TSEC rx DMA putting in wrong spot?

[zwilcox]

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 15.9.8.2 - 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 15.9.8.3 - 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? :smileysad:
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 15.9.8.3 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]);
}
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[bpe]

P1020RM, Table Table 15-1108 states:

The receive buffer pointer, which always points to the first location of the associated data
buffer, must be 8-byte aligned. For best performance, use 64-byte aligned receive buffer
pointer addresses.

Regards,

Platon

[zwilcox]

Thank you sir.