Re: [PATCH] PCI: Only enable IO window if supported

[Guenter Roeck]

On Wed, May 27, 2015 at 04:04:47PM -0500, Bjorn Helgaas wrote:
> [+cc Lorenzo, Suravee, Will]
> 
> I cc'd Lorenzo, Suravee, and Will because Lorenzo is working on calling
> pci_read_bases() from the PCI core instead of from arch code, and there are
> likely some dependencies between these two things.
> 
> On Fri, May 22, 2015 at 05:52:16PM -0700, Guenter Roeck wrote:
> > The PCI subsystem always assumes that I/O is supported on PCIe bridges
> > and tries to assign an I/O window to each port even if that is not
> > the case.
> > 
> > This may result in messages such as
> > 
> > pcieport 0000:02:00.0: res[7]=[io  0x1000-0x0fff]
> > 					get_res_add_size add_size 1000
> > pcieport 0000:02:00.0: BAR 7: no space for [io  size 0x1000]
> > pcieport 0000:02:00.0: BAR 7: failed to assign [io  size 0x1000]
> > 
> > for each bridge port, even if a port or its parent does not support
> > I/O in the first place.
> > 
> > To avoid this message, check if a port supports I/O before trying to
> > enable it. Also check if port's parent supports I/O, and only modify
> > a port's I/O resource size if both the port and its parent support I/O.
> > 
> > If IO is disabled after the initial port scan, the IO base and size
> > registers are set to 0x00f0 to indicate that IO is disabled. A later
> > rescan interprets this as "IO supported" and enables the IO range,
> > even if the parent does not support IO. Handle this situation as well.
> > 
> > Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> > ---
> >  drivers/pci/probe.c     | 14 ++++++++++++++
> >  drivers/pci/setup-bus.c |  4 ++--
> >  include/linux/pci.h     |  9 +++++++++
> >  3 files changed, 25 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> > index 6675a7a1b9fc..f4944ef45148 100644
> > --- a/drivers/pci/probe.c
> > +++ b/drivers/pci/probe.c
> > @@ -354,6 +354,20 @@ static void pci_read_bridge_io(struct pci_bus *child)
> >  	base = (io_base_lo & io_mask) << 8;
> >  	limit = (io_limit_lo & io_mask) << 8;
> >  
> > +	/* If necessary, check if the bridge supports an I/O aperture */
> > +	if (!io_base_lo && !io_limit_lo) {
> > +		u16 io;
> > +
> > +		if (!pci_parent_supports_io(child))
> > +			return;
> > +
> > +		pci_write_config_word(dev, PCI_IO_BASE, 0xe0f0);
> > +		pci_read_config_word(dev, PCI_IO_BASE, &io);
> > +		pci_write_config_word(dev, PCI_IO_BASE, 0x0);
> > +		if (!io)
> > +			return;
> > +	}
> 
> I really like the idea of pushing this into pci_read_bridge_io().
> 
> I wonder if we can do the same with pci_read_bridge_mmio_pref(), and
> somehow get rid of pci_bridge_check_ranges() altogether?
> 
Sure, I just figured I'd start with IO, and do the rest after
I have a better idea if I am going into the right direction.

> I think I looked at doing that a while back, and it seems like there was
> some wrinkle, but I don't remember what it was.
> 
> It does make sense that if the bridge supports an I/O aperture, but there's
> no possibility of I/O resources on the primary side, we should pretend the
> bridge has no I/O aperture.  But I think it might be nice to emit a
> diagnostic about *why* we're ignoring it.  Otherwise there's a little
> discrepancy between dmesg and lspci.
> 
Ok, makes sense. Would you want to see that message for every port ?
Guess I can check how it looks like, to make sure that I don't end up
getting a lot of noise again.

> > +
> >  	if ((io_base_lo & PCI_IO_RANGE_TYPE_MASK) == PCI_IO_RANGE_TYPE_32) {
> >  		u16 io_base_hi, io_limit_hi;
> >  
> > diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
> > index 4fd0cacf7ca0..963b31a109a9 100644
> > --- a/drivers/pci/setup-bus.c
> > +++ b/drivers/pci/setup-bus.c
> > @@ -750,12 +750,12 @@ static void pci_bridge_check_ranges(struct pci_bus *bus)
> >  	b_res[1].flags |= IORESOURCE_MEM;
> >  
> >  	pci_read_config_word(bridge, PCI_IO_BASE, &io);
> > -	if (!io) {
> > +	if (!io && pci_parent_supports_io(bus)) {
> >  		pci_write_config_word(bridge, PCI_IO_BASE, 0xe0f0);
> >  		pci_read_config_word(bridge, PCI_IO_BASE, &io);
> >  		pci_write_config_word(bridge, PCI_IO_BASE, 0x0);
> >  	}
> > -	if (io)
> > +	if (io && (io != 0x00f0 || pci_parent_supports_io(bus)))
> 
> I *think* this 0x00f0 depends on what pci_setup_bridge_io() writes to
> PCI_IO_BASE when it disables an I/O aperture.  Depending on that particular

Correct. I could have checked if io is disabled (limit < base),
but at least for the time being I wanted the impact to be minimal.
So far the code auto-enables IO if it was disabled (eg by the BIOS)
but the bridge chip supports it. I only wanted to keep it disabled
if it was likely that it was disabled by pci_setup_bridge_io().

Of course,
	if (io && pci_parent_supports_io(bus))
might just be sufficient.

> value here is sort of ugly and would need at least a comment if we can't
> figure out a better way to do it.
> 
> But it would be ideal if we could get rid of pci_bridge_check_ranges()
> altogether and have the rule that we read bridge window characteristics
> (IORESOURCE_IO, IORESOURCE_MEM, IORESOURCE_PREFETCH, IORESOURCE_MEM_64)
> once when we enumerate the bridge.  After that, the only changes would be
> to change res->start and res->end and update the hardware correspondingly.
> 
Would be great - this should solve the above problem automatically.
I was hesitant to do that, because I don't know if there would be side
effects. I could take out the io handling from pci_bridge_check_ranges()
and see what happens, but obviously my test coverage would be somewhat
limited.

> I'd like res->flags to reflect the capabilities of the hardware, not
> whether the window is currently enabled.
> 
Flag bits seem to be all taken. Could we use IORESOURCE_DISABLED for that
purpose, or could that cause conflicts elsewhere ?

> >  		b_res[0].flags |= IORESOURCE_IO;
> >  
> >  	/*  DECchip 21050 pass 2 errata: the bridge may miss an address
> > diff --git a/include/linux/pci.h b/include/linux/pci.h
> > index 353db8dc4c6e..f3de9e24aab1 100644
> > --- a/include/linux/pci.h
> > +++ b/include/linux/pci.h
> > @@ -489,6 +489,15 @@ static inline bool pci_is_root_bus(struct pci_bus *pbus)
> >  	return !(pbus->parent);
> >  }
> >  
> > +/*
> > + * Returns true if the parent bus supports an I/O aperture.
> > + */
> > +static inline bool pci_parent_supports_io(struct pci_bus *pbus)
> > +{
> > +	return pci_is_root_bus(pbus) || pci_is_root_bus(pbus->parent) ||
> > +	       (pbus->parent->resource[0]->flags & IORESOURCE_IO);
> 
> This is not obvious to me.  There are host bridges that do not have I/O
> apertures, so I don't see what the pci_is_root_bus() tests have to do with
> this.  The resource[0]->flags & IORESOURCE_IO part does make sense to me.
> 
More a matter of me not knowing what I need to do. resource[0] is NULL
for the root bus, at least on the powerpc system I used for testing.

> I think at the root bus, we'd have to iterate through all the host bridge
> resources to figure out whether there are any I/O apertures.
> 
Can you give me a hint on how to do that, hopefully in a platform
independent way ? Walk through bus->resources and search for an
IO resource ? Or does resource[0] == NULL already indicate
that there is no IO aperture ?

Thanks,
Guenter
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