Ed Sutter wrote:
Ok, I see what you're talking about in pbuf_header(). I don't know the history
of the code to comment on whether or not the change should be in the master
code base; but it would seem to me that if this is forcing a driver to use
the memcpy loop to build the pbuf chain, then it would be a good improvement.
The reason pbuf_header can't expand to the front is that it doesn't know
where the memory area that the PBUF_REF points to starts. While
shrinking works as long as p->len is > 0, expanding to the front doesn't
work without another pointer in the pbuf struct that tells us the
original ->payload pointer.
For this reason, it is not a good thing to give this 'feature' to
everyone as this could very easily lead to memory corruption.
The way to prevent memcpy is to allocate a PBUF_POOL (of the maximum
size) and pass the ->payload pointer to the MAC which can store the
receive data directly to that location. This method has some
disadvantages though: a) with most MACs, you cannot use chained pbufs
for receiving and b) you cannot use this method with MACs that have
internal receive buffers.
Just so you know: changing this _is_ on the to-do-list somewhere, but I
think don't even know whether there is a feature-request for this
somewhere...
Simon
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