Re: [net-next PATCH v7 1/6] Documentation: ACPI: DSD: Document MDIO PHY

[Florian Fainelli <f.fainelli@gmail.com>]

On 7/28/2020 7:53 PM, Jeremy Linton wrote:
> Hi,
> 
> On 7/28/20 7:39 PM, Florian Fainelli wrote:
>> On 7/28/2020 3:30 PM, Jeremy Linton wrote:
>>> Hi,
>>>
>>> On 7/28/20 3:06 AM, Dan Callaghan wrote:
>>>> Excerpts from Andrew Lunn's message of 2020-07-24 21:14:36 +02:00:
>>>>> Now i could be wrong, but are Ethernet switches something you expect
>>>>> to see on ACPI/SBSA platforms? Or is this a legitimate use of the
>>>>> escape hatch?
>>>>
>>>> As an extra data point: right now I am working on an x86 embedded
>>>> appliance (ACPI not Device Tree) with 3x integrated Marvell switches.
>>>> I have been watching this patch series with great interest, because
>>>> right now there is no way for me to configure a complex switch topology
>>>> in DSA without Device Tree.
>>>
>>> DSA though, the switch is hung off a normal MAC/MDIO, right? (ignoring
>>> whether that NIC/MAC is actually hug off PCIe for the moment).
>>
>> There is no specific bus, we have memory mapped, MDIO, SPI, I2C swiches
>> all supported within the driver framework right now.
>>
>>>
>>> It just has a bunch of phy devices strung out on that single MAC/MDIO.
>>
>> It has a number of built-in PHYs that typically appear on a MDIO bus,
>> whether that bus is the switch's internal MDIO bus, or another MDIO bus
>> (which could be provided with just two GPIOs) depends on how the switch
>> is connected to its management host.
>>
>> When the switch is interfaced via MDIO the switch also typically has a
>> MDIO interface called the pseudo-PHY which is how you can actually tap
>> into the control interface of the switch, as opposed to reading its
>> internal PHYs from the MDIO bus.
>>
>>> So in ACPI land it would still have a relationship similar to the one
>>> Andrew pointed out with his DT example where the eth0->mdio->phy are all
>>> contained in their physical parent. The phy in that case associated with
>>> the parent adapter would be the first direct decedent of the mdio, the
>>> switch itself could then be represented with another device, with a
>>> further string of device/phys representing the devices. (I dislike
>>> drawing acsii art, but if this isn't clear I will, its also worthwhile
>>> to look at the dpaa2 docs for how the mac/phys work on this device for
>>> contrast.).
>>
>> The eth0->mdio->phy relationship you describe is the simple case that
>> you are well aware of which is say what we have on the Raspberry Pi 4
>> with GENET and the external Broadcom PHY.
>>
>> For an Ethernet switch connected to an Ethernet MAC, we have 4 different
>> types of objects:
>>
>> - the Ethernet MAC which sits on its specific bus
>>
>> - the Ethernet switch which also sits on its specific bus
>>
>> - the built-in PHYs of the Ethernet switch which sit on whatever
>> bus/interface the switch provides to make them accessible
>>
>> - the specific bus controller that provides access to the Ethernet switch
>>
>> and this is a simplification that does not take into account Physical
>> Coding Sublayer devices, pure MDIO devices (with no foot in the Ethernet
>> land such as PCIe, USB3 or SATA PHYs), SFP, SFF and other pluggable
>> modules.
> 
> Which is why I've stayed away from much of the switch discussion. There
> are a lot of edge cases to fall into, because for whatever reason
> networking seems to be unique in all this non-enumerable customization
> vs many other areas of the system. Storage, being an example i'm more
> familiar with which has very similar problems yet, somehow has managed
> to avoid much of this, despite having run on fabrics significantly more
> complex than basic ethernet (including running on a wide range of hot
> pluggable GBIC/SFP/QSFP/etc media layers).
> 
> ACPI's "problem" here is that its strongly influenced by the "Plug and
> Play" revolution of the 1990's where the industry went from having
> humans describing hardware using machine readable languages, to hardware
> which was enumerable using standard protocols. ACPI's device
> descriptions are there as a crutch for the remaining non plug an play
> hardware in the system.
> 
> So at a basic level, if your describing hardware in ACPI rather than
> abstracting it, that is a problem.

I suppose that is a good summary, my impression from this patch series
is that we want the description part, not the abstraction, whether it is
on purpose or because there is a misunderstanding of what ACPI is
intended for, or higher powers have decided this must be done otherwise
nothing gets sold, who knows?
-- 
Florian