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Date:	Mon, 10 Dec 2012 16:59:04 +0530
From:	Mugunthan V N <>
To:	Christian Riesch <>
CC:	<>, <>,
	<>, <>
Subject: Re: [PATCH 1/1] net: ethernet: davinci_cpdma: Add boundary for rx
 and tx descriptors

On 12/10/2012 1:54 PM, Christian Riesch wrote:
> Hi again,
> On Mon, Dec 10, 2012 at 8:37 AM, Mugunthan V N <> wrote:
>> When there is heavy transmission traffic in the CPDMA, then Rx descriptors
>> memory is also utilized as tx desc memory this leads to reduced rx desc memory
>> which leads to poor performance.
> "poor performance" is an understatement, see Sascha's description of
> his patch. At initialization of the driver, half of the descriptors in
> the pool are allocated for rx. When a packet arrives, one of the rx
> descriptors is released and a new one is allocated. If tx allocates
> this descriptor in the meantime, it is lost for rx forever! If tx
> consumes all rx descriptors this way, the rx channel is dead!
> Regards, Christian
>> This patch adds boundary for tx and rx descriptors in bd ram dividing the
>> descriptor memory to ensure that during heavy transmission tx doesn't use
>> rx descriptors.
>> This patch is already applied to davinci_emac driver, since CPSW and
>> davici_dmac uses the same CPDMA, moving the boundry seperation from
>> Davinci EMAC driver to CPDMA driver which was done in the following
>> commit
>> commit 86d8c07ff2448eb4e860e50f34ef6ee78e45c40c
>> Author: Sascha Hauer <>
>> Date:   Tue Jan 3 05:27:47 2012 +0000
>>      net/davinci: do not use all descriptors for tx packets
>>      The driver uses a shared pool for both rx and tx descriptors.
>>      During open it queues fixed number of 128 descriptors for receive
>>      packets. For each received packet it tries to queue another
>>      descriptor. If this fails the descriptor is lost for rx.
>>      The driver has no limitation on tx descriptors to use, so it
>>      can happen during a nmap / ping -f attack that the driver
>>      allocates all descriptors for tx and looses all rx descriptors.
>>      The driver stops working then.
>>      To fix this limit the number of tx descriptors used to half of
>>      the descriptors available, the rx path uses the other half.
>>      Tested on a custom board using nmap / ping -f to the board from
>>      two different hosts.
>> Signed-off-by: Mugunthan V N <>
Will change the commit description and resubmit the patch.

Mugunthan V N
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