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

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Message-ID: <50BF6366.8080600@atmel.com>
Date:	Wed, 5 Dec 2012 16:08:22 +0100
From:	Nicolas Ferre <nicolas.ferre@...el.com>
To:	David Miller <davem@...emloft.net>
CC:	<netdev@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
	<linux-kernel@...r.kernel.org>, <manabian@...il.com>,
	<plagnioj@...osoft.com>
Subject: Re: [PATCH] net/macb: increase RX buffer size for GEM

On 12/04/2012 07:22 PM, David Miller :
> From: Nicolas Ferre <nicolas.ferre@...el.com>
> 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@...el.com>
> 
> 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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