Re: [Linaro-acpi] [PATCH V7 00/11] Support for generic ACPI based PCI host controller

-- Computer Architect | Sent from my 64-bit #ARM Powered phone > On May 23, 2016, at 19:39, Bjorn Helgaas helgaas@kernel.org wrote: > >> On Mon, May 23, 2016 at 03:16:01PM +0000, Gabriele Paoloni wrote: >> Hi Lorenzo >> >>> -----Original Message----- >>> From: Lorenzo Pieralisi [mailto:lorenzo.pieralisi@arm.com] >>> Sent: 23 May 2016 11:57 >>> To: Ard Biesheuvel >>> Cc: Gabriele Paoloni; Jon Masters; Tomasz Nowicki; helgaas@kernel.org; >>> 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 >>> >>>> On Fri, May 20, 2016 at 11:14:03AM +0200, Ard Biesheuvel wrote: >>>> On 20 May 2016 at 10:40, Gabriele Paoloni >>> gabriele.paoloni@huawei.com wrote: >>>>> Hi Ard >>>>> >>>>>> -----Original Message----- >>>>>> From: Ard Biesheuvel [mailto:ard.biesheuvel@linaro.org] >>>> [...] >>>>>> >>>>>> Is the PCIe root complex so special that you cannot simply >>> describe an >>>>>> implementation that is not PNP0408 compatible as something else, >>> under >>>>>> its own unique HID? If everybody is onboard with using ACPI, how >>> is >>>>>> this any different from describing other parts of the platform >>>>>> topology? Even if the SBSA mandates generic PCI, they already >>> deviated >>>>>> from that when they built the hardware, so pretending that it is a >>>>>> PNP0408 with quirks really does not buy us anything. >>>>> >>>>> From my understanding we want to avoid this as this would allow >>> each >>>>> vendor to come up with his own code and it would be much more >>> effort >>>>> for the PCI maintainer to rework the PCI framework to accommodate >>> X86 >>>>> and "all" ARM64 Host Controllers... >>>>> >>>>> I guess this approach is too risky and we want to avoid this. >>> Through >>>>> standardization we can more easily maintain the code and scale it >>> to >>>>> multiple SoCs... >>>>> >>>>> So this is my understanding; maybe Jon, Tomasz or Lorenzo can give >>>>> a bit more explanation... >>>> >>>> OK, so that boils down to recommending to vendors to represent known >>>> non-compliant hardware as compliant, just so that we don't have to >>>> change the code to support additional flavors of ECAM ? It's fine to >>>> be pragmatic, but that sucks. >>>> >>>> We keep confusing the x86 case with the ARM case here: for x86, they >>>> needed to deal with broken hardware *after* the fact, and all they >>>> could do is find /some/ distinguishing feature in order to guess >>> which >>>> exact hardware they might be running on. For arm64, it is the >>> opposite >>>> case. We are currently in a position where we can demand vendors to >>>> comply with the standards they endorsed themselves, and (ab)using >>> ACPI >>>> + DMI as a de facto platform description rather than plain ACPI makes >>>> me think the DT crowd were actually right from the beginning. It >>>> *directly* violates the standardization principle, since it requires >>> a >>>> priori knowledge inside the OS that a certain 'generic' device must >>> be >>>> driven in a special way. >>>> >>>> So can anyone comment on the feasibility of adding support for >>> devices >>>> with vendor specific HIDs (and no generic CIDs) to the current ACPI >>>> ECAM driver in Linux? > > 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. > > If we only expect a few non-standard devices, maybe it's enough to > have DMI quirks to statically set up ECAM and just live with the > inconvenience of requiring a kernel change for every new non-standard > device. > >>> Host bridges in ACPI are handled through PNP0A08/PNP0A03 ids, and >>> most of the arch specific code is handled in the respective arch >>> directories (X86 and IA64, even though IA64 does not rely on ECAM/MCFG >>> for >>> PCI ops), it is not a driver per-se, PNP0A08/PNP0A03 are detected >>> through >>> ACPI scan handlers and the respective arch code (ie pci_acpi_scan_root) >>> sets-up resources AND config space on an arch specific basis. >>> >>> X86 deals with that with code in arch/x86 that sets-up the pci_raw_ops >>> on a platform specific basis (and it is not nice, but it works because >>> as you all know the number of platforms in X86 world is contained). >>> >>> Will this happen for ARM64 in arch/arm64 based on vendor specific >>> HIDs ? >>> >>> No. >>> >>> So given the current state of play (we were requested to move the >>> arch/arm64 specific ACPI PCI bits to arch/arm64), we would end up >>> with arch/arm64 code requiring code in /drivers to set-up pci_ops >>> in a platform specific way, it is horrible, if feasible at all. >>> >>> The only way this can be implemented is by pretending that the >>> ACPI/PCI arch/arm64 implementation is generic code (that's what this >>> series does), move it to /drivers (where it is in this series), and >>> implement _DSD vendor specific bindings (per HID) to set-up the pci >>> operations; whether this solution should go upstream, given that it >>> is just a short-term solution for early platforms bugs, it is another >>> story and my personal answer is no. > > It seems like there should be a way to look for a _DSM before we call > acpi_pci_root_get_mcfg_addr() to look for _CBA. > > Currently we call acpi_pci_root_get_mcfg_addr() (to read _CBA) from > the generic acpi_pci_root_add(), but the result (root->mcfg_addr) is > only used in x86-specific code. I think it would be nicer if the > lookup and the use were together. Then it would be easier to override > it because the mapping assumptions would all be in one place. > >> I think it shouldn't be too bad to move quirk handling mechanism to >> arch/arm64. Effectively we would not move platform specific code into >> arch/arm64 but just the mechanism checking if there is any quirk that >> is defined. >> >> i.e.: >> >> extern struct pci_cfg_fixup __start_acpi_mcfg_fixups[]; >> extern struct pci_cfg_fixup __end_acpi_mcfg_fixups[]; >> >> static struct pci_ecam_ops *pci_acpi_get_ops(struct acpi_pci_root *root) >> { >> int bus_num = root->secondary.start; >> int domain = root->segment; >> struct pci_cfg_fixup *f; >> >> /* >> * Match against platform specific quirks and return corresponding >> * CAM ops. >> * >> * First match against PCI topology domain:bus then use DMI or >> * custom match handler. >> */ >> for (f = __start_acpi_mcfg_fixups; f < __end_acpi_mcfg_fixups; f++) { >> if ((f->domain == domain || f->domain == PCI_MCFG_DOMAIN_ANY) && >> (f->bus_num == bus_num || f->bus_num == PCI_MCFG_BUS_ANY) && >> (f->system ? dmi_check_system(f->system) : 1) && >> (f->match ? f->match(f, root) : 1)) >> return f->ops; >> } >> /* No quirks, use ECAM */ >> return &pci_generic_ecam_ops; >> } >> >> Such quirks will be defined anyway in drivers/pci/host/ in the vendor >> specific quirk implementations. >> >> e.g. in HiSilicon case we would have >> >> DECLARE_ACPI_MCFG_FIXUP(NULL, hisi_pcie_match, &hisi_pcie_ecam_ops, >> PCI_MCFG_DOMAIN_ANY, PCI_MCFG_BUS_ANY); >> >> in "drivers/pci/host/pcie-hisi-acpi.c " >> >> Thanks >> >> Gab > > Sorry Gab, I guess I was really responding to earlier messages :) > > I don't really have much to say here, except that it doesn't seem > right to have an MCFG that describes a non-standard ECAM mapping. > I suppose there's already firmware in the field that does that, > though? > > Bjorn