Re: [PATCH v3 00/10] ARM: tegra: Add PCIe device tree support

[Stephen Warren <swarren@wwwdotorg.org>]

On 08/14/2012 03:55 PM, Bjorn Helgaas wrote:
> On Tue, Aug 14, 2012 at 12:58 PM, Thierry Reding
> <thierry.reding@avionic-design.de> wrote:
>> On Tue, Aug 14, 2012 at 01:39:23PM -0600, Stephen Warren wrote:
>>> On 08/13/2012 05:18 PM, Bjorn Helgaas wrote:
>>>> On Mon, Aug 13, 2012 at 11:47 AM, Stephen Warren <swarren@wwwdotorg.org> wrote:
>>> ...
>>>>> whereas for a device tree boot:
>>>>>
>>>>> (same):
>>>>>> [    2.112217] pci 0000:01:00.0: reg 10: [io  0x0000-0x00ff]
>>>>>> [    2.117635] pci 0000:01:00.0: reg 18: [mem 0x00000000-0x00000fff 64bit pref]
>>>>>> [    2.124690] pci 0000:01:00.0: reg 20: [mem 0x00000000-0x00003fff 64bit pref]
>>>>>> [    2.131731] pci 0000:01:00.0: reg 30: [mem 0x00000000-0x0001ffff pref]
>>>>> ... (request region happens early)
>>>>>> [    2.179838] r8169 0000:01:00.0: BAR 0: requesting [io  0x0000-0x00ff]
>>>>>> [    2.193312] r8169 0000:01:00.0: BAR 2: requesting [mem 0x00000000-0x00000fff 64bit pref]
>>>>>> [    2.201397] r8169 0000:01:00.0: BAR 2: can't reserve [mem 0x00000000-0x00000fff 64bit pref]
>>>>>> [    2.209742] r8169 0000:01:00.0: (unregistered net_device): could not request regions
>>>>> ... (same, just happens too late)
>>>>>> [    2.236818] pci 0000:01:00.0: BAR 6: assigned [mem 0xa0000000-0xa001ffff pref]
>>>>>> [    2.244027] pci 0000:01:00.0: BAR 4: assigned [mem 0xa0020000-0xa0023fff 64bit pref]
>>>>>> [    2.251794] pci 0000:01:00.0: BAR 2: assigned [mem 0xa0024000-0xa0024fff 64bit pref]
>>>>>> [    2.259542] pci 0000:01:00.0: BAR 0: assigned [io  0x1000-0x10ff]
>>>>>
>>>>> I suspect this is all still related to the PCI devices themselves being
>>>>> probed much earlier in the overall PCI initialization sequence when the
>>>>> PCI controller is probed later in the boot sequence, whereas PCI device
>>>>> probe is deferred until the overall PCI initialization sequence is
>>>>> complete if the PCI controller is probed very early in the boot sequence.
>>>>
>>>> I don't know what to apply your patches to (they don't apply cleanly
>>>> to v3.6-rc2), so I can't see exactly what you're doing.  But it looks
>>>> like you might be calling pci_bus_add_devices() before
>>>> pci_bus_assign_resource(), which isn't going to work.
>>>
>>> Yes, that's exactly what is happening.
>>>
>>> PCIe initialization starts in arch/arm/mach-tegra/pci.e
>>> tegra_pcie_init() which calls arch/arm/kernel/bios32.c
>>> pci_common_init(). That function first calls pcibios_init_hw() (in the
>>> same file, more about this later) and then loops over PCI buses, calling
>>> amongst other things pci_bus_assign_resources() then pci_bus_add_devices().
>>>
>>> The problem is that ARM's pcibios_init_hw() calls pci_scan_root_bus()
>>> (or a host-driver-specific function which that also calls
>>> pci_scan_root_bus() in Tegra's case) which in turn calls
>>> pci_bus_add_devices() right at the end, before control has returned to
>>> pci_common_init() and hence before pci_bus_assign_resources() has been
>>> called.
>>>
>>> If I modify pci_scan_root_bus() and remove the call to
>>> pci_bus_add_devices(), everything works as expected.
>>>
>>> So, I guess the question is: Should ARM's pcibios_init_hw() not be
>>> calling pci_scan_root_bus(), or at least presumably the ARM PCI code
>>> needs to do things in a slightly different order?
> 
> I think you need to do something like this instead of using pci_scan_root_bus():
> 
>     pci_create_root_bus()
>     pci_scan_child_bus()
>     pci_bus_assign_resources()
>     pci_bus_add_devices()
> 
> This is the effective order used by most of the pci_create_root_bus() callers.

That would pretty much duplicate everything in pci_scan_root_bus(). That
might cause divergence down the road.

Can't we make the call to pci_bus_add_devices() optional in
pci_scan_root_bus() somehow; one of:

* Add a parameter to pci_scan_root_bus() controlling this.

(rather a large patch)

* Split pci_scan_root_bus() into pci_scan_root_bus() and
pci_scan_root_bus_no_add(), such that pci_scan_root_bus() is just a
wrapper that calls pci_scan_root_bus_no_add() then pci_bus_add_devices().

(very simple patch, and the new function can easily be used as/when it's
needed, e.g. enabled just for Tegra in 3.6 to reduce risk of regressions)

* Add a flag to struct pci_bus that requests pci_scan_root_bus() skip
the call to pci_bus_add_devices().

(a flag in the bus struct just for one function seems a little
circuitous, but perhaps OK)

* ifdef out the call to pci_bus_add_devices(), if building for ARM.

(very simple, and probably correct)

Actually, I'm not totally convinced some other archs shouldn't skip this
too; while I couldn't find any other arch that explicitly calls
pci_bus_assign_resources() and pci_bus_add_devices() after
pci_scan_root_bus(), I did see some that call
pci_assign_unassigned_resources() which seems like it might be due to a
similar situation?

* Add another pcibios_*() callback that pci_scan_root_bus() calls to
determine whether to call pci_bus_add_devices(), with default
implementation.