Re: [PATCH] arm64: dts: marvell: armada-37xx: Set linux,pci-domain to zero

["Pali Rohár" <pali@kernel.org>]

On Friday 23 April 2021 10:33:38 Rob Herring wrote:
> On Sat, Apr 17, 2021 at 9:49 AM Pali Rohár <pali@kernel.org> wrote:
> >
> > On Thursday 15 April 2021 10:13:17 Rob Herring wrote:
> > > On Thu, Apr 15, 2021 at 3:45 AM Marek Behun <marek.behun@nic.cz> wrote:
> > > >
> > > > On Thu, 15 Apr 2021 10:36:40 +0200
> > > > Pali Rohár <pali@kernel.org> wrote:
> > > >
> > > > > On Tuesday 13 April 2021 13:17:29 Rob Herring wrote:
> > > > > > On Mon, Apr 12, 2021 at 7:41 AM Pali Rohár <pali@kernel.org> wrote:
> > > > > > >
> > > > > > > Since commit 526a76991b7b ("PCI: aardvark: Implement driver 'remove'
> > > > > > > function and allow to build it as module") PCIe controller driver for
> > > > > > > Armada 37xx can be dynamically loaded and unloaded at runtime. Also driver
> > > > > > > allows dynamic binding and unbinding of PCIe controller device.
> > > > > > >
> > > > > > > Kernel PCI subsystem assigns by default dynamically allocated PCI domain
> > > > > > > number (starting from zero) for this PCIe controller every time when device
> > > > > > > is bound. So PCI domain changes after every unbind / bind operation.
> > > > > >
> > > > > > PCI host bridges as a module are relatively new, so seems likely a bug to me.
> > > > >
> > > > > Why a bug? It is there since 5.10 and it is working.
> > >
> > > I mean historically, the PCI subsystem didn't even support host
> > > bridges as a module. They weren't even proper drivers and it was all
> > > arch specific code. Most of the host bridge drivers are still built-in
> > > only. This seems like a small detail that was easily overlooked.
> > > unbind is not a well tested path.
> >
> > Ok! Just to note that during my testing I have not spotted any issue.
> >
> > > > > > > Alternative way for assigning PCI domain number is to use static allocated
> > > > > > > numbers defined in Device Tree. This option has requirement that every PCI
> > > > > > > controller in system must have defined PCI bus number in Device Tree.
> > > > > >
> > > > > > That seems entirely pointless from a DT point of view with a single PCI bridge.
> > > > >
> > > > > If domain id is not specified in DT then kernel uses counter and assigns
> > > > > counter++. So it is not pointless if we want to have stable domain id.
> > > >
> > > > What Rob is trying to say is that
> > > > - the bug is that kernel assigns counter++
> > > > - device-tree should not be used to fix problems with how kernel does
> > > >   things
> > > > - if a device has only one PCIe controller, it is pointless to define
> > > >   it's pci-domain. If there were multiple controllers, then it would
> > > >   make sense, but there is only one
> > >
> > > Yes. I think what we want here is a domain bitmap rather than a
> > > counter and we assign the lowest free bit. That could also allow for
> > > handling a mixture of fixed domain numbers and dynamically assigned
> > > ones.
> >
> > Currently this code is implemented in pci_bus_find_domain_nr() function.
> > IIRC domain number is 16bit integer, so plain bitmap would consume 8 kB
> > of memory. I'm not sure if it is fine or some other tree-based structure
> > for allocated domain numbers is needed.
> 
> It's an atomic_t but then shortened (potentially) to an 'int'. Surely
> we don't need 8k (or 2^31) host bridges? Seems like we could start
> with 64 and bump it as needed. Or the idr route is another option if
> that works. We'd need to get the lowest free value and be able to
> reserve values (when specified by firmware).

Seems that idr_alloc() supports both required operations as you can ask
idr_alloc() for allocating specific id (if is available).

> > > You could create scenarios where the numbers change on you, but it
> > > wouldn't be any different than say plugging in USB serial adapters.
> > > You get the same ttyUSBx device when you re-attach unless there's been
> > > other ttyUSBx devices attached/detached.
> >
> > This should be fine for most scenarios. Dynamically attaching /
> > detaching PCI domain is not such common action...
> >
> > Will you implement this new feature?
> 
> Yes, after I find a DT binding co-maintainer.
> 
> Rob