Re: [PATCH v2 0/2] Attach DT nodes to existing PCI devices

[Herve Codina <herve.codina@bootlin.com>]

On Wed, 10 Apr 2024 16:41:43 -0500
Rob Herring <robh@kernel.org> wrote:

> On Tue, Mar 19, 2024 at 11:34 AM Herve Codina <herve.codina@bootlin.com> wrote:
> >
> > Hi Bjorn,
> >
> > On Tue, 19 Mar 2024 10:25:13 -0500
> > Bjorn Helgaas <helgaas@kernel.org> wrote:
> >  
> > > [+cc Krzysztof]
> > >
> > > On Fri, Dec 15, 2023 at 02:52:07PM +0100, Herve Codina wrote:  
> > > > On Mon, 4 Dec 2023 07:59:09 -0600
> > > > Rob Herring <robh@kernel.org> wrote:  
> > > > > On Mon, Dec 4, 2023 at 6:43 AM Herve Codina <herve.codina@bootlin.com> wrote:  
> > > > > > On Fri, 1 Dec 2023 16:26:45 -0600
> > > > > > Rob Herring <robh@kernel.org> wrote:  
> > > > > > > On Thu, Nov 30, 2023 at 10:57 AM Herve Codina <herve.codina@bootlin.com> wrote:
> > > > > > > ...  
> > >  
> > > > > > > --- a/drivers/pci/of.c
> > > > > > > +++ b/drivers/pci/of.c
> > > > > > > @@ -31,6 +31,8 @@ int pci_set_of_node(struct pci_dev *dev)
> > > > > > >                 return 0;
> > > > > > >
> > > > > > >         node = of_pci_find_child_device(dev->bus->dev.of_node, dev->devfn);
> > > > > > > +       if (!node && pci_is_bridge(dev))
> > > > > > > +               of_pci_make_dev_node(dev);
> > > > > > >         if (!node)
> > > > > > >                 return 0;  
> > > > > >
> > > > > > Maybe it is too early.
> > > > > > of_pci_make_dev_node() creates a node and fills some properties based on
> > > > > > some already set values available in the PCI device such as its struct resource
> > > > > > values.
> > > > > > We need to have some values set by the PCI infra in order to create our DT node
> > > > > > with correct values.  
> > > > >
> > > > > Indeed, that's probably the issue I'm having. In that case,
> > > > > DECLARE_PCI_FIXUP_HEADER should work. That's later, but still before
> > > > > device_add().
> > > > >
> > > > > I think modifying sysfs after device_add() is going to race with
> > > > > userspace. Userspace is notified of a new device, and then the of_node
> > > > > link may or may not be there when it reads sysfs. Also, not sure if
> > > > > we'll need DT modaliases with PCI devices, but they won't work if the
> > > > > DT node is not set before device_add().  
> > > >
> > > > DECLARE_PCI_FIXUP_HEADER is too early as well as doing the DT node creation
> > > > just before the device_add() call.
> > > > Indeed, in order to fill the DT properties, resources need to be assigned
> > > > (needed for the 'ranges' property used for addresses translation).
> > > > The resources assignment is done after the call to device_add().  
> > >
> > > Do we need to know the actual address *value* before creating the
> > > sysfs file, or is it enough to know that the file should *exist*, even
> > > if the value may be changed later?  
> >
> > I think, the problematic file is 'of_node'.
> > This file is a symlink present in the device directory pointing to the
> > node in a device-tree subdir.
> >
> > How can we create this of_node symlink without having the device-tree
> > subdir available ?
> >  
> > >  
> > > > Some PCI sysfs files are already created after adding the device by the
> > > > pci_create_sysfs_dev_files() call:
> > > >   https://elixir.bootlin.com/linux/v6.6/source/drivers/pci/bus.c#L347
> > > >
> > > > Is it really an issue to add the of_node link to sysfs on an already
> > > > present device ?  
> > >
> > > Yes, I think this would be an issue.  We've been trying to get rid of
> > > pci_create_sysfs_dev_files() altogether because there's a long history
> > > of race issues related to it:
> > >
> > >   https://lore.kernel.org/r/1271099285.9831.13.camel@localhost/ WARNING: at fs/sysfs/dir.c:451 sysfs_add_one: sysfs: cannot create duplicate filename '/devices/pci0000:00/0000:00:01.0/slot'
> > >   https://lore.kernel.org/r/19461.26166.427857.612983@pilspetsen.it.uu.se/ [2.6.35-rc1 regression] sysfs: cannot create duplicate filename ... XVR-600 related?
> > >   https://lore.kernel.org/r/20200716110423.xtfyb3n6tn5ixedh@pali/ PCI: Race condition in pci_create_sysfs_dev_files
> > >   https://lore.kernel.org/r/m3eebg9puj.fsf@t19.piap.pl/ PCI: Race condition in pci_create_sysfs_dev_files (can't boot)
> > >   https://bugzilla.kernel.org/show_bug.cgi?id=215515 sysfs: cannot create duplicate filename '.../0000:e0'
> > >
> > > And several previous attempts to fix them:
> > >
> > >   https://lore.kernel.org/r/4469eba2-188b-aab7-07d1-5c77313fc42f@gmail.com/ Guard pci_create_sysfs_dev_files with atomic value
> > >   https://lore.kernel.org/r/20230316103036.1837869-1-alexander.stein@ew.tq-group.com PCI/sysfs: get rid of pci_sysfs_init late_initcall
> > >   https://lore.kernel.org/r/1702093576-30405-1-git-send-email-ssengar@linux.microsoft.com/ PCI/sysfs: Fix race in pci sysfs creation
> > >  
> >
> > I am not sure we are facing in the same kind of issues.
> > The ones you mentioned are related to some sysfs duplication.
> > In the of_node case, the issue (if any) is that the symlink will be created
> > after the other device's file. Not sure that it can lead to some file
> > duplication.  
> 
> Again, if you notify userspace and it wants to make some decisions
> based on of_node, then it has to be there when the notification
> happens. As Greg says frequently, you've raced with userspace and
> lost.
> 
> I imagine Bjorn won't like it, but may we need another fixup point?
> 
> Rob

I'am not sure that a fixup point can fix the issue.

In order to have the of_node available before the notification, we need
to a have the of_node set and filled before the device_add() call.

In order to create the 'ranges' property in the DT node, we need PCI
resources computed. These resources are computed after the device_add()
call.

Hervé