[ECOS] Avoiding memcpy in ethernet drivers

[Edgar Grimberg]

Hi Bart,

Thanks for the response. The reason why I started the conversation is 
that I did a bit of profiling on the working system and memcpy is on the 
top of the list. I was expecting this and, in order to optimize, I 
wanted to do something about the biggest CPU consumer. In other systems 
I would have been linking the code into a faster RAM area, but in this 
case I have none available. So my next idea was to minimize the usage of 
memcpy.
Some comments inlined:

>     Edgar> Having to write a driver for an ethernet device, I noticed
>     Edgar> that there are 2 memcpy()s in most of the drivers, one when
>     Edgar> sending and one when receiving. This usually happens from
>     Edgar> and to the sg_list passed from the hardware independent
>     Edgar> ethernet driver. Now, is this really necessary?
>
> Ethernet hardware varies widely, but there are two main categories:
> fifo vs. DMA.
>   
My case is, of course, DMA based ethernet.
> Changing the current API between drivers and higher-level code would
> impact existing drivers. There are a number of different ways of
> tackling that problem, but nothing simple.
>   
This is what I'm afraid of. I tried to hack my way around it for a short 
while, but I don't see an easy way to do it without breaking the 
existing code and to keep the drivers independent from the stack 
implementation.
> The current code works well enough for most people. Realistically, it
> is unlikely that anything would happen in this area except possibly as
> part of a re-import of the BSD stack. Just the re-import would be a
> big job, never mind changing the device driver interface. It may
> happen some day, most likely if/when somebody really needs maximum
> performance and is willing to pay for the work.
>   
In my case, the clients are spinning their own tcp/ip stack with zero 
memcpy and some FPGA help. I just wanted to provide a "reference" system 
for measurements, the fastest one I can get with minimal modifications.

Regards,
Edgar

-- 
Edgar Grimberg
System Developer
Zylin AS
Phone: (+47) 51 63 25 00


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