Host Data Buffer Alignment; Buffer Size Calculations; Dma Arbiter - ST STM32F101xx Reference Manual

Ethernet (ETH): media access control (MAC) with DMA controller
28.6.3

Host data buffer alignment

The transmit and receive data buffers do not have any restrictions on start address
alignment. In our system with 32-bit memory, the start address for the buffers can be aligned
to any of the four bytes. However, the DMA always initiates transfers with address aligned to
the bus width with dummy data for the byte lanes not required. This typically happens during
the transfer of the beginning or end of an Ethernet frame.
Example of buffer read:
If the Transmit buffer address is 0x0000 0FF2, and 15 bytes need to be transferred,
then the DMA will read five full words from address 0x0000 0FF0, but when transferring
data to the Transmit FIFO, the extra bytes (the first two bytes) will be dropped or
ignored. Similarly, the last 3 bytes of the last transfer will also be ignored. The DMA
always ensures it transfers a full 32-bit data items to the Transmit FIFO, unless it is the
end of frame.
Example of buffer write:
If the Receive buffer address is 0x0000 0FF2, and 16 bytes of a received frame need to
be transferred, then the DMA will write five full 32-bit data items from address
0x0000 0FF0. But the first 2 bytes of the first transfer and the last 2 bytes of the third
transfer will have dummy data.
28.6.4

Buffer size calculations

The DMA does not update the size fields in the transmit and receive descriptors. The DMA
updates only the status fields (xDES0) of the descriptors. The driver has to calculate the
sizes. The transmit DMA transfers the exact number of bytes (indicated by buffer size field in
TDES1) towards the MAC core. If a descriptor is marked as first (FS bit in TDES0 is set),
then the DMA marks the first transfer from the buffer as the start of frame. If a descriptor is
marked as last (LS bit in TDES0), then the DMA marks the last transfer from that data buffer
as the end of frame. The receive DMA transfers data to a buffer until the buffer is full or the
end of frame is received. If a descriptor is not marked as last (LS bit in RDES0), then the
buffer(s) that correspond to the descriptor are full and the amount of valid data in a buffer is
accurately indicated by the buffer size field minus the data buffer pointer offset when the
descriptor's FS bit is set. The offset is zero when the data buffer pointer is aligned to the
databus width. If a descriptor is marked as last, then the buffer may not be full (as indicated
by the buffer size in RDES1). To compute the amount of valid data in this final buffer, the
driver must read the frame length (FL bits in RDES0[29:16]) and subtract the sum of the
buffer sizes of the preceding buffers in this frame. The receive DMA always transfers the
start of next frame with a new descriptor.
Note: Even when the start address of a receive buffer is not aligned to the system databus width
the system should allocate a receive buffer of a size aligned to the system bus width. For
example, if the system allocates a 1024 byte (1 KB) receive buffer starting from address
0x1000, the software can program the buffer start address in the receive descriptor to have
a 0x1002 offset. The receive DMA writes the frame to this buffer with dummy data in the first
two locations (0x1000 and 0x1001). The actual frame is written from location 0x1002. Thus,
the actual useful space in this buffer is 1022 bytes, even though the buffer size is
programmed as 1024 bytes, due to the start address offset.
28.6.5

DMA arbiter

The arbiter inside the DMA takes care of the arbitration between transmit and receive
channel accesses to the AHB master interface. Two types of arbitrations are possible:
858/959
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