Re: [PATCH V8 7/9] acpi: Add generic MCFG table handling

[Tomasz Nowicki]

On 08.06.2016 15:17, Bjorn Helgaas wrote:
> On Wed, Jun 08, 2016 at 02:21:30PM +0200, Tomasz Nowicki wrote:
> > On 08.06.2016 03:56, Bjorn Helgaas wrote:
> > > On Mon, May 30, 2016 at 05:14:20PM +0200, Tomasz Nowicki wrote:
> > > > In order to handle PCI config space regions properly in ACPI, new MCFG
> > > > interface is defined which does sanity checks on MCFG table and keeps its
> > > > root pointer. The user is able to lookup MCFG regions based on
> > > > host bridge root structure and domain:bus_start:bus_end touple.
> > > > Use pci_mmcfg_late_init old prototype to avoid another function name.
>
> > > > +	/* found matching entry, bus range check */
> > > > +	if (entry->end_bus_number != bus_res->end) {
> > > > +		resource_size_t bus_end = min_t(resource_size_t,
> > > > +					entry->end_bus_number, bus_res->end);
> > > > +		pr_warn("%04x:%pR bus end mismatch, using %02lx\n",
> > > > +			root->segment, bus_res, (unsigned long)bus_end);
> > > > +		bus_res->end = bus_end;
> > > > +	}
> >
> > What about bus end mismatch case? Should we trim the host bridge bus
> > range or expect MCFG entry covers that range? Sometimes we get
> > _BBN-0xFF bus range, not from _CRS.
>
> Lack of a bus range in _CRS is a firmware defect.  There's a comment
> about this in acpi_pci_root_add().  On x86, we probably had to live
> with firmware in the field that had this defect.  I think we should
> expect all ARM64 systems to provide a bus number range in _CRS, and
> fail the attach if it's not there.
>
> I don't think we should warn about an MCFG entry that covers more than
> the _CRS bus range.  On x86, it's common to have something like:
>
>    ACPI: PCI Root Bridge [PCI0] (domain 0000 [bus 00-7f])
>    ACPI: PCI Root Bridge [PCI1] (domain 0000 [bus 80-ff])
>
> with a single MCFG entry that covers [bus 00-ff].  That seems
> reasonable and I don't think it's worth warning about it.
>
> If the MCFG entry doesn't cover all of a _CRS bus range, we should
> just fail so we can find and fix broken firmware.

Make sense to me.

> > > > +/* Interface called by ACPI - parse and save MCFG table */
> > >
> > > I think we save a *pointer* to the MCFG table, not the table itself.
> >
> > Right, the comment is broken.
> >
> > > And acpi_table_parse() calls early_acpi_os_unmap_memory() immediately
> > > after it calls pci_mcfg_parse(), so I'm doubtful that the pointer
> > > remains valid.
> >
> > At this stage early_acpi_os_unmap_memory() is doing nothing since
> > acpi_early_init() set acpi_gbl_permanent_mmap to 1 way before. The
> > pointer is fine then.
>
> Hmmm...  I see your argument, but this is a problem waiting to happen.
> We should not depend on the internal implementation of
> early_acpi_os_unmap_memory().  The pattern of:
>
>    y = x;
>    unmap(x);
>    z = *y;
>
> is just broken and we shouldn't expect readers to recognize that "oh,
> unmap() isn't really unmapping anything in this special case, so this
> looks wrong but is really fine."

Right, so we are back to MCFG cache.

Thanks,
Tomasz