Re: [PATCH 3/7] PCI: Distribute available buses to hotplug capable PCIe downstream ports

[Mika Westerberg]

On Wed, Oct 11, 2017 at 06:32:43PM -0500, Bjorn Helgaas wrote:
> On Tue, Sep 26, 2017 at 05:17:16PM +0300, Mika Westerberg wrote:
> > System BIOS sometimes allocates extra bus space for hotplug capable PCIe
> > root/downstream ports. This space is needed if the device plugged to the
> > port will have more hotplug capable downstream ports. A good example of
> > this is Thunderbolt. Each Thunderbolt device contains a PCIe switch and
> > one or more hotplug capable PCIe downstream ports where the daisy chain
> > can be extended.
> > 
> > Currently Linux only allocates minimal bus space to make sure all the
> > enumerated devices barely fit there. The BIOS reserved extra space is
> > not taken into consideration at all. Because of this we run out of bus
> > space pretty quickly when more PCIe devices are attached to hotplug
> > downstream ports in order to extend the chain.
> > 
> > This modifies PCI core so that we distribute the available BIOS
> > allocated bus space equally between hotplug capable PCIe downstream
> > ports to make sure there is enough bus space for extending the
> > hierarchy later on.
> 
> I think this makes sense in general.  It's a fairly complicated patch,
> so my comments here are just a first pass.

Thanks for the comments!

> Why do you limit it to PCIe?  Isn't it conceivable that one could
> hot-add a conventional PCI card that contained a bridge leading to
> another hotplug slot?  E.g., a PCI card with PCMCIA slot or something
> on it?

I guess this could be generalized to such configurations but I wanted to
restrict this with PCIe for a couple of reasons. Firstly, I'm able to
actually test this ;-) Second, the rules in PCIe are quite simple,
whenever you have hotplug bridge (downstream port with a hotplug
capability set) you distribute the available bus space with it. With a
conventional PCI it is not so clear (at least to me).

> > Signed-off-by: Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>
> > ---
> >  drivers/pci/hotplug-pci.c |  13 +++++-
> >  drivers/pci/probe.c       | 114 ++++++++++++++++++++++++++++++++++++++--------
> >  include/linux/pci.h       |  19 ++++++--
> >  3 files changed, 123 insertions(+), 23 deletions(-)
> > 
> > diff --git a/drivers/pci/hotplug-pci.c b/drivers/pci/hotplug-pci.c
> > index c68366cee6b7..deb06fe22b26 100644
> > --- a/drivers/pci/hotplug-pci.c
> > +++ b/drivers/pci/hotplug-pci.c
> > @@ -8,6 +8,7 @@ int pci_hp_add_bridge(struct pci_dev *dev)
> >  {
> >  	struct pci_bus *parent = dev->bus;
> >  	int pass, busnr, start = parent->busn_res.start;
> > +	unsigned int available_buses = 0;
> >  	int end = parent->busn_res.end;
> >  
> >  	for (busnr = start; busnr <= end; busnr++) {
> > @@ -19,8 +20,18 @@ int pci_hp_add_bridge(struct pci_dev *dev)
> >  				pci_name(dev));
> >  		return -1;
> >  	}
> > +
> > +	/*
> > +	 * In case of PCIe the hierarchy can be extended through hotplug
> > +	 * capable downstream ports. Distribute the available bus
> > +	 * numbers between them to make extending the chain possible.
> > +	 */
> > +	if (pci_is_pcie(dev))
> > +		available_buses = end - busnr;
> > +
> >  	for (pass = 0; pass < 2; pass++)
> > -		busnr = pci_scan_bridge(parent, dev, busnr, pass);
> > +		busnr = pci_scan_bridge_extend(parent, dev, busnr,
> > +					       available_buses, pass);
> >  	if (!dev->subordinate)
> >  		return -1;
> >  
> > diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> > index f285cd74088e..ae0bf3c807f5 100644
> > --- a/drivers/pci/probe.c
> > +++ b/drivers/pci/probe.c
> > @@ -960,6 +960,17 @@ static void pci_enable_crs(struct pci_dev *pdev)
> >  }
> >  
> >  /*
> > + * pci_scan_bridge_extend() - Scan buses behind a bridge
> > + * @bus: Parent bus the bridge is on
> > + * @dev: Bridge itself
> > + * @max: Starting subordinate number of buses behind this bridge
> > + * @available_buses: Total number of buses available for this bridge and
> > + *		     the devices below. After the minimal bus space has
> > + *		     been allocated the remaining buses will be
> > + *		     distributed equally between hotplug capable bridges.
> > + * @pass: Either %0 (scan already configured bridges) or %1 (scan bridges
> > + *        that need to be reconfigured.
> > + *
> >   * If it's a bridge, configure it and scan the bus behind it.
> >   * For CardBus bridges, we don't scan behind as the devices will
> >   * be handled by the bridge driver itself.
> > @@ -969,7 +980,8 @@ static void pci_enable_crs(struct pci_dev *pdev)
> >   * them, we proceed to assigning numbers to the remaining buses in
> >   * order to avoid overlaps between old and new bus numbers.
> >   */
> > -int pci_scan_bridge(struct pci_bus *bus, struct pci_dev *dev, int max, int pass)
> > +int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev, int max,
> > +			   unsigned int available_buses, int pass)
> >  {
> >  	struct pci_bus *child;
> >  	int is_cardbus = (dev->hdr_type == PCI_HEADER_TYPE_CARDBUS);
> > @@ -1080,6 +1092,9 @@ int pci_scan_bridge(struct pci_bus *bus, struct pci_dev *dev, int max, int pass)
> >  						bus->busn_res.end);
> >  		}
> >  		max++;
> > +		if (available_buses)
> > +			available_buses--;
> > +
> >  		buses = (buses & 0xff000000)
> >  		      | ((unsigned int)(child->primary)     <<  0)
> >  		      | ((unsigned int)(child->busn_res.start)   <<  8)
> > @@ -1101,7 +1116,7 @@ int pci_scan_bridge(struct pci_bus *bus, struct pci_dev *dev, int max, int pass)
> >  
> >  		if (!is_cardbus) {
> >  			child->bridge_ctl = bctl;
> > -			max = pci_scan_child_bus(child);
> > +			max = pci_scan_child_bus_extend(child, available_buses);
> >  		} else {
> >  			/*
> >  			 * For CardBus bridges, we leave 4 bus numbers
> > @@ -1169,7 +1184,7 @@ int pci_scan_bridge(struct pci_bus *bus, struct pci_dev *dev, int max, int pass)
> >  
> >  	return max;
> >  }
> > -EXPORT_SYMBOL(pci_scan_bridge);
> > +EXPORT_SYMBOL(pci_scan_bridge_extend);
> >  
> >  /*
> >   * Read interrupt line and base address registers.
> > @@ -2397,9 +2412,24 @@ void __weak pcibios_fixup_bus(struct pci_bus *bus)
> >         /* nothing to do, expected to be removed in the future */
> >  }
> >  
> > -unsigned int pci_scan_child_bus(struct pci_bus *bus)
> > +/**
> > + * pci_scan_child_bus_extend() - Scan devices below a bus
> > + * @bus: Bus to scan for devices
> > + * @available_buses: Total number of buses available (%0 does not try to
> > + *		     extend beyond the minimal)
> 
> What does "%0" mean?  Is that kernel-doc for something?  0?

"%" means constant in kernel-doc comments. I've been using it with
numbers as well as with NULL and so on.

> > + * Scans devices below @bus including subordinate buses. Returns new
> > + * subordinate number including all the found devices. Passing
> > + * @available_buses causes the remaining bus space to be distributed
> > + * equally between hotplug capable bridges to allow future extension of
> > + * the hierarchy.
> > + */
> > +unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
> > +				       unsigned int available_buses)
> >  {
> > -	unsigned int devfn, pass, max = bus->busn_res.start;
> > +	unsigned int used_buses, hotplug_bridges = 0;
> > +	unsigned int start = bus->busn_res.start;
> > +	unsigned int devfn, cmax, max = start;
> >  	struct pci_dev *dev;
> >  
> >  	dev_dbg(&bus->dev, "scanning bus\n");
> > @@ -2409,7 +2439,8 @@ unsigned int pci_scan_child_bus(struct pci_bus *bus)
> >  		pci_scan_slot(bus, devfn);
> >  
> >  	/* Reserve buses for SR-IOV capability. */
> > -	max += pci_iov_bus_range(bus);
> > +	used_buses = pci_iov_bus_range(bus);
> > +	max += used_buses;
> >  
> >  	/*
> >  	 * After performing arch-dependent fixup of the bus, look behind
> > @@ -2421,23 +2452,68 @@ unsigned int pci_scan_child_bus(struct pci_bus *bus)
> >  		bus->is_added = 1;
> >  	}
> >  
> > -	for (pass = 0; pass < 2; pass++)
> > -		list_for_each_entry(dev, &bus->devices, bus_list) {
> > -			if (pci_is_bridge(dev))
> > -				max = pci_scan_bridge(bus, dev, max, pass);
> 
> Thanks for getting rid of this "for (pass = 0; ...)" loop.  Much nicer
> to just have it spelled out.  It might even be worth pulling this
> looping change into a separate patch to simplify *this* patch a little
> bit.

OK, I'll make a separate patch of it.

> I'd be happy if you did the same for the similar loop in
> pci_hp_add_bridge().

Sure, I'll do the same for pci_hp_add_bridge().

> > +	/*
> > +	 * First pass. Bridges that are already configured. We don't touch
> > +	 * these unless they are misconfigured (which we will do in second
> > +	 * pass).
> > +	 */
> > +	list_for_each_entry(dev, &bus->devices, bus_list) {
> > +		if (pci_is_bridge(dev)) {
> > +			/*
> > +			 * Calculate how many hotplug bridges there are on
> > +			 * this bus. We will distribute the additional
> > +			 * available buses between them.
> > +			 */
> > +			if (dev->is_hotplug_bridge)
> > +				hotplug_bridges++;
> 
> Maybe pull this out into a third list traversal, since it's not
> related to the "scan bridge" functionality?

OK.

> > +
> > +			cmax = max;
> > +			max = pci_scan_bridge_extend(bus, dev, max, 0, 0);
> > +			used_buses += cmax - max;
> > +		}
> > +	}
> 
> I'm trying to relate the "Bridges that are already configured" comment
> to this code.  I don't see any test here for whether the bridge is
> already configured.  Oh, I see -- the last parameter (0) to
> pci_scan_bridge_extend() means this is the first pass, and it
> basically just checks for secondary and subordinate being set.
> I thought "first pass" was related to the list_for_each_entry() loop
> here, but it's actually related to the internals of
> pci_scan_bridge_extend().  I think maybe rewording the comment can
> address this.

I'll update the comment.

> > +	/* Second pass. Bridges that need to be configured. */
> > +	list_for_each_entry(dev, &bus->devices, bus_list) {
> > +		if (pci_is_bridge(dev)) {
> > +			unsigned int buses = 0;
> > +
> > +			if (pcie_upstream_port(dev)) {
> > +				/* Upstream port gets all available buses */
> > +				buses = available_buses;
> 
> I guess this relies on the assumption that there can only be one
> upstream port on a bus?  Is that true?  Typically a switch only has a
> function 0 upstream port, but something niggles at me like the spec
> admits the possibility of a switch with multiple functions of upstream
> ports?  I don't know where that is right now (or if it exists), but
> I'll try to find it.

My understanding is that there can be only one upstream port on a bus.
That said I looked at the spec again and there is this in chapter 7.3.1
of PCIe 3.1 spec:

  Switches, and components wishing to incorporate more than eight
  Functions at their Upstream Port, are permitted to implement one or
  more âvirtual switchesâ represented by multiple Type 1 (PCI-PCI
  Bridge) Configuration Space headers as illustrated in Figure 7-2.
  These virtual switches serve to allow fan-out beyond eight Functions.
  Since Switch Downstream Ports are permitted to appear on any Device
  Number, in this case all address information fields (Bus, Device, and
  Function Numbers) must be completely decoded to access the correct
  register. Any Configuration Request targeting an unimplemented Bus,
  Device, or Function must return a Completion with Unsupported Request
  Completion Status.

Not sure what it actually means, though. A "virtual switch" to me says
it is a switch with one upstream port and multiple downstream ports,
just like normal switch. Is this what you meant? Do you understand it so
that there can be multiple upstream ports connected to a bus?

> > +			} else if (dev->is_hotplug_bridge) {
> > +				/*
> > +				 * Distribute the extra buses between
> > +				 * hotplug bridges if any.
> > +				 */
> > +				buses = available_buses / hotplug_bridges;
> > +				buses = min(buses, available_buses - used_buses);
> > +			}
> > +
> > +			cmax = max;
> > +			max = pci_scan_bridge_extend(bus, dev, cmax, buses, 1);
> > +			used_buses += max - cmax;
> >  		}
> > +	}
> >  
> >  	/*
> >  	 * Make sure a hotplug bridge has at least the minimum requested
> > -	 * number of buses.
> > +	 * number of buses but allow it to grow up to the maximum available
> > +	 * bus number of there is room.
> >  	 */
> > -	if (bus->self && bus->self->is_hotplug_bridge && pci_hotplug_bus_size) {
> > -		if (max - bus->busn_res.start < pci_hotplug_bus_size - 1)
> > -			max = bus->busn_res.start + pci_hotplug_bus_size - 1;
> > -
> > -		/* Do not allocate more buses than we have room left */
> > -		if (max > bus->busn_res.end)
> > -			max = bus->busn_res.end;
> > +	if (bus->self && bus->self->is_hotplug_bridge) {
> > +		used_buses = max_t(unsigned int, available_buses,
> > +				   pci_hotplug_bus_size - 1);
> > +		if (max - start < used_buses) {
> > +			max = start + used_buses;
> > +
> > +			/* Do not allocate more buses than we have room left */
> > +			if (max > bus->busn_res.end)
> > +				max = bus->busn_res.end;
> > +
> > +			dev_dbg(&bus->dev, "%pR extended by %#02x\n",
> > +				&bus->busn_res, max - start);
> > +		}
> >  	}
> >  
> >  	/*
> > @@ -2450,7 +2526,7 @@ unsigned int pci_scan_child_bus(struct pci_bus *bus)
> >  	dev_dbg(&bus->dev, "bus scan returning with max=%02x\n", max);
> >  	return max;
> >  }
> > -EXPORT_SYMBOL_GPL(pci_scan_child_bus);
> > +EXPORT_SYMBOL_GPL(pci_scan_child_bus_extend);
> 
> Does this need to be exported?  The only callers I see are
> pci_scan_bridge_extend() (already in the same module) and
> pci_scan_child_bus() (now an inline in linux/pci.h).
> 
> I'd rather move pci_scan_child_bus() back to probe.c and make
> pci_scan_child_bus_extend() static.

OK.

> Same with pci_scan_bridge_extend(), although that looks harder because
> it's called from hotplug-pci.c.  Really, I'm not sure hotplug-pci.c
> deserves to exist.  I think the whole file (one function) could be
> folded into pci/probe.c (as a separate patch all by itself).

No problem, I'll move it to drivers/pci/probe.c and make
pci_scan_bridge_extend() static instead.