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    SubjectRe: [PATCH] PCI: Only enable IO window if supported
    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@roeck-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 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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