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List:       linux-tegra
Subject:    Re: [PATCH V2 5/9] PCI: Export pci_flags
From:       Manikanta Maddireddy <mmaddireddy () nvidia ! com>
Date:       2017-11-30 19:50:56
Message-ID: 1edda714-5dba-3673-1f98-a6c5f2869e41 () nvidia ! com
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On 01-Dec-17 12:12 AM, Bjorn Helgaas wrote:
> On Thu, Nov 30, 2017 at 10:24:37AM +0000, Lorenzo Pieralisi wrote:
>> On Wed, Nov 29, 2017 at 11:01:33AM -0600, Bjorn Helgaas wrote:
>>> [+cc Lorenzo]
>>>
>>> On Sun, Nov 26, 2017 at 01:02:09AM +0530, Manikanta Maddireddy wrote:
>>>> pci_flags variable is used in inline functions in 'pci.h', Tegra PCIe
>>>> driver use one of these functions pci_add_flags() and includes 'pci.h'.
>>>> Export pci_flags to allow Tegra PCIe host controller driver to be
>>>> compiled as loadable kernel module.
>>>
>>> Here's the usage in tegra_pcie_probe():
>>>
>>>   pci_add_flags(PCI_REASSIGN_ALL_RSRC | PCI_REASSIGN_ALL_BUS);
>>>
>>> We've probably had this discussion before, but  I don't know why Tegra
>>> needs PCI_REASSIGN_ALL_RSRC and PCI_REASSIGN_ALL_BUS.
>>>
>>> I would prefer to drop this usage of pci_add_flags() if possible.  It
>>> seems to be just an arm/powerpc thing and I'm not convinced it's
>>> really necessary.
>>
>> It is hard to say if it is really necessary (because it depends
>> on firmware configuration - ie pci_scan_bridge()), that's the
>> problem.
>>
>> I suspect it can trigger regressions if we do not set it (since
>> it affects what pcibios_assign_all_busses() returns on eg arm/arm64).
>>
>> There are two things we can do:
>>
>> 1) Set it unconditionally in arch code (in a hook to be defined)
>> 2) We remove it on a per-host bridge basis and ask for testing
>>
>> I agree this may have trickled from host bridge to host bridge through
>> copy'n'paste and it is not based on any firmware assumtpion but I can't
>> say if it is really needed.
> 
> My basic position is that if resources are not assigned correctly, the
> PCI core should automatically try to assign them, regardless of
> whether PCI_REASSIGN_ALL_RSRC or PCI_REASSIGN_ALL_BUS is set.  If that
> doesn't work, I think there's something wrongin the PCI core and we
> should fix that.
> 
> This might be an opportunity to try removing the use of
> pci_add_flags() and see what breaks.
> 
> Bjorn
> 

As per the Tegra TRM primary, secondary and subordinate default
bus numbers are 0 and it is expecting SW to program these numbers.
I believe this is the reason for adding PCI_REASSIGN_ALL_RSRC and
PCI_REASSIGN_ALL_BUS flags.

Looking at the function pci_scan_bridge_extend(), if secondary and
subordinate bus numbers are 0 it is assigning bus numbers even if
these flags are not set.

In the basic testing with one endpoint PCIe link up is working
fine without these flags, however I would like test with PCIe switch
and multiple endpoints connected to Tegra.

Manikanta
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