Re: [lwip-users] Memory alignment and DMA controllers

[address@hidden]

On 12.01.2018 13:20, Will Wykeham wrote:
> [..]
> The possible ways of fixing it that I have come up with are:
>
> (1)
> Change the argument in pbuf_alloc where it passes the pointer to 
> pbuf_init_alloced_pbuf from:
> LWIP_MEM_ALIGN((void *)((u8_t *)p + SIZEOF_STRUCT_PBUF + offset))
> To:
> LWIP_MEM_ALIGN((void *)((u8_t *)p + SIZEOF_STRUCT_PBUF)) + offset
> This aligns the data space after the pbuf, and then adds on the offset. The 
> payload pointer starts of unaligned, but becomes aligned when wound back by 14 
> bytes.
> This is fine as long as the 'layer' value is correct, but my understanding is 
> that it is a conservative maximum and when more stack layers are involved it 
> may be an over estimate, so we're not guaranteed the payload pointer will be 
> wound back to where it needs to be. Also, that's me patching the stack which 
> I'd rather not do.
> [..]
> Am I missing something or is this just a limitation of how the stack works?

That's still a 'todo', unfortunately. Mainly because never really
started working on it. I guess in most devices it either works or high
ethernet throughput is not the main goal.

Let's start working on this. Your solution 1 looks good, let's try that.
Are you working with the git master version of lwIP? That would ensure
we could share the code.

As an addition, you need to flush the cache before TX. To do that
safely, your buffers need to start on a cache line (not only on a 32 bit
boundary), so we need another define for alignment.

In the end, you always need to implement a memcpy fallback in your
driver. A netif needs to be able to send*all*  packets, as you never
know how bogus an lwIP callback application allocates a pbuf. This is
also true when forwarding a packet , e.g. from PPP to ETH.

Simon