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

[Guenter Roeck]

On 06/02/2015 07:55 AM, Lorenzo Pieralisi wrote:
> Bjorn, Guenter,
>
> On Wed, May 27, 2015 at 10:04:47PM +0100, 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?
> >
> > 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.

After looking into this some more, I think the wrinkle may be that
pci_read_bridge_bases() and thus pci_read_bridge_io() isn't called
on probe-only systems (if PCI_PROBE_ONLY is set). A secondary
problem is that pci_read_bridge_io() does not enable a resource
if it is explicitly disabled (base > limit), but the subsequent call
to pci_bridge_check_ranges() unconditionally enables it.

Not really sure how to address this; my current code checks IO support
in both pci_read_bridge_io() and pci_bridge_check_ranges(). And since
pci_read_bridge_io() is not always called, I don't see how it might
be possible to get rid of pci_bridge_check_ranges(), or even the check
for IO support in pci_bridge_check_ranges().

> While at it, do you think it is reasonable to also claim the bridge
> windows (resources) in the respective pci_read_bridge_* calls ?
>
> Is there a reason why we don't/can't do it ? I noticed that on
> PROBE_ONLY systems on ARM/ARM64 at the moment we do not claim
> the bridge apertures and this is not correct, see below:
>
> [5.980127] pcieport 0000:00:02.1: can't enable device: BAR 8
> [mem 0xbff00000 - 0xbfffffff] not claimed
> [5.988056] pcieport: probe of 0000:00:02.1 failed with error -22
Is this when trying my patches or with the current upstream code ?

Thanks,
Guenter

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