netdev - Re: [PATCH net-next 1/2] ixgbe: register a mdiobus

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Message-ID: <51115bcc-17cd-c6d1-b2b0-17293f3e01e0@silicom-usa.com>
Date:   Fri, 30 Nov 2018 17:34:25 +0000
From:   Steve Douthit <stephend@...icom-usa.com>
To:     Andrew Lunn <andrew@...n.ch>
CC:     Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 1/2] ixgbe: register a mdiobus

On 11/30/18 11:34 AM, Andrew Lunn wrote:
>> Yep, registering multiple interfaces is wrong.  The first board I tested
>> against only had a single MAC enabled (they can be disabled/hidden via
>> straps) so it just happened to work.
> 
> Hi Steve
> 
> Can you hide any/all via straps, or is 00.0 always guaranteed to
> exist?

You can hide all the devices, but if function 1 is enabled then function
0 must also be enabled, so not all combinations are valid.

>> The Intel C3xxx family of SoCs have up to four ixgbe MACs.  These are
>> structured as two devices of two functions each on fixed internal root
>> ports.
>>
>> from lspci:
>> <snip>
>>              +-16.0-[05]--+-00.0
>>              |            \-00.1
>>              +-17.0-[06]--+-00.0
>>              |            \-00.1
>> <snip>
> 
> Is there any other hardware resource which is shared between the MAC
> interfaces? I'm just wondering if the driver has already solved this
> once. Is there an EEPROM per interface for the MAC address, or one
> shared EEPROM?

NVM is shared, it's actually the same SPI flash that holds the BIOS for
this SoC.  Access is serialized by the swfw_sync semaphore.  I think the
device firmware is automagically handling offset translation.

I don't think that helps for this case.

There might be a better match for shared resources, but nothing springs
to mind.

> Ah, how about using the 'cards_found' found variable. It is not
> perfect, in that it is not decremented in ixgb_remove(), and i wonder
> about race conditions since there does not appear to be any lock when
> it is incremented. But if cards_found == 0, register the MDIO bus.

'cards_found' doesn't exist for the ixgbe driver.  I could add it and
fix the race/decrement issues you mention, but it'd have to be a device
type specific count.  It's still possible there are other non-x550em_a
ixgbe devices in external PCIe slots that have different resource
sharing setups.

It's still a lighter weight solution than poking around the parent bus
so I'll add a 'x550em_a_devs_found' counter to v2.