Message-ID: <20150528124112.GJ10210@google.com>
Date:	Thu, 28 May 2015 07:41:12 -0500
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>,
	Will Deacon <will.deacon@....com>
Subject: Re: [PATCH] PCI: Only enable IO window if supported

On Wed, May 27, 2015 at 07:23:32PM -0700, Guenter Roeck wrote:
> 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@...ck-us.net>
> > > ---
> > >  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 definitely think this is the right direction :)

> > 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.

I was thinking once per port.  We currently print a line for every enabled
bridge window, so it shouldn't be too much.  In fact, we often print the
bridge windows several times (which I think is overkill; I'd prefer to
print it once when we discover it and again only if we change something
later).

> > > +
> > >  	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().

OK, I see.  What I think we *should* do is:

  - If the I/O window was enabled by the BIOS, leave it that way unless we
    need to change it

  - If the I/O window was left disabled by the BIOS, enable it only if we
    need it, i.e., there's I/O space available on the primary side of the
    bridge and one of the following is true:

      1) the bridge supports hotplug
      2) a downstream bridge supports hotplug
      3) a downstream device needs I/O space

> > 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'm willing to take the risk :)  Of course, we'll need to analyze it as
much as we can to make sure we believe it is correct.

> > 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 ?

Yes, I think IORESOURCE_DISABLED would be appropriate for any I/O windows
below a host bridge that doesn't support I/O space.

> > > +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 ?

Use pci_bus_for_each_resource() and look for one with IORESOURCE_IO set.

Bjorn
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