Re: [PATCH V4 0/6] PCI, x86: update MMCFG information when hot-plugging PCI host bridges

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]


On Mon, Apr 23, 2012 at 12:50 PM, Don Dutile <ddutile@xxxxxxxxxx> wrote:
> On 04/23/2012 01:41 PM, Bjorn Helgaas wrote:
>>
>> On Mon, Apr 16, 2012 at 11:54 AM, Don Dutile<ddutile@xxxxxxxxxx>  wrote:
>>>
>>> On 04/16/2012 12:09 PM, Jiang Liu wrote:
>>>>
>>>>
>>>> Hi Don,
>>>>        Thanks for your comments and please refer to inline comments
>>>> below.
>>>
>>>
>>>
>>> Thanks for the info below; couple quick replies below.. - Don
>>>
>>>
>>>> On 04/16/2012 11:30 PM, Don Dutile wrote:
>>>>>
>>>>>
>>>>> On 04/13/2012 10:33 AM, Jiang Liu wrote:
>>>>>>
>>>>>>
>>>>>> On 04/13/2012 06:48 PM, Kenji Kaneshige wrote:
>>>>>>>
>>>>>>>
>>>>>>> (2012/04/12 9:06), Bjorn Helgaas wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> On Wed, Apr 11, 2012 at 9:34 AM, Jiang Liu<liuj97@xxxxxxxxx>
>>>>>>>>  wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On 04/11/2012 08:05 PM, Kenji Kaneshige wrote:
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> (2012/04/11 13:02), Bjorn Helgaas wrote:
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> On Tue, Apr 10, 2012 at 6:10 PM, Jiang Liu<liuj97@xxxxxxxxx>
>>>>>>>>>>>  wrote:
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> This patchset enhance pci_root driver to update MMCFG
>>>>>>>>>>>> information
>>>>>>>>>>>> when
>>>>>>>>>>>> hot-plugging PCI root bridges. It applies to Yinghai's tree at
>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> git://git.kernel.org/pub/scm/linux/kernel/git/yinghai/linux-yinghai.git
>>>>>>>>>>>> for-pci-root-bus-hotplug
>>>>>>>>>>>>
>>>>>>>>>>>> The second patch is based on Taku Izumi work with some
>>>>>>>>>>>> enhancements to
>>>>>>>>>>>> correctly handle PCI host bridges without _CBA method.
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> I'm sorry I won't have time to really review these for a couple
>>>>>>>>>>> weeks.
>>>>>>>>>>>
>>>>>>>>>>> It always seemed wrong to me that we parse MCFG and set things up
>>>>>>>>>>> before we even look at PNP0A03/PNP0A08 devices.  It would make
>>>>>>>>>>> more
>>>>>>>>>>> sense to me to have something in acpi_pci_root_add() to set up
>>>>>>>>>>> MMCONFIG using _CBA if available, and falling back to parsing
>>>>>>>>>>> MCFG
>>>>>>>>>>> if
>>>>>>>>>>> it's not.
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> I think your idea could make the code (design) much cleaner.
>>>>>>>>>> Do you have any other reason why you think "It always seemed
>>>>>>>>>> wrong..."?
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> The current scheme is just an ugly design.  Does I need more
>>>>>>>> reasons?
>>>>>>>>  :)
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> Ok, I just wanted to know if I'm missing anything we need to
>>>>>>> take into account when re-factoring the code.
>>>>>>>
>>>>>>> By the way, the following code makes me think there could be
>>>>>>> some hardwares that need a fixup using mmconfig access before
>>>>>>> scanning the PCI tree. If this is the case, we would need
>>>>>>> something to enable early mmconfig initialization for those
>>>>>>> hardwares.
>>>>>>>
>>>>>>> static __init int pci_arch_init(void)
>>>>>>> {
>>>>>>>      ...
>>>>>>>          if (!(pci_probe&     PCI_PROBE_NOEARLY))
>>>>>>>                  pci_mmcfg_early_init();
>>>>>>>
>>>>>>>
>>>>>>> Regards,
>>>>>>> Kenji Kaneshige
>>>>>>
>>>>>>
>>>>>>
>>>>>> If MMCFG could be treated as an optional configuration space access
>>>>>> method,
>>>>>> we can refine the MMCFG code according to Bjorn's suggestion. And as
>>>>>> Kenji
>>>>>> has mentioned, there may be some risks ahead. So need more
>>>>>> confirmation
>>>>>> from other PCI experts here.
>>>>>>
>>>>> I looked at the thread, but didn't know which suggestion of Bjorn's you
>>>>> were referring to.
>>>>> But, mmcfg access to PCI config space is need for any cap structure
>>>>> greater than 256 byte offset.  A number of devices have cap structures
>>>>> in this upper PCI config space, esp. SRIOV devices.
>>>>> So, if 'optional MMCFG' only means at the beginning of kernel scanning
>>>>> of
>>>>> PCI (pass-0 scanning), that should be ok, but in-depth, pass-1 scanning
>>>>> of PCIe devices may require MMCFG for full functional support.
>>>>
>>>>
>>>> For mainstream systems with support of ACPI and MMCFG, the booting
>>>> sequences are about:
>>>> 1) Probe for legacy PCI configuration access mechanism, such as CONF1,
>>>> CONF2, BIOS
>>>> 2) Start ACPICA/ACPI subsystem with the legacy PCI configuration access
>>>> mechanism
>>>> 3) Enumerate PCI root bridges (PNP0A03/PNP0A08) in ACPI namespace and
>>>> bind
>>>> pci_root
>>>>    driver to them
>>>> 4) pci_root driver calls into arch code to add MMCFG information for the
>>>> host bridge
>>>> 5) pci_root driver calls PCI core to enumerate all PCI devices under the
>>>> host bridge
>>>>
>>>> The above flow should work for SRIOV case. But still need to check
>>>> following cases:
>>>> 1) ACPICA/ACPI subsystem has no dependency on MMCFG
>>>> 2) Systems implementing SFI instead of ACPI work as expected
>>>> 3) ACPI has been disabled by user (Bjorn points out we could ignore this
>>>> case)
>>>
>>>
>>> Agreed. My least favorite bz: "I set boot param to noacpi and can't scan
>>> entire PCI space.... duh!
>>>
>>>
>>>> 4) Some host bridges are not reported by ACPI (Bjorn points out we
>>>> should
>>>> eventually
>>>>    get rid of the blind probing logic)
>>>
>>>
>>> And depend on BIOS-ACPI to be correct all the time? ....hahahahahaha  ...
>>> sorry.... you hit my funny bone! ;-)
>>> Is blind probing problematic ?
>>> Seems like a pci-fixup/quirk can be implemented under arch/<>/pci to
>>> handle
>>> these cases, and thus, depend on ACPI for host-bridge info... wait! did I
>>> just
>>> say depend on ACPI?!?!   :)
>>
>>
>> Hope your funny bone has stopped tingling by now :)
>>
> Not when ACPI's always there to bang it again! ;-)
>
>> When we probe blindly, we don't know what resources are available on
>> the bus (except for AMD systems).  Therefore, we can't do reliable
>> assignment, and we have to rely on whatever the BIOS did.
>>
>> Blind probing finds devices not exposed by the BIOS.  This might be a
>> BIOS bug, or it might be a conscious decision to hide the devices from
>> the OS.  Some OEMs hide devices to reduce the likelihood of users
>> messing things up with setpci.
>>
>> It would be interesting and relatively easy to figure out whether
>> Windows ever discovers a device behind an unreported host bridge.  My
>> guess is "no," but I haven't had time to verify this.
>>
> Well, call me crazy(again!), but why put a host-bridge device on a system
> and then (try to) hide it with software(BIOS/ACPI) ?
> Sounds like a recipe for disaster if the OS tries to
> reconfigure address space or bus-number space.....

Some OEMs do leave host bridges unreported.  Here's an example, from
an HP DL380 G7:

  ACPI: PCI Root Bridge [PCI0] (domain 0000 [bus 00-1a])
  pci 0000:00:00.0: [8086:3406] type 0 class 0x000600
  pci 0000:00:01.0: [8086:3408] type 1 class 0x000604
  pci 0000:00:02.0: [8086:3409] type 1 class 0x000604
  ...
  PCI: Discovered peer bus 3e
  pci 0000:3e:00.0: [8086:2c70] type 0 class 0x000600
  pci 0000:3e:00.1: [8086:2d81] type 0 class 0x000600
  pci 0000:3e:02.0: [8086:2d90] type 0 class 0x000600
  ...
  PCI: Discovered peer bus 3f
  pci 0000:3f:00.0: [8086:2c70] type 0 class 0x000600
  pci 0000:3f:00.1: [8086:2d81] type 0 class 0x000600
  pci 0000:3f:02.0: [8086:2d90] type 0 class 0x000600

ACPI did not report host bridges leading to buses 3e and 3f; we found
those devices by probing blindly.

In this case, these are Intel CPU uncore devices, and they don't
consume MMIO or IO port resources.  But you're absolutely right that
we can't safely reconfigure anything we find this way.

> Hiding a PCI device is as simple as not loading its driver... :)

Sure, if it's the OS that wants to hide it.  In this case, it's the
OEM that wants to hide it, and the only mechanism for doing that is to
refrain from telling the OS how to find it.  I'm pretty confident that
the DL380 omission of those host bridges is an intentional choice by
the BIOS writers.

Bjorn
--
To unsubscribe from this list: send the line "unsubscribe linux-pci" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html


[Home]     [Linux USB Devel]     [Video for Linux]     [Linux Audio Users]     [Photo]     [Yosemite News]    [Yosemite Photos]    [Free Online Dating]     [Linux Kernel]     [Linux SCSI]     [XFree86]

Add to Google Powered by Linux