On Mon, Nov 21, 2016 at 08:52:52AM +0000, Gabriele Paoloni wrote:

Hi Bjorn

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-----Original Message-----
From: Bjorn Helgaas [mailto:helgaas at kernel.org]
Sent: 18 November 2016 17:54
To: Gabriele Paoloni
Cc: Bjorn Helgaas; linux-pci at vger.kernel.org; linux-
acpi at vger.kernel.org; linux-kernel at vger.kernel.org; linux-arm-
kernel at lists.infradead.org; linaro-acpi at lists.linaro.org
Subject: Re: [PATCH] PCI: Add information about describing PCI in ACPI

On Fri, Nov 18, 2016 at 05:17:34PM +0000, Gabriele Paoloni wrote:

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-----Original Message-----
From: linux-kernel-owner at vger.kernel.org [mailto:linux-kernel-
owner at vger.kernel.org] On Behalf Of Bjorn Helgaas
Sent: 17 November 2016 18:00

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+Static tables like MCFG, HPET, ECDT, etc., are *not* mechanisms

for

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+reserving address space!  The static tables are for things the OS
+needs to know early in boot, before it can parse the ACPI

namespace.

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+If a new table is defined, an old OS needs to operate correctly

even

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+though it ignores the table.  _CRS allows that because it is

generic

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+and understood by the old OS; a static table does not.

Right so if my understanding is correct you are saying that resources
described in the MCFG table should also be declared in PNP0C02

devices

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so that the PNP driver can reserve these resources.

Yes.

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On the other side the PCI Root bridge driver should not reserve such
resources.

Well if my understanding is correct I think we have a problem here:
http://lxr.free-electrons.com/source/drivers/pci/ecam.c#L74

As you can see pci_ecam_create() will conflict with the pnp driver
as it will try to reserve the resources from the MCFG table...

Maybe we need to rework pci_ecam_create() ?

I think it's OK as it is.

The pnp/system.c driver does try to reserve PNP0C02 resources, and it
marks them as "not busy".  That way they appear in /proc/iomem and
won't be allocated for anything else, but they can still be requested
by drivers, e.g., pci/ecam.c, which will mark them "busy".

This is analogous to what the PCI core does in pci_claim_resource().
This is really a function of the ACPI/PNP *core*, which should reserve
all _CRS resources for all devices (not just PNP0C02 devices).  But
it's done by pnp/system.c, and only for PNP0C02, because there's a
bunch of historical baggage there.

You'll also notice that in this case, things are out of order:
logically the pnp/system.c reservation should happen first, but in
fact the pci/ecam.c request happens *before* the pnp/system.c one.
That means the pnp/system.c one might fail and complain "[mem ...]
could not be reserved".

Correct me if I am wrong...

So currently we are relying on the fact that pci_ecam_create() is called
before the pnp driver.
If the pnp driver came first we would end up in pci_ecam_create() failing
here:
http://lxr.free-electrons.com/source/drivers/pci/ecam.c#L76

I am not sure but it seems to me like a bit weak condition to rely on...
what about removing the error condition in pci_ecam_create() and logging
just a dev_info()?

Huh.  I'm confused.  I *thought* it would be safe to reverse the
order, which would effectively be this:

  system_pnp_probe
    reserve_resources_of_dev
      reserve_range
        request_mem_region([mem 0xb0000000-0xb1ffffff])
  ...
  pci_ecam_create
    request_resource_conflict([mem 0xb0000000-0xb1ffffff])


but I experimented with the patch below on qemu, and it failed as you
predicted:

  ** res test **
  requested [mem 0xa0000000-0xafffffff]
  can't claim ECAM area [mem 0xa0000000-0xafffffff]: conflict with ECAM PNP [mem 0xa0000000-0xafffffff]

I expected the request_resource_conflict() to succeed since it's
completely contained in the "ECAM PNP" region.  But I guess I don't
understand kernel/resource.c well enough.

I'm not sure we need to fix anything yet, since we currently do the
ecam.c request before the system.c one, and any change there would be
a long ways off.  If/when that *does* change, I think the correct fix
would be to change ecam.c so its request succeeds (by changing the way
it does the request, fixing kernel/resource.c, or whatever) rather
than to reduce the log level and ignore the failure.

Bjorn

diff --git a/arch/x86/pci/init.c b/arch/x86/pci/init.c
index adb62aa..5a35638 100644
--- a/arch/x86/pci/init.c
+++ b/arch/x86/pci/init.c

@@ -7,6 +7,8 @@
    in the right sequence from here. */
 static __init int pci_arch_init(void)
 {
+	struct resource *res, *conflict;
+	static struct resource cfg;
 #ifdef CONFIG_PCI_DIRECT
 	int type = 0;
 

@@ -39,6 +41,26 @@ static __init int pci_arch_init(void)
 
 	dmi_check_skip_isa_align();
 
+	printk("\n** res test **\n");
+
+	res = request_mem_region(0xa0000000, 0x10000000, "ECAM PNP");
+	printk("requested %pR\n", res);
+	if (!res)
+		return 0;
+	res->flags &= ~IORESOURCE_BUSY;
+
+	cfg.start = 0xa0000000;
+	cfg.end = 0xafffffff;
+	cfg.flags = IORESOURCE_MEM | IORESOURCE_BUSY;
+	cfg.name = "PCI ECAM";
+
+	conflict = request_resource_conflict(&iomem_resource, &cfg);
+	if (conflict)
+		printk("can't claim ECAM area %pR: conflict with %s %pR\n",
+		    &cfg, conflict->name, conflict);
+
+	printk("\n");
+
 	return 0;
 }
 arch_initcall(pci_arch_init);