RE: [PATCH V7 00/11] Support for generic ACPI based PCI host controller

[Gabriele Paoloni]

Hi Bjorn many thanks for your suggestions

> -----Original Message-----
> From: Bjorn Helgaas [mailto:helgaas@xxxxxxxxxx]
> Sent: 24 May 2016 20:01
> To: Lorenzo Pieralisi
> Cc: Gabriele Paoloni; Ard Biesheuvel; Jon Masters; Tomasz Nowicki;
> arnd@xxxxxxxx; will.deacon@xxxxxxx; catalin.marinas@xxxxxxx;
> rafael@xxxxxxxxxx; hanjun.guo@xxxxxxxxxx; okaya@xxxxxxxxxxxxxx;
> jchandra@xxxxxxxxxxxx; linaro-acpi@xxxxxxxxxxxxxxxx; linux-
> pci@xxxxxxxxxxxxxxx; dhdang@xxxxxxx; Liviu.Dudau@xxxxxxx;
> ddaney@xxxxxxxxxxxxxxxxxx; jeremy.linton@xxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; linux-acpi@xxxxxxxxxxxxxxx;
> robert.richter@xxxxxxxxxxxxxxxxxx; Suravee.Suthikulpanit@xxxxxxx;
> msalter@xxxxxxxxxx; Wangyijing; mw@xxxxxxxxxxxx;
> andrea.gallo@xxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [PATCH V7 00/11] Support for generic ACPI based PCI host
> controller
> 
> On Tue, May 24, 2016 at 06:24:23PM +0100, Lorenzo Pieralisi wrote:
> > Hi Bjorn,
> >
> > On Mon, May 23, 2016 at 06:39:18PM -0500, Bjorn Helgaas wrote:
> >
> > [...]
> >
> > > On Mon, May 23, 2016 at 03:16:01PM +0000, Gabriele Paoloni wrote:
> > > I don't think of ECAM support itself as a "driver".  It's just a
> > > service available to drivers, similar to OF resource parsing.
> > >
> > > Per PCI Firmware r3.2, sec 4.1.5, "PNP0A03" means a PCI/PCI-X/PCIe
> > > host bridge.  "PNP0A08" means a PCI-X Mode 2 or PCIe bridge that
> > > supports extended config space.  It doesn't specify how we access
> that
> > > config space, so I think hardware with non-standard ECAM should
> still
> > > have PNP0A03 and PNP0A08 in _CID or _HID.
> > >
> > > "ECAM" as used in the specs (PCIe r3.0, sec 7.2.2, and PCI Firmware
> > > r3.2, sec 4.1) means:
> > >
> > >   (a) a memory-mapped model for config space access, and
> > >   (b) a specific mapping of address bits to bus/device/function/
> > >       register
> > >
> > > MCFG and _CBA assume both (a) and (b), so I think a device with
> > > non-standard ECAM mappings should not be described in MCFG or _CBA.
> > >
> > > If a bridge has ECAM with non-standard mappings, I think either a
> > > vendor-specific _HID or a device-specific method, e.g., _DSM, could
> > > communicate that.
> > >
> > > Jon, I agree that we should avoid describing non-standardized
> hardware
> > > in Linux-specific ways.  Is there a mechanism in use already?  How
> > > does Windows handle this?  DMI is a poor long-term solution because
> it
> > > requires ongoing maintenance for new platforms, but I think it's OK
> > > for getting started with platforms already shipping.
> > >
> > > A _DSM has the advantage that once it is defined and supported,
> OEMs
> > > can ship new platforms without requiring a new quirk or a new _HID
> to
> > > be added to a driver.
> > >
> > > There would still be the problem of config access before the
> namespace
> > > is available, i.e., the MCFG use case.  I don't know how important
> > > that is.  Defining an MCFG extension seems like the most obvious
> > > solution.
> >
> > Your summary above is a perfect representation of the situation.
> >
> > We had an opportunity to sync-up on the current status of ACPI PCI
> > for ARM64 (and talked about a way forward for this series, which
> > includes quirks handling), let me summarize it here for everyone
> > involved so that we can agree on a way forward.
> >
> > 1) ACPI PCI support for PNP0A03/PNP0A08 host bridges on top of MCFG
> >    ECAM for config space is basically ready (Tomasz and JC addressed
> >    Rafael's concerns in relation to ARM64 specific code, and managed
> >    to find a way to allocate domain numbers in preparation for Arnd
> >    pci_create_root_bus() clean-up, v8 to be posted shortly and should
> >    be final). This provides support for de-facto ACPI/PCI ECAM base
> >    standard for ARM64 (with a clean-split between generic code and
> ARM64
> >    bits, where ARM64, like X86 and IA64, manages in arch code IO
> space and
> >    PCI resources, to be further consolidated in the near future).
> >    I do not think anyone can complain about the generality of what we
> >    achieved, for systems that are PCI standard (yes, PCI STANDARD)
> this
> >    would just be sufficient.
> 
> Sounds good to me.
> 
> > 2) In a real world (1) is not enough. Some ARM64 platforms, not
> entirely
> >    ECAM compliant, already shipped with the corresponding firmware
> that
> >    we can't update. HW has ECAM quirks and to work around it in the
> kernel
> >    we put forward many solutions to the problem, it is time we found
> a
> >    solution (when, of course, (1) is completed and upstream).
> >    Using the MCFG table OEMID matching floated around in this thread
> >    would work fine for most of the platforms (and cross-OS) that have
> >    shipped with HW ECAM quirks, so I think that's the starting point
> for
> >    our solution and that's how we can sort this out, _today_.
> >
> >    The solution is a trivial look-up table:
> >    MCFG OEMID <-> PCI config space ops
> 
> Sounds reasonable to me.
> 
> > 3) (2) does not just work on some platforms (and we can't predict the
> >    future either - actually I can, it is three letters, ECAM), simply
> >    because MCFG OEMID matching does not provide a way to attach
> further
> >    data to the MCFG (eg if config space for, say, bus 0 domain 0, is
> not
> >    ECAM compliant, the config region can't be handled and must not be
> >    handled through a corresponding MCFG region.
> 
> Couldn't this be handled by custom pci_ops that do something special
> for bus 0 domain 0, and default to some different pci_ops for the
> rest?

My idea was to remove bus 0 from domain 0 MCFG (and also the other
buses corresponding to the root complexes ports)

So the driver would use the ECAM access with addresses retrieved from
MCFG for any devices except the RCs.

For the RC we could retrieve special addresses from the "PNP0C02"
reserved resource... 


> 
> >    That's the problem Gabriele is facing and wants to solve through
> >    something like:
> >
> >    https://lkml.org/lkml/2016/3/9/91
> >
> >    in the respective ACPI tables-bindings. It may be an idea worth
> >    pursuing, it does not solve (2) simply because that FW has
> shipped,
> >    we can't patch it any longer.
> 
> (2) is for quirks to deal with MCFG.  Gabriele's post is a proposal
> for ACPI namespace.  We can't use anything in the namespace to
> implement MCFG quirks because MCFG is needed before the namespace is
> available.

Honestly I have to dig better into this (and I will once V8 is out),
However from my current understanding so far we have look-up where
MCFG OEMID is going to tell which specific pci-ops are going to be used.

Now from my quirk idea I need to retrieve the "special address" from the
ACPI namespace before cfg rd/wr take place...if this is not doable then
I need to find a different solution...than can be the one you proposed
below (many thanks for this).

I will look into details later on once v8 is out.

Thanks

Gab

> 
> I think Gabriele's post is a good proposal for the namespace, but I
> would propose the following modifications:
> 
>   - Add PNP0A08 to the PCI1 _CID since this is a PCIe host bridge
>   - Add a PCI1 _DSM describing the ECAM space
>   - Remove the HISI0081 _HID
> 
> The result would be:
> 
>   Device (PCI1) {
>     Name(_HID, "HISI0080")
>     Name(_CID, "PNP0A03,PNP0A08")
>     Method(_CRS) { ... }
>     Method(_DSM) { <describe ECAM base and mapping function> }
>   }
>   Device (RES0) {
>     Name(_HID, "PNP0C02")
>     Name(_CRS) { <describe ECAM base and size> }
>   }
> 
> RES0 could also be contained within PCI1, as Gabriele suggested.  I
> don't really care whether it's contained or not, and making it
> contained might make it easier for firmware, because addition/removal
> of PCI1 and RES0 should always happen together.
> 
> I think the _DSM is important because it is really ugly if the
> HISI0080 driver has to look for a separate HISI0081 device to learn
> about the ECAM space.  There are several PCI drivers that do something
> similar, using for_each_pci_dev(), and I cringe every time I see them,
> because this totally screws up the driver model.  A driver should
> claim a device via a .probe() method called by the core, and it
> shouldn't look at devices it hasn't claimed.  This is required to make
> hotplug work correctly.
> 
> > Hence to finally support ACPI PCI on ARM64 I suggest we carry out the
> > following steps, in order:
> >
> > - Let's complete/merge (1), that's fundamental to this whole thread
> > - On top of (1) we apply a quirking mechanism based on (2) that
> allows
> >   us to boot mainline with boxes shipping today with no FW update
> required.
> > - We devise a way to handle quirks that is more generic than (2) so
> that
> >   can we can accomodate further platforms that can't rely on (2) but
> >   have more leeway in terms of FW updates.
> >
> > I hope that's a reasonable plan, Tomasz's v8 series coming to kick it
> off.
> 
> Sounds very good to me; I'm looking forward to v8.
> 
> Bjorn