XMC4500 TIP of the day: DMA descriptor in Ethernet Module

[Travis]

The DMA in the Ethernet subsystem transfers data based on a linked list of Tx/Rx descriptors using the Ring or Chain structure.
(See attachment)

The Tx/Rx descriptors are located in the RAM area and the base address of the descriptors are initialised in the
- TRANSMIT_DESCRIPTOR_LIST_ADDRESS
- RECEIVE_DESCRIPTOR_LIST_ADDRESS

The descriptor contain a set of Tx/Rx Status and Control bits, 2 Buffer length and 2 Buffer address pointer.

Important descriptor bit field
OWN – Own Bit (1:DMA, 0:CPU)
FS – First Descriptor
LS – Last Descriptor
Buffer 1 or 2 Address Pointer – Points to Tx/Rx buffer

[Travis]

Question and Answer ...

Any body knows the size of a ETH DMA descriptor?

[Travis]

Hi all,

A Tx/Rx descriptor is make up of 4 words (32bits) and arrays of Tx/Rx descriptors are normally used for DMA link list purposes.

The declaration of ETH DMA descriptor can be found in "DMA_Descriptors.c" of ETH001

/** Tx DMA Descriptot List */
DmaDesc Eth_TxDMADescList;
uint8_t Eth_TxBuffer[RX_BUFFER_SIZE];


uint8_t Eth_RxBuffer[RX_BUFFER_SIZE];
/** Rx DMA Descriptot List */
DmaDesc Eth_RxDMADescList[RECEIVE_DESC_SIZE];

[#4]

In version 1.0.22 of ETH001 the DMA_Descriptors are linked to section "ETH_RAM", but there is no such section in the linker file. So the arrays are linked to normal BSS section.

In the linker file (V1.1) there is only the section "Communication_Buffers" which should be used for, or shouldn´t it?

/** Tx DMA Descriptot List */
DmaDesc __attribute__((section ("ETH_RAM"))) Eth_TxDMADescList __attribute__ ((aligned (4)));
uint8_t __attribute__((section ("ETH_RAM"))) Eth_TxBuffer[TX_BUFFER_SIZE] __attribute__ ((aligned (4)));

/** Rx DMA Descriptot List */
DmaDesc __attribute__((section ("ETH_RAM"))) Eth_RxDMADescList[RECEIVE_DESC_SIZE] __attribute__ ((aligned (4)));
uint8_t __attribute__((section ("ETH_RAM"))) Eth_RxBuffer[RX_BUFFER_SIZE] __attribute__ ((aligned (4)));

[Travis]

Hi Hissi,

I am not well verse with the linker script, but definitely the DMA descriptor list and Tx/Rx buffer should be allocated in the RAM area.

Best Regards
Travis