netdev - Re: [PATCH 2/7] net: dcbnl, add multicast group for DCB

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Date:	Mon, 20 Jun 2011 09:16:22 -0700
From:	John Fastabend <john.r.fastabend@...el.com>
To:	Shmulik Ravid <shmulikr@...adcom.com>
CC:	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH 2/7] net: dcbnl, add multicast group for DCB

On 6/19/2011 11:14 AM, Shmulik Ravid wrote:
> 
> On Fri, 2011-06-17 at 14:16 -0700, John Fastabend wrote:
>> Now that dcbnl is being used in many cases by more
>> than a single agent it is beneficial to be notified
>> when some entity either driver or user space has
>> changed the DCB attributes.
>>
>> Today applications either end up polling the interface
>> or relying on a user space database to maintain the DCB
>> state and post events. Polling is a poor solution for
>> obvious reasons. And relying on a user space database
>> has its own downside. Namely it has created strange
>> boot dependencies requiring the database be populated
>> before any applications dependent on DCB attributes
>> starts or the application goes into a polling loop.
>> Populating the database requires negotiating link
>> setting with the peer and can take anywhere from less
>> than a second up to a few seconds depending on the switch
>> implementation.
>>
>> Perhaps more importantly if another application or an
>> embedded agent sets a DCB link attribute the database
>> has no way of knowing other than polling the kernel.
>> This prevents applications from responding quickly to
>> changes in link events which at least in the FCoE case
>> and probably any other protocols expecting a lossless
>> link may result in IO errors.
>>
>> By adding a multicast group for DCB we have clean way
>> to disseminate kernel DCB link attributes up to user
>> space. Avoiding the need for user space to maintain
>> a coherant database and disperse events that potentially
>> do not reflect the current link state.
>>
>> Signed-off-by: John Fastabend <john.r.fastabend@...el.com>
>> ---
>>
>>  include/linux/rtnetlink.h |    2 
>>  net/dcb/dcbnl.c           |  228 +++++++++++++++++++++++++++++----------------
>>  2 files changed, 147 insertions(+), 83 deletions(-)
>>
> [...]
>> +static int dcbnl_notify(struct net_device *dev, int event, int cmd,
>> +			u32 seq, u32 pid)
> [...]
> A driver supporting an embedded dcbx stack would want to directly invoke
> the notification routine, therefore it should be exported. I'd like also

We can export this function when we add support to the driver.

> to add support for a CEE notification. Also when the driver invokes the
> notification shouldn't it use a new event type something like
> RTM_NEWDCB?
>  

I'm not sure a RTM_NEWDCB event is needed. How would user space be expected
to handle the two events? FCoE needs to know if the link is lossless or not
it shouldn't care if a firmware agent is running or a user space LLDP agent.
Other applications (multipathd, iscsid) probably just want to know the current
DCB attributes.

Maybe I missed your point could you provide more details?

Thanks,
John
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