Re: [PATCH v3] PCI/ACPI: xgene: Add ECAM quirk for X-Gene PCIe controller

[Jon Masters]

Hi Bjorn, Duc, Mark,

I switched my brain to the on mode and went and read some specs, and a few
tables, so here's my 2 cents on this...

On 12/01/2016 06:22 PM, Duc Dang wrote:
> On Thu, Dec 1, 2016 at 3:07 PM, Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
>> On Thu, Dec 01, 2016 at 02:10:10PM -0800, Duc Dang wrote:

>>>>> The SoC provide some number of RC bridges, each with a different base
>>>>> for some mmio registers. Even if segment is legitimate in MCFG, there
>>>>> is still a problem if a platform doesn't use the segment ordering
>>>>> implied by the code. But the PNP0A03 _CRS does have this base address
>>>>> as the first memory resource, so we could get it from there and not
>>>>> have hard-coded addresses and implied ording in the quirk code.
>>>>
>>>> I'm confused.  Doesn't the current code treat every item in PNP0A03
>>>> _CRS as a window?  Do you mean the first resource is handled
>>>> differently somehow?  The Consumer/Producer bit could allow us to do
>>>> this by marking the RC MMIO space as "Consumer", but I didn't think
>>>> that strategy was quite working yet.

Let's see if I summarized this correctly...

1. The MMIO registers for the host bridge itself need to be described
   somewhere, especially if we need to find those in a quirk and poke
   them. Since those registers are very much part of the bridge device,
   it makes sense for them to be in the _CRS for PNP0A08/PNP0A03.

2. The address space covering these registers MUST be described as a
   ResourceConsumer in order to avoid accidentally exposing them as
   available for use by downstream devices on the PCI bus.

3. The ACPI specification allows for resources of the type "Memory32Fixed".
   This is a macro that doesn't have the notion of a producer or consumer.
   HOWEVER various interpretations seem to be that this could/should
   default to being interpreted as a consumed region.

4. At one point, a regression was added to the kernel:

   63f1789ec716 ("x86/PCI/ACPI: Ignore resources consumed by
   host bridge itself")

   Which lead to a series on conversations about what should happen
   for bridge resources (e.g. https://lkml.org/lkml/2015/3/24/962 )

5. This resulted in the following commit reverting point 4:

   2c62e8492ed7 ("x86/PCI/ACPI: Make all resources except [io 0xcf8-0xcff]
   available on PCI bus")

   Which also stated that:

   "This solution will also ease the way to consolidate ACPI PCI host
    bridge common code from x86, ia64 and ARM64"

End of summary.

So it seems that generally there is an aversion to having bridge resources
be described in this manner and you would like to require that they be
described e.g. using QWordMemory with a ResourceConsumer type?

BUT if we were to do that, it would break existing shipping systems since
there are quirks out there that use this form to find the base CSR:

       if (acpi_res->type == ACPI_RESOURCE_TYPE_FIXED_MEMORY32) {
                fixed32 = &acpi_res->data.fixed_memory32;
                port->csr_base = ioremap(fixed32->address,
                                         fixed32->address_length);
                return AE_CTRL_TERMINATE;
        }

That's what's shipping in at least RHEL(SA) today, and probably in other
distros. So if we get vendors to take that out, existing stuff will break,
which will have the downside that customers will have to choose between
whether to run a given distro or be able to use upstream kernels. In
that sense, to me, there are shipping platforms out there, which may well
be doing the "wrong" thing, but they are deployed and they are doing it.

Which makes me wonder a couple of things (I think should NOT be done):

1. What would happen if we had both. A FixedMemory32 and the same region
   described again using the longer form as a consumed region. I doubt
   that's legal, and the current code would still add the region if it
   saw the FixedMemory32 first when walking the tree. I don't like it,
   but I'm mentioning it in case that leads to some helpful thinking.

2. What would happen if we had a difference policy on arm64 for such
   resources. x86 has an "exception" for accessing the config space
   using IO port 0xCF8-0xCFF (a fairly reasonable exception!) and
   we can make the rules for a new platform (i.e. actually prescribe
   exactly what the behavior is, rather than have it not be defined).
   This is of course terrible in that existing BIOS vendors and so on
   won't necessarily know this when working on ARM ACPI later on.

I don't like either of these obviously. I'm hoping there's some way we
can say that this is tolerated in this one quirk (allow the use of
FixedMemory32 in this case) on the grounds that the driver claims
this bridge region and can be annotated to explain such.

Once you let us know what you prefer, we will go and update the ARM
SBBR to spell out that future platforms should not make this mistake
again. We can prescribe whatever you'd like in terms of how bridge
resources consumed by the bridge are exposed. I have spoken about
this kind of situation within MS in the past, but they didn't have
specific guidance since they don't really tolerate such quirks. I
can, however, consult them before we change the SBBR as well.

>>> The first resource is defined like below. It was introduced long time
>>> ago to use with older version of X-Gene ECAM quirks.
>>> Memory32Fixed(ReadWrite, 0x1F2B0000, 0x10000, )

Indeed. And in the case of m400, it is currently this in shipping systems:

               Memory32Fixed (ReadWrite,
                    0x1F500000,         // Address Base
                    0x00010000,         // Address Length
                    )

The spec isn't clear on whether these are produced or consumed but the
implication is that these are consumed resources in most cases. Not that
this changes any of the above, but one can understand why it happened.

>>> [    0.822990] pci_bus 0000:00: root bus resource [mem 0x1f2b0000-0x1f2bffff]
>>
>> I think this is wrong.  The PCI core thinks [mem 0x1f2b0000-0x1f2bffff]
>> is available for use by devices on bus 0000:00, but I think you're
>> saying it is consumed by the bridge itself, not forwarded down to PCI.

Indeed.

>> What's in your /proc/iomem?  I see that your quirks do call
>> devm_ioremap_resource(), which calls devm_request_mem_region()
>> internally, so the driver does at least request that region, which
>> should keep us from assigning it to PCI devices.

I'm hoping you can grant an exception on the grounds that the quirk will
keep the region from actually being used. And then somehow we document
this in the driver.

>> But it still isn't quite right to tell the PCI core that the region is
>> available on the root bus.
> 
> This is /proc/iomem output on my Mustang board. The 64K "PCIe CSR"
> region is consumed completely.
> 1f2b0000-1f2bffff : PCI Bus 0000:00
>   1f2b0000-1f2bffff : PCIe CSR
> 
> e040000000-e07fffffff : PCI Bus 0000:00
>   e040000000-e0401fffff : PCI Bus 0000:01
>     e040000000-e0400fffff : 0000:01:00.0
>       e040000000-e0400fffff : mlx4_core
>     e040100000-e0401fffff : 0000:01:00.0
> e0d0000000-e0dfffffff : PCI ECAM
> f000000000-ffffffffff : PCI Bus 0000:00
>   f000000000-f001ffffff : PCI Bus 0000:01
>     f000000000-f001ffffff : 0000:01:00.0
>       f000000000-f001ffffff : mlx4_core
> 
> Using hard-coded resources for mmio space make the quirk rely on the
> segment number passing from the firmware. Using Mark's method or
> acpi_get_rc_resource can discover the mmio space and consume all of
> the space, but as you mentioned, it leaves the defect that PCI core
> considers the mmio space as available resource for secondary devices
> although it will never allocate the mmio space to secondary devices as
> the RC already reserves and consumes all of the space.

Indeed. It's not clean, but perhaps we can get away with it on the
grounds that there are existing systems out there and this won't
be allowed to happen again in the future :)

Jon.

-- 
Computer Architect | Sent from my Fedora powered laptop