[lwip-users] NXP LPC 17xx lwip port: use of pbuf for rx

[Giuseppe Modugno]

I'm using LPC1769 MCU. I'm using source code from NXP SDK that works.

I need to free some RAM to use mbedTLS libraries, so I was studying the 
code better. I found that NXP Ethernet driver preallocates a number of 
pbufs for RX DMA descriptors. The code uses:

    p = pbuf_alloc(PBUF_RAW, (u16_t) ENET_ETH_MAX_FLEN, PBUF_RAM);

When a new frame is received, a new pbuf is allocated with the same 
instruction. The pbuf of received frame is finally freed.

With PBUF_RAM parameter, this code allocates pbuf with mem_malloc, that 
is standard malloc in my case. However ENET_ETH_MAX_FLEN is 1552 bytes. 
With so many alloc and free of 1552 bytes, I will have big fragmentation.

So I replace PBUF_RAM with PBUF_POOL, to use memory pools that don't 
suffer from fragmentation. I tried and it works, but I don't know if 
there's some drawbacks in doing so.