Subject: Re: [RFC PATCH V3 14/17] ppc/pci: create/release dev-tree node for VFs

From: Wei Yang <[email-masked]>
Date: Fri, 20 Jun 2014 11:46:58 +0800

On Thu, Jun 19, 2014 at 09:30:47AM +0100, Grant Likely wrote:
>On Thu, Jun 19, 2014 at 3:46 AM, Wei Yang <[email-masked]> wrote:
>> On Wed, Jun 18, 2014 at 07:26:27PM +0100, Grant Likely wrote:
>>>On Tue, Jun 10, 2014 at 2:56 AM, Wei Yang <[email-masked]> wrote:
>>>> Currently, powernv platform is not aware of VFs. This means no dev-node
>>>> represents a VF. Also, VF PCI device is created when PF driver want to enable
>>>> it. This leads to the pdn->pdev and pdn->pe_number an invalid value.
>>>>
>>>> This patch create/release dev-node for VF and fixs this when a VF's pci_dev
>>>> is created.
>>>>
>>>> Signed-off-by: Wei Yang <[email-masked]>
>>>
>>>I don't think this is the right way to handle this. Unless it is a
>>>fixup to a buggy devicetree provided by firmware, I don't want to see
>>>any code modifying the devicetree to describe stuff that is able to be
>>>directly enumerated. Really the pci code should handle the lack of a
>>>device_node gracefully. If it cannot then it should be fixed.
>>
>> Grant,
>>
>> Glad to see your comment.
>>
>> I will fix this in the firmware.
>
>That's not really what I meant. The kernel should be able to deal with
>virtual functions even if firmware doesn't know how, and the kernel
>should not require modifying the device tree to support them.
>
>I'm saying fix the kernel so that a device node is not necessary for
>virtual functions.
>
>g.

Grant,

After doing some investigation, I found there are two places might highly rely
on these information. And not only VFs, but also PFs.

1. pnv_pci_read_config()/pnv_pci_cfg_read()
   When doing config space read, this needs the information of the phb.
   In commit 61305a96, the phb is retrived from the bus, and in commit
   9bf41be6 it turns to use the device node for EEH hotplug case. Also VF may
   face similar case for EEH hotplug.(This is under dev)

   To get rid of the device node/pci_dn, we need a special handling for VFs.
   Hmm... it looks not nice.

2. pnv_pci_ioda_dma_dev_setup()/pnv_pci_ioda_dma_set_mask()
   In pci_dn, there is a field: pe_number. This is used to retrive the correct
   PE this pci device associated with.

   If we don't have a pci_dn for a VF, we need to store this information to
   another place. Like in the PF's pci_dn? Hmm... looks not nice neither.

Generally, we could find a workaround make the VFs work without device
node/pci_dn, but it would do some harm to the infrastructure, make it not
consistant and not easy to read/maintain.

Currently I don't find a neat way to just get rid of device node/pci_dn for
VFs only. May require a careful restructure to do so.

BTW, my understanding may not be correct. If you have better idea, please let
me know :-) Thanks a lot.

-- 
Richard Yang
Help you, Help me