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

[Gabriele Paoloni <gabriele.paoloni@huawei.com>]

Hi Lorenzo

> -----Original Message-----
> From: Lorenzo Pieralisi [mailto:lorenzo.pieralisi@arm.com]
> Sent: 24 May 2016 18:24
> To: Bjorn Helgaas
> Cc: Gabriele Paoloni; Ard Biesheuvel; Jon Masters; Tomasz Nowicki;
> arnd@arndb.de; will.deacon@arm.com; catalin.marinas@arm.com;
> rafael@kernel.org; hanjun.guo@linaro.org; okaya@codeaurora.org;
> jchandra@broadcom.com; linaro-acpi@lists.linaro.org; linux-
> pci@vger.kernel.org; dhdang@apm.com; Liviu.Dudau@arm.com;
> ddaney@caviumnetworks.com; jeremy.linton@arm.com; linux-
> kernel@vger.kernel.org; linux-acpi@vger.kernel.org;
> robert.richter@caviumnetworks.com; Suravee.Suthikulpanit@amd.com;
> msalter@redhat.com; Wangyijing; mw@semihalf.com;
> andrea.gallo@linaro.org; linux-arm-kernel@lists.infradead.org
> Subject: Re: [PATCH V7 00/11] Support for generic ACPI based PCI host
> controller
> 
> 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.
> 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
> 
> 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.
>    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.
> 
> 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.

Thanks for summarizing.

100% agree on the summary and next steps.

Cheers

Gab


> 
> Thank you,
> Lorenzo