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

[Calvin Johnson <calvin.johnson@oss.nxp.com>]

On Wed, Jul 29, 2020 at 10:43:34AM +0200, Jon Nettleton wrote:
> On Wed, Jul 29, 2020 at 4:53 AM Jeremy Linton <jeremy.linton@arm.com> 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.
> >
> This is also my first run with ACPI and this discussion needs to be
> brought back to the powers that be regarding sorting this out.  This
> is where I see it from an Armada and Layerscape perspective.  This
> isn't a silver bullet fix but the small things I think that need to be
> done to move this forward.
> 
> From Microsoft's documentation.
> 
> Device dependencies
> 
> Typically, there are hardware dependencies between devices on a
> particular platform. Windows requires that all such dependencies be
> described so that it can ensure that all devices function correctly as
> things change dynamically in the system (device power is removed,
> drivers are stopped and started, and so on). In ACPI, dependencies
> between devices are described in the following ways:
> 
> 1) Namespace hierarchy. Any device that is a child device (listed as a
> device within the namespace of another device) is dependent on the
> parent device. For example, a USB HSIC device is dependent on the port
> (parent) and controller (grandparent) it is connected to. Similarly, a
> GPU device listed within the namespace of a system memory-management
> unit (MMU) device is dependent on the MMU device.
> 
> 2) Resource connections. Devices connected to GPIO or SPB controllers
> are dependent on those controllers. This type of dependency is
> described by the inclusion of Connection Resources in the device's
> _CRS.
> 
> 3) OpRegion dependencies. For ASL control methods that use OpRegions
> to perform I/O, dependencies are not implicitly known by the operating
> system because they are only determined during control method
> evaluation. This issue is particularly applicable to GeneralPurposeIO
> and GenericSerialBus OpRegions in which Plug and Play drivers provide
> access to the region. To mitigate this issue, ACPI defines the
> OpRegion Dependency (_DEP) object. _DEP should be used in any device
> namespace in which an OpRegion (HW resource) is referenced by a
> control method, and neither 1 nor 2 above already applies for the
> referenced OpRegion's connection resource. For more information, see
> section 6.5.8, "_DEP (Operation Region Dependencies)", of the ACPI 5.0
> specification.
> 
> We can forget about 3 because even though _DEP would solve many of our
> problems, and Intel has kind of used it for some of their
> architectures, according to the ACPI spec it should not be used this
> way.
> 
> 1) can be achievable on some platforms like the LX2160a.  We have the
> mcbin firmware which is the bus (the ACPI spec does allow you to
> define a platform defined bus), which has MACs as the children, which
> then can have phy's or SFP modules as their children.  This works okay
> for enumeration and parenting but how do they talk?
> 
> This is where 2) comes into play.  The big problem is that MDIO isn't
> designated as a SPB
> (https://docs.microsoft.com/en-us/windows-hardware/drivers/bringup/simple-peripheral-bus--spb-)
> We have GPIO, I2C, SPI, UART, MIPI and a couple of others.  While not
> a silver bullet I think getting MDIO added to the spec would be the
> next step forward to being able to implement this in Linux with
> phylink / phylib in a sane manner.  Currently SFP definitions are
> using the SPB to designate the various GPIO and I2C interfaces that
> are needed to probe devices and handle interrupts.
> 
> The other alternatives is the ACPI maintainers agree on the _DSD
> method (would be quickest and should be easy to migrate to SBP if MDIO
> were adopter), or nothing is done at all (which I know seems to be a
> popular opinion).
> 

Before other ACPI experts miss any further discussion let me add them to the
loop.

Hi ACPI maintainers,  please have a look at the discussion and some conclusions
in this thread:
https://lore.kernel.org/linux-acpi/20200715090400.4733-1-calvin.johnson@oss.nxp.com/T/#t

Discussion is around adding ACPI support into the PHY subsystem.

Thanks
Calvin