Re: [PATCH v2 07/10] ARM: tegra: pcie: Add device tree support

[Arnd Bergmann <arnd@arndb.de>]

On Friday 22 June 2012, Thierry Reding wrote:
> On Fri, Jun 22, 2012 at 05:46:52AM -0600, Bjorn Helgaas wrote:
> > On Fri, Jun 22, 2012 at 5:00 AM, Thierry Reding
> > <thierry.reding@avionic-design.de> wrote:
> > >> Is the bus number aperture included somewhere?  How do we know what
> > >> bus numbers are available for allocation under each bridge?
> > >
> > > Not yet. I don't think DT imposes a bus number allocation on PCI
> > > bridges. However the matching of DT nodes to PCI bridges is done based
> > > on the bus number. For that you provide a bus-ranges property which
> > > defines the bus aperture of the given PCI bridge. The DT matching code
> > > compares the first cell of this property with the primary bus number of
> > > the bridge.
> > 
> > I don't fully understand this, but I can tell you that things don't
> > work very well if we don't know the aperture.  We can make
> > assumptions, like the root bus is 00, and then enumerate everything
> > reachable from there.  But then all we know is the largest bus number
> > actually reachable from bus 00, which is usually smaller then the end
> > of the aperture.  We don't know how many unused bus numbers there are
> > in the aperture, so we can't safely allocate any for hot-added
> > devices.

I believe the assumption today is that all bus numbers are ok on each
root port, which is usually the case. There was something about a Power
Mac that faked a single PCI bus number space across both an AGP and
a PCIe domain in pmac_pci_fixup_u4_of_node(), but I think that is the
exception.

Is there any requirement to use a bus aperture on non-PC hardware?
 
> I think DT support for PCI is lacking in a lot of areas. PowerPC seems
> to be the only architecture actively setting up busses according to the
> bus-range property specified in the DT. All other architectures seem to
> not pre-allocate bus apertures and go with the default instead. From
> what you say that means hot-plugging is out. Although I don't think the
> Tegra PCIe controller even supports hot-plugging.

If the default is allowing any bus numbers, and that matches the
hardware capabilities, I think hot-plugging would work.

	Arnd