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

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Date:	Wed, 05 Dec 2012 12:58:47 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	nicolas.ferre@...el.com
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

From: Nicolas Ferre <nicolas.ferre@...el.com>
Date: Wed, 5 Dec 2012 16:08:22 +0100

> 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...

I almost think you have to.
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