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

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Date:   Mon, 4 Dec 2023 16:30:14 +0100
From:   Herve Codina <herve.codina@...tlin.com>
To:     Rob Herring <robh@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lizhi Hou <lizhi.hou@....com>, Max Zhen <max.zhen@....com>,
        Sonal Santan <sonal.santan@....com>,
        Stefano Stabellini <stefano.stabellini@...inx.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        PCI <linux-pci@...r.kernel.org>,
        Allan Nielsen <allan.nielsen@...rochip.com>,
        Horatiu Vultur <horatiu.vultur@...rochip.com>,
        Steen Hegelund <steen.hegelund@...rochip.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH v2 0/2] Attach DT nodes to existing PCI devices

Hi Rob,

On Mon, 4 Dec 2023 07:59:09 -0600
Rob Herring <robh@...nel.org> wrote:

[...]

> > > diff --git a/drivers/pci/bus.c b/drivers/pci/bus.c
> > > index 9c2137dae429..46b252bbe500 100644
> > > --- a/drivers/pci/bus.c
> > > +++ b/drivers/pci/bus.c
> > > @@ -342,8 +342,6 @@ void pci_bus_add_device(struct pci_dev *dev)
> > >          */
> > >         pcibios_bus_add_device(dev);
> > >         pci_fixup_device(pci_fixup_final, dev);
> > > -       if (pci_is_bridge(dev))
> > > -               of_pci_make_dev_node(dev);
> > >         pci_create_sysfs_dev_files(dev);
> > >         pci_proc_attach_device(dev);
> > >         pci_bridge_d3_update(dev);
> > > diff --git a/drivers/pci/of.c b/drivers/pci/of.c
> > > index 51e3dd0ea5ab..e15eaf0127fc 100644
> > > --- 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().

Ok, we can try using DECLARE_PCI_FIXUP_HEADER.
On your side, is moving from DECLARE_PCI_FIXUP_EARLY to DECLARE_PCI_FIXUP_HEADER
fix your QEMU unittest ?

We have to note that between the pci_fixup_device(pci_fixup_header, dev) call
and the device_add() call, the call to pci_set_msi_domain() is present.
MSIs are not supported currently but in the future ...

Related to DT modaliases, I don't think they are needed.
All drivers related to PCI device should be declared as pci_driver.
Correct me if I am wrong but I think that the core PCI will load the correct
module without any DT modalias.

Best regards,
Hervé

-- 
Hervé Codina, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com