Re: [PATCH 0/7] MdePkg, OvmfPkg, ArmVirtPkg: add and use PCI(E) Capabilities Library

[Laszlo Ersek <lersek@redhat.com>]

On 05/14/18 17:06, Gao, Liming wrote:
> Laszlo:
>   I have not fully understood its goal and design. Could you give me one more week to review it?

Sure! Thank you.

Laszlo

>> -----Original Message-----
>> From: Laszlo Ersek [mailto:lersek@redhat.com]
>> Sent: Monday, May 14, 2018 9:16 PM
>> To: edk2-devel-01 <edk2-devel@lists.01.org>
>> Cc: Kinney, Michael D <michael.d.kinney@intel.com>; Justen, Jordan L <jordan.l.justen@intel.com>; Gao, Liming
>> <liming.gao@intel.com>; Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> Subject: Re: [edk2] [PATCH 0/7] MdePkg, OvmfPkg, ArmVirtPkg: add and use PCI(E) Capabilities Library
>>
>> ping
>>
>> On 05/04/18 23:36, Laszlo Ersek wrote:
>>> Repo:   https://github.com/lersek/edk2.git
>>> Branch: pci_cap
>>>
>>> In message [1], Jordan suggested a general "capabilities helper lib that
>>> could iterate & read the PCI capability structs". Patch #1 introduces
>>> that library (class and central BASE instance) to MdePkg; the library
>>> class is called PciCapLib.
>>>
>>> [1] http://mid.mail-archive.com/150696619010.2454.9266470245604846575@jljusten-skl
>>>
>>> PciCapLib offers APIs to parse capabilities lists, and to locate,
>>> describe, read and write capabilities.
>>>
>>> The library class targets a wide range of client module types; for
>>> example, platform PEIMs, platform DXE drivers, and UEFI drivers that
>>> follow the UEFI Driver Model. For this reason, the library class is
>>> designed with PCI config access abstracted away, even beyond the PPI /
>>> Protocol level, since those cannot cross the PEI / DXE boundary.
>>>
>>> This (minimal) config space access method abstraction is called
>>> PCI_CAP_DEV. It is *conceptually* a protocol. Client modules plug
>>> PCI_CAP_DEV instances into PciCapLib for config space access, similarly
>>> to how DiskIo and BlockIo instances are plugged into filesystem drivers.
>>> Similarly to the filesystem driver example, PciCapLib is an example for
>>> the "strategy pattern".
>>>
>>> Patches #2 and #3 introduce small "factory" libraries that help client
>>> modules produce PCI_CAP_DEV instances:
>>>
>>> - PciCapPciSegmentLib -- incl. class and central BASE instance -- from
>>>   patch #2 produces PCI_CAP_DEV instances on top of PciSegmentLib, using
>>>   the "Segment:Bus:Device.Function" notation. This is meant for platform
>>>   modules (PEIMs and DXE drivers), which generally have a working
>>>   PciSegmentLib instance available.
>>>
>>> - PciCapPciIoLib -- incl. class and central UEFI instance -- from patch
>>>   #3 lets client modules produce PCI_CAP_DEV instances on top of
>>>   EFI_PCI_IO_PROTOCOL instances. This is meant for UEFI drivers.
>>>
>>> Patches #4 and #5 resolve the three new lib classes to their respective
>>> central instances in the OvmfPkg and ArmVirtPkg DSC files.
>>>
>>> Patch #6 converts OvmfPkg/PciHotPlugInitDxe to the new APIs, from manual
>>> capability list parsing. Because PciHotPlugInitDxe is a platform DXE
>>> driver, this conversion uses PciCapPciSegmentLib and PciCapLib.
>>>
>>> Patch #7 converts OvmfPkg/Virtio10Dxe to the new APIs, from manual
>>> capability list parsing. Because Virtio10Dxe is a UEFI driver, this
>>> conversion uses PciCapPciIoLib and PciCapLib.
>>>
>>> ----[ jump to the patches now, and return if you have questions: I
>>>       have answers, but you might find them too verbose in advance ]----
>>>
>>> Q1: If PCI config access needs to be abstracted from PciCapLib, then why
>>>     don't we provide multiple instances of PciCapLib? In other words,
>>>     why the PCI_CAP_DEV "protocol" rather than multiple instances of
>>>     PciCapLib?
>>>
>>> A1: The point of lib classes and instances is that the class provides a
>>>     common interface, and the instances provide different
>>>     implementations. We have the opposite use case here (hence the
>>>     "strategy pattern"): the implementation of the higher level
>>>     algorithm is common, and the objects that plug into it differ at the
>>>     *interface* level ("Segment:Bus:Device.Function" notation versus
>>>     PciIo protocol).
>>>
>>>     None of those notations are suitable for inclusion in the PciCapLib
>>>     *class*; a more abstract interface is needed for identifying the PCI
>>>     device to the library, for parsing of capabilities lists.
>>>
>>> Q2: OK, let's say we can't (and shouldn't) include both notations in the
>>>     PciCapLib class; can we settle on one of them, rather than
>>>     PCI_CAP_DEV?
>>>
>>> A2: No, we can't. PciIo is obviously unusable in PEIMs (and in quite a
>>>     few platform DXE drivers too).
>>>
>>>     It takes a bit longer to explain why the other choice, i.e. adopting
>>>     the "Segment:Bus:Device.Function" notation for the PciCapLib
>>>     interface, is not good either. It has to do with the portability of
>>>     UEFI drivers that bind PciIo protocol instances:
>>>
>>>     It is indeed easy enough to call PciIo->GetLocation() and plug the
>>>     output values into a theoretical "Segment:Bus:Device.Function"
>>>     interface. However, about the only two things that could consume
>>>     such location identifiers and actually *implement* config space
>>>     access would be:
>>>
>>>     (a) the EFI_PCI_ROOT_BRIDGE_IO_PROTOCOL.Pci.Read/Write member
>>>         functions, where the member functions themselves take the
>>>         "Bus:Device.Function" part, and the "Segment" part must be used
>>>         for selecting the EFI_PCI_ROOT_BRIDGE_IO_PROTOCOL instance
>>>         itself, through the SegmentNumber member;
>>>
>>>     (b) the PciSegmentLib interfaces.
>>>
>>>     Building a PciCapLib instance that uses (a) into a UEFI driver is a
>>>     bad idea, because there are platforms that provide PciIo instances
>>>     -- hence the driver can generally run on them -- but don't provide
>>>     PciRootBridgeIo instances.
>>>
>>>     Building a PciCapLib instance that uses (b) into a UEFI driver is
>>>     worse. The PciSegmentLib interfaces are not capable of returning
>>>     errors. For example, if a device appears as PCI Express (implying
>>>     its extended config space exists, potentially containing extended
>>>     capabilities), but the underlying platform does not support extended
>>>     config space access (for example because it only supports
>>>     0xCF8/0xCFC, and not ECAM), then peeking at the extended config
>>>     space of the device may invoke undefined behavior within
>>>     PciSegmentLib, and the PciCapLib instance will be none the wiser.
>>>
>>>     (In fact, for the same reason, even PciCapPciSegmentLib has to
>>>     extend the "Segment:Bus:Device.Function" notation slightly; so that
>>>     such issues are caught *before* the call is made to PciSegmentLib.)
>>>
>>> Q3: Wait, I thought it wasn't possible for a PCI Express device to
>>>     appear on a platform that lacked ECAM.
>>>
>>> A3: Is that a question? :) Yes, it is plenty possible [2]. Same way as
>>>     garbage / looped config spaces are [3].
>>>
>>> [2] https://github.com/tianocore/edk2/commit/014b472053ae3
>>> [3] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=02d578e5edd9
>>>
>>> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>>> Cc: Jordan Justen <jordan.l.justen@intel.com>
>>> Cc: Liming Gao <liming.gao@intel.com>
>>> Cc: Michael D Kinney <michael.d.kinney@intel.com>
>>>
>>> Thanks,
>>> Laszlo
>>>
>>> Laszlo Ersek (7):
>>>   MdePkg: introduce PciCapLib
>>>   MdePkg: introduce PciCapPciSegmentLib
>>>   MdePkg: introduce PciCapPciIoLib
>>>   OvmfPkg: resolve PciCapLib, PciCapPciSegmentLib, PciCapPciIoLib
>>>   ArmVirtPkg: resolve PciCapLib, PciCapPciSegmentLib, PciCapPciIoLib
>>>   OvmfPkg/PciHotPlugInitDxe: convert to PciCapLib
>>>   OvmfPkg/Virtio10Dxe: convert to PciCapLib
>>>
>>>  ArmVirtPkg/ArmVirt.dsc.inc                                         |    3 +
>>>  MdePkg/Include/Library/PciCapLib.h                                 |  429 +++++++++
>>>  MdePkg/Include/Library/PciCapPciIoLib.h                            |   58 ++
>>>  MdePkg/Include/Library/PciCapPciSegmentLib.h                       |   82 ++
>>>  MdePkg/Library/BasePciCapLib/BasePciCapLib.c                       | 1007 ++++++++++++++++++++
>>>  MdePkg/Library/BasePciCapLib/BasePciCapLib.h                       |   60 ++
>>>  MdePkg/Library/BasePciCapLib/BasePciCapLib.inf                     |   37 +
>>>  MdePkg/Library/BasePciCapPciSegmentLib/BasePciCapPciSegmentLib.c   |  226 +++++
>>>  MdePkg/Library/BasePciCapPciSegmentLib/BasePciCapPciSegmentLib.h   |   47 +
>>>  MdePkg/Library/BasePciCapPciSegmentLib/BasePciCapPciSegmentLib.inf |   34 +
>>>  MdePkg/Library/UefiPciCapPciIoLib/UefiPciCapPciIoLib.c             |  243 +++++
>>>  MdePkg/Library/UefiPciCapPciIoLib/UefiPciCapPciIoLib.h             |   44 +
>>>  MdePkg/Library/UefiPciCapPciIoLib/UefiPciCapPciIoLib.inf           |   35 +
>>>  MdePkg/MdePkg.dec                                                  |   14 +
>>>  MdePkg/MdePkg.dsc                                                  |    3 +
>>>  OvmfPkg/Include/IndustryStandard/Virtio10.h                        |    7 +-
>>>  OvmfPkg/OvmfPkgIa32.dsc                                            |    3 +
>>>  OvmfPkg/OvmfPkgIa32X64.dsc                                         |    3 +
>>>  OvmfPkg/OvmfPkgX64.dsc                                             |    3 +
>>>  OvmfPkg/PciHotPlugInitDxe/PciHotPlugInit.c                         |  267 ++----
>>>  OvmfPkg/PciHotPlugInitDxe/PciHotPlugInit.inf                       |    5 +
>>>  OvmfPkg/Virtio10Dxe/Virtio10.c                                     |  135 +--
>>>  OvmfPkg/Virtio10Dxe/Virtio10.inf                                   |    2 +
>>>  23 files changed, 2485 insertions(+), 262 deletions(-)
>>>  create mode 100644 MdePkg/Include/Library/PciCapLib.h
>>>  create mode 100644 MdePkg/Include/Library/PciCapPciIoLib.h
>>>  create mode 100644 MdePkg/Include/Library/PciCapPciSegmentLib.h
>>>  create mode 100644 MdePkg/Library/BasePciCapLib/BasePciCapLib.c
>>>  create mode 100644 MdePkg/Library/BasePciCapLib/BasePciCapLib.h
>>>  create mode 100644 MdePkg/Library/BasePciCapLib/BasePciCapLib.inf
>>>  create mode 100644 MdePkg/Library/BasePciCapPciSegmentLib/BasePciCapPciSegmentLib.c
>>>  create mode 100644 MdePkg/Library/BasePciCapPciSegmentLib/BasePciCapPciSegmentLib.h
>>>  create mode 100644 MdePkg/Library/BasePciCapPciSegmentLib/BasePciCapPciSegmentLib.inf
>>>  create mode 100644 MdePkg/Library/UefiPciCapPciIoLib/UefiPciCapPciIoLib.c
>>>  create mode 100644 MdePkg/Library/UefiPciCapPciIoLib/UefiPciCapPciIoLib.h
>>>  create mode 100644 MdePkg/Library/UefiPciCapPciIoLib/UefiPciCapPciIoLib.inf
>>>
>