Re: [PATCH] net/macb: increase RX buffer size for GEM

From: Nicolas Ferre
Date: Wed Dec 05 2012 - 10:08:10 EST

On 12/04/2012 07:22 PM, David Miller :
> From: Nicolas Ferre <nicolas.ferre@xxxxxxxxx>
> Date: Mon, 3 Dec 2012 13:15:43 +0100
>> Macb Ethernet controller requires a RX buffer of 128 bytes. It is
>> highly sub-optimal for Gigabit-capable GEM that is able to use
>> a bigger DMA buffer. Change this constant and associated macros
>> with data stored in the private structure.
>> I also kept the result of buffers per page calculation to lower the
>> impact of this move to a variable rx buffer size on rx hot path.
>> RX DMA buffer size has to be multiple of 64 bytes as indicated in
>> DMA Configuration Register specification.
>> Signed-off-by: Nicolas Ferre <nicolas.ferre@xxxxxxxxx>
> This looks like it will waste a couple hundred bytes for 1500 MTU
> frames, am I right?

Yep! But buffers get recycled, and with the current memory management by
pages, it seems that I have to rework some part of it to optimize this
memory usage (8KB memory blocks split into 5 buffers each as David said...).

Do you think it is worth digging this way or may I rework the rx buffer
management in case of the GEM interface. If I implement a different path
for GEM interface, I will have the possibility to tailor rx DMA buffers
from 1500 Bytes up to 10KB jumbo frames...

Best regards,
Nicolas Ferre
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