Re: [PATCH] PCI: check bridge->bus in pci_host_common_remove

[Peng Fan <peng.fan@oss.nxp.com>]

Hi Rob,

On Mon, Dec 02, 2024 at 07:55:27AM -0600, Rob Herring wrote:
>On Mon, Dec 2, 2024 at 3:29 AM Manivannan Sadhasivam
><manivannan.sadhasivam@linaro.org> wrote:
>>
>> On Wed, Nov 27, 2024 at 01:56:50PM -0600, Rob Herring wrote:
>> > On Fri, Nov 15, 2024 at 08:17:20PM +0530, Manivannan Sadhasivam wrote:
>> > > On Fri, Nov 15, 2024 at 10:14:10AM +0000, Peng Fan wrote:
>> > > > Hi Manivannan,
>> > > >
>> > > > > Subject: Re: [PATCH] PCI: check bridge->bus in
>> > > > > pci_host_common_remove
>> > > > >
>> > > > > On Mon, Oct 28, 2024 at 04:46:43PM +0800, Peng Fan (OSS) wrote:
>> > > > > > From: Peng Fan <peng.fan@nxp.com>
>> > > > > >
>> > > > > > When PCI node was created using an overlay and the overlay is
>> > > > > > reverted/destroyed, the "linux,pci-domain" property no longer exists,
>> > > > > > so of_get_pci_domain_nr will return failure. Then
>> > > > > > of_pci_bus_release_domain_nr will actually use the dynamic IDA,
>> > > > > even
>> > > > > > if the IDA was allocated in static IDA. So the flow is as below:
>> > > > > > A: of_changeset_revert
>> > > > > >     pci_host_common_remove
>> > > > > >      pci_bus_release_domain_nr
>> > > > > >        of_pci_bus_release_domain_nr
>> > > > > >          of_get_pci_domain_nr      # fails because overlay is gone
>> > > > > >          ida_free(&pci_domain_nr_dynamic_ida)
>> > > > > >
>> > > > > > With driver calls pci_host_common_remove explicity, the flow
>> > > > > becomes:
>> > > > > > B pci_host_common_remove
>> > > > > >    pci_bus_release_domain_nr
>> > > > > >     of_pci_bus_release_domain_nr
>> > > > > >      of_get_pci_domain_nr      # succeeds in this order
>> > > > > >       ida_free(&pci_domain_nr_static_ida)
>> > > > > > A of_changeset_revert
>> > > > > >    pci_host_common_remove
>> > > > > >
>> > > > > > With updated flow, the pci_host_common_remove will be called
>> > > > > twice, so
>> > > > > > need to check 'bridge->bus' to avoid accessing invalid pointer.
>> > > > > >
>> > > > > > Fixes: c14f7ccc9f5d ("PCI: Assign PCI domain IDs by ida_alloc()")
>> > > > > > Signed-off-by: Peng Fan <peng.fan@nxp.com>
>> > > > >
>> > > > > I went through the previous discussion [1] and I couldn't see an
>> > > > > agreement on the point raised by Bjorn on 'removing the host bridge
>> > > > > before the overlay'.
>> > > >
>> > > > This patch is an agreement to Bjorn's idea.
>> > > >
>> > > > I have added pci_host_common_remove to remove host bridge
>> > > > before removing overlay as I wrote in commit log.
>> > > >
>> > > > But of_changeset_revert will still runs into pci_host_
>> > > > common_remove to remove the host bridge again. Per
>> > > > my view, the design of of_changeset_revert to remove
>> > > > the device tree node will trigger device remove, so even
>> > > > pci_host_common_remove was explicitly used before
>> > > > of_changeset_revert. The following call to of_changeset_revert
>> > > > will still call pci_host_common_remove.
>> > > >
>> > > > So I did this patch to add a check of 'bus' to avoid remove again.
>> > > >
>> > >
>> > > Ok. I think there was a misunderstanding. Bjorn's example driver,
>> > > 'i2c-demux-pinctrl' applies the changeset, then adds the i2c adapter for its
>> > > own. And in remove(), it does the reverse.
>> > >
>> > > But in your case, the issue is with the host bridge driver that gets probed
>> > > because of the changeset. While with 'i2c-demux-pinctrl' driver, it only
>> > > applies the changeset. So we cannot compare both drivers. I believe in your
>> > > case, 'i2c-demux-pinctrl' becomes 'jailhouse', isn't it?
>> > >
>> > > So in your case, changeset is applied by jailhouse and that causes the
>> > > platform device to be created for the host bridge and then the host bridge
>> > > driver gets probed. So during destroy(), you call of_changeset_revert() that
>> > > removes the platform device and during that process it removes the host bridge
>> > > driver. The issue happens because during host bridge remove, it calls
>> > > pci_remove_root_bus() and that tries to remove the domain_nr using
>> > > pci_bus_release_domain_nr().
>> > >
>> > > But pci_bus_release_domain_nr() uses DT node to check whether to free the
>> > > domain_nr from static IDA or dynamic IDA. And because there is no DT node exist
>> > > at this time (it was already removed by of_changeset_revert()), it forces
>> > > pci_bus_release_domain_nr() to use dynamic IDA even though the IDA was initially
>> > > allocated from static IDA.
>> >
>> > Putting linux,pci-domain in an overlay is the same problem as aliases in
>> > overlays[1]. It's not going to work well.
>> >
>> > IMO, you can have overlays, or you can have static domains. You can't
>> > have both.
>> >
>>
>> Okay.
>>
>> > > I think a neat way to solve this issue would be by removing the OF node only
>> > > after removing all platform devices/drivers associated with that node. But I
>> > > honestly do not know whether that is possible or not. Otherwise, any other
>> > > driver that relies on the OF node in its remove() callback, could suffer from
>> > > the same issue. And whatever fix we may come up with in PCI core, it will be a
>> > > band-aid only.
>> > >
>> > > I'd like to check with Rob first about his opinion.
>> >
>> > If the struct device has an of_node set, there should be a reference
>> > count on that node. But I think that only prevents the node from being
>> > freed. It does not prevent the overlay from being detached. This is one
>> > of many of the issues with overlays Frank painstakingly documented[2].
>> >
>>
>> Ah, I do remember this page as Frank ended up creating it based on my
>> continuous nudge to add CONFIG_FS interface for applying overlays.
>>
>> So why are we applying overlays in kernel now?
>
>That's been the case for some time. Mostly it's been for fixups of old
>to new bindings, but those all got dropped at some point. The in
>kernel users are very specific use cases where we know something about
>what's in the overlay. In contrast, configfs interface allows for any
>change to any node or property with no control over it by the kernel.
>Never say never, but I just don't see that ever happening upstream.

So should I switch to use configfs for jailhouse case? Currently
we use overlay to add a virtual pci node to kernel device tree.

Thanks
Peng

>
>Rob