD ring

dev->regs->rbase = (uint32_t)(&dev->rxbd[0]);