On Wed, 17 Feb 2016 02:38:24 +0530
Jayachandran Chandrashekaran Nair
<jayachandran.chandrasheka...@broadcom.com> wrote:
> On Tue, Feb 16, 2016 at 1:09 AM, Alex Williamson
> <alex.william...@redhat.com> wrote:
> > On Mon, 15 Feb 2016 12:20:23 -0600
> > Bjorn Helgaas <helg...@kernel.org> wrote:
> >
> >> [+cc Alex, iommu list]
> >>
> >> On Mon, Feb 15, 2016 at 03:35:00AM +0530, Jayachandran C wrote:
> >> > Add a new flag PCI_DEV_FLAGS_BRIDGE_SKIP_ALIAS to indicate bridges
> >> > that should not be considered during DMA alias search. This is
> >> > to support hardware (in this case Broadcom Vulcan PCIe subsystem)
> >> > that has internal bridges which have either missing or wrong PCIe
> >> > capabilities.
> >
> > I figured this would come at some point, the right answer is of course
> > to follow the PCIe spec and implement the required PCIe capability in
> > the hardware.
>
> There are PCIe controllers on the chip that follows the spec, the issue is
> how it is integrated to the northbridge (equivalent) on the chip, I have
> tried to explain it below.
>
> >> This needs more explanation, like what exactly is wrong with this
> >> device? A missing PCIe capability might cause other problems.
> >>
> >> What problem does this fix? Without these patches, do we merely add
> >> aliases that are unnecessary? Do we crash because something goes
> >> wrong in the pci_pcie_type() switch because of the incorrect
> >> capability?
>
> Here's how (for example) how the PCI enumeration of a 2 node Vulcan
> processor will look like:
>
>
> [0] +-0.0.0--[1]---+--1.a.0----[2]-----2.0.0---[3]----3.0.0
> | +--1.a.1----[4]-----4.0.0---[5]----5.0.0
> | .
> | ... etc...
> |
> +-0.0.1--[10]--+-10.a.0----[11]---11.0.0---[12]---12.0.0
> +-10.a.1----[13]---13.0.0---[14]---14.0.0
> .
> ... etc...
So we have:
"Glue Bridge"---"Glue Bridges"---Root Ports---Endpoints
(no pcie) (pci/x-pcie)
>
> The devices 0.0.x and x.a.x are glue bridges that are there to
> bring the real PCIe controllers (pcie cap type 4) 2.0.0, 4.0.0,
> 11.0.0, 13.0.0 under a single PCI hierarchy. The x.a.x bridges
> have a PCIe capability (type 8) and 0.0.x does not have any pcie
> capability.
>
> In case of Vulcan, both the GICv3 ITS driver code and the SMMUv3
> driver code does dma alias walk to find the device id to use
> in ITS and SMMU. In both cases it will ignore the x.0.0 bridges
> since they are type root port, but will continue on up and end
> up with incorrect device id.
>
> The flag I have added is to make the pci_for_each_dma_alias()
> ignore the last 2 levels of glue/internal bridges.
Per the PCIe spec, I'm not even sure what you have is a valid
hierarchy, root ports sit on root complexes, not behind a PCI-to-PCIe
bridge. So really you're pretending that the downstream "glue bridge"
is your host bridge and therefore root complex, but the PCI topology
would clearly dictate that the top-most bus is conventional PCI and
therefore everything is an alias of everything else and the DMA alias
code is doing exactly what it should.
Also note that the caller of pci_for_each_dma_alias() owns the callback
function triggered for each alias, that function could choose to prune
out these "glue bridges" itself if that's the appropriate thing to do.
Where do the SMMU and ITS actually reside in the above diagram? You
can use the callout function to stop the traversal anywhere you wish,
it's free to return a -errno to stop or positive number, which the
caller can interpret as error or non-failure stop condition.
You could even think about changing what Linux considers to be the host
bridge. Maybe these glue bridges shouldn't even be visible to Linux,
would that also solve the issue with the broken BAR?
> > The change takes the same code path as it would for a real PCIe bridge
> > port (downstream/upstream/root), which means they want to skip adding
> > this bridge as an alias of the device. So we're adding in aliases that
> > don't exist, the bridge itself.
> >
> > If anything I'd suggest a flag that actually tries to address the
> > problem rather than a symptom of the problem. For example, maybe the
> > flag should be PCI_DEV_FLAGS_IS_PCIE. Maybe pci_is_pcie() should even
> > take that into account. That has some trickle through for
> > pci_pcie_type() and all the accessor functions, but maybe it's a
> > cleaner solution overall (or maybe it explodes further). Thanks,
>
> I didn't really want to mark the glue bridges as PCIe or have fake
> PCIe capability there, the obvious simple solution was to add
> the flag PCI_DEV_FLAGS_BRIDGE_SKIP_ALIAS
>
> Any suggestions/comments on how to do this better would be welcome.
I definitely don't think either flag idea is the right solution, I
think you've actually already got the tools you need to solve it by
putting the intelligence in the callback function or by going further
down the path of pretending the glue bridge is really a host bridge.
Thanks,
Alex
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