[PATCH] PCI: Add information about describing PCI in ACPI

From: Gabriele Paoloni <hidden>
Date: 2016-11-21 17:23:26
Also in: linux-acpi, linux-pci, lkml

Hi Bjorn

-----Original Message-----
From: linux-pci-owner at vger.kernel.org [mailto:linux-pci-
owner at vger.kernel.org] On Behalf Of Bjorn Helgaas
Sent: 21 November 2016 16:47
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 Mon, Nov 21, 2016 at 08:52:52AM +0000, Gabriele Paoloni wrote:

quoted

Hi Bjorn

quoted

-----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

quoted

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

quoted

-----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

quoted

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

mechanisms

quoted

for

quoted

+reserving address space!  The static tables are for things the

OS

quoted

+needs to know early in boot, before it can parse the ACPI

namespace.

quoted

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

correctly

quoted

even

quoted

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

generic

quoted

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

Right so if my understanding is correct you are saying that

resources

quoted

described in the MCFG table should also be declared in PNP0C02

devices

quoted

so that the PNP driver can reserve these resources.

Yes.

quoted

On the other side the PCI Root bridge driver should not reserve

such

quoted

resources.

Well if my understanding is correct I think we have a problem

here:

quoted

http://lxr.free-electrons.com/source/drivers/pci/ecam.c#L74

As you can see pci_ecam_create() will conflict with the pnp

driver

quoted

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

quoted

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

quoted

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().

quoted

This is really a function of the ACPI/PNP *core*, which should

reserve

quoted

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

quoted

before the pnp driver.
If the pnp driver came first we would end up in pci_ecam_create()

failing

quoted

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...

quoted

what about removing the error condition in pci_ecam_create() and

logging

quoted

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 think it fails because effectively the PNP driver is populating the
iomem_resource resource tree and therefore pci_ecam_create() finds that
it cannot add the cfg resource to the same hierarchy as it is already
there...

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.

Well in my mind I didn't want just to make the error disappear...
If all the resources should be reserved by the PNP driver then ideally
we could take away request_resource_conflict() from pci_ecam_create(),
but this would make buggy some systems with an already shipped BIOS
that relied on pci_ecam_create() reservation rather than PNP reservation.

Just removing the error condition and converting dev_err() into
dev_info() seems to me like accommodating already shipped BIOS images
and flagging a reservation that is already done by somebody else
without compromising the functionality of the PCI Root bridge driver
(so far the only reason why I can see the error condition there is
to catch a buggy MCFG with overlapping addresses; so if this is the
case maybe we need to have a different diagnostic check to make sure
that the MCFG table is alright)

BTW if you think that so far we can keep this as it is I am ok.

Many Thanks

Gab

quoted hunk ↗ jump to hunk

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);

--

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