Re: [patch net-next RFC 00/10] introduce line card support for modular switch

From: Jiri Pirko <jiri@resnulli.us>
To: David Ahern <dsahern@gmail.com>
Cc: Jakub Kicinski <kuba@kernel.org>, Andrew Lunn <andrew@lunn.ch>,
	netdev@vger.kernel.org, davem@davemloft.net,
	jacob.e.keller@intel.com, roopa@nvidia.com, mlxsw@nvidia.com
Subject: Re: [patch net-next RFC 00/10] introduce line card support for modular switch
Date: Fri, 22 Jan 2021 09:05:55 +0100	[thread overview]
Message-ID: <20210122080555.GI3565223@nanopsycho.orion> (raw)
In-Reply-To: <971e9eff-0b71-8ff9-d72c-aebe73cab599@gmail.com>

Thu, Jan 21, 2021 at 05:38:40PM CET, dsahern@gmail.com wrote:
>On 1/21/21 8:32 AM, Jiri Pirko wrote:
>> Thu, Jan 21, 2021 at 12:41:58AM CET, kuba@kernel.org wrote:
>>> On Wed, 20 Jan 2021 14:56:46 +0100 Andrew Lunn wrote:
>>>>> No, the FW does not know. The ASIC is not physically able to get the
>>>>> linecard type. Yes, it is odd, I agree. The linecard type is known to
>>>>> the driver which operates on i2c. This driver takes care of power
>>>>> management of the linecard, among other tasks.  
>>>>
>>>> So what does activated actually mean for your hardware? It seems to
>>>> mean something like: Some random card has been plugged in, we have no
>>>> idea what, but it has power, and we have enabled the MACs as
>>>> provisioned, which if you are lucky might match the hardware?
>>>>
>>>> The foundations of this feature seems dubious.
>>>
>>> But Jiri also says "The linecard type is known to the driver which
>>> operates on i2c." which sounds like there is some i2c driver (in user
>>> space?) which talks to the card and _does_ have the info? Maybe I'm
>>> misreading it. What's the i2c driver?
>> 
>> That is Vadim's i2c kernel driver, this is going to upstream.
>> 
>
>This pre-provisioning concept makes a fragile design to work around h/w
>shortcomings. You really need a way for the management card to know

Not really. As I replied to you in the other part of this thread, the
linecard is basically very similar to a splitter cable. In a way, it is
a splitter cable. And should be threated in a similar way. As a phy. Not
as a device. Cables are replaceble without netdevice reappearing. This
linecards are the same. Therefore, the concept of provisioning makes
sense for them, as it does for splitter cable.

>exactly what was plugged in to a slot so the control plane S/W can
>respond accordingly. Surely there is a way for processes on the LC to
>communicate with a process on the management card - even if it is inband
>packets with special headers.

If a device is capable of splitter cable/linecard hotplug, sure, that
may be implemented. But the user has to configure it as such, to be
aware that "cable change" may move netdevices around.