[SeaBIOS] Fwd: [Qemu-devel] [RFC PATCH 0/2] Allow RedHat PCI bridges reserve more buses than necessary during init

[Alexander Bezzubikov]

2017-07-19 21:18 GMT+03:00 Konrad Rzeszutek Wilk <konrad.wilk at oracle.com>:

> On Wed, Jul 19, 2017 at 05:14:41PM +0000, Alexander Bezzubikov wrote:
> > ср, 19 июля 2017 г. в 16:57, Konrad Rzeszutek Wilk <
> konrad.wilk at oracle.com>:
> >
> > > On Wed, Jul 19, 2017 at 04:20:12PM +0300, Aleksandr Bezzubikov wrote:
> > > > Now PCI bridges (and PCIE root port too) get a bus range number in
> > > system init,
> > > > basing on currently plugged devices. That's why when one wants to
> > > hotplug another bridge,
> > > > it needs his child bus, which the parent is unable to provide.
> > >
> > > Could you explain how you trigger this?
> >
> >
> > I'm trying to hot plug pcie-pci bridge into pcie root port, and Linux
> says
> > 'cannot allocate bus number for device bla-bla'. This obviously does not
> > allow me to use the bridge at all.
> >
> > >
> > >
> > > > The suggested workaround is to have vendor-specific capability in
> RedHat
> > > generic pcie-root-port
> > > > that contains number of additional bus to reserve on BIOS PCI init.
> > >
> > > But wouldn't the proper fix be for the PCI bridge to have the
> subordinate
> > > value be extended to fit more bus ranges?
> >
> >
> > What do you mean? This is what I'm trying to do. Do you suppose to get
> rid
> > of vendor-specific cap and use original register value instead of it?
>
> I would suggest a simple fix - each bridge has a a number of bus devices
> it can use. You have up to 255 - so you split the number of northbridge
> numbers by the amount of NUMA nodes (if that is used) - so for example
> if you have 4 NUMA nodes, each bridge would cover 63 bus numbers.
>
> Meaning the root bridge would cover 0->63 bus, 64->128, and so on.
> That gives you enough space to plug in your plugged in devices
> (up to 63).
>
> And if you need sub-briges then carve out a specific range.
>

The problem is that we don't know at the init moment how many subbridges we
may need,
and how deep the whole device tree will be. The key moment - PCI bridge
hotplugging
needs either rescan all buses on each bridge device addition, or reserve
space in advance during BIOS init.
In this series the second way was chosen.


>
>
> >
> > >
> > > >
> > > > Aleksandr Bezzubikov (2):
> > > >   pci: add support for direct usage of bdf for capability lookup
> > > >   pci: enable RedHat pci bridges to reserve more buses
> > > >
> > > >  src/fw/pciinit.c   | 12 ++++++++++--
> > > >  src/hw/pcidevice.c | 24 ++++++++++++++++++++++++
> > > >  src/hw/pcidevice.h |  1 +
> > > >  3 files changed, 35 insertions(+), 2 deletions(-)
> > > >
> > > > --
> > > > 2.7.4
> > > >
> > > >
> > >
> > --
> > Alexander Bezzubikov
>



-- 
Alexander Bezzubikov
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