Hi Bjorn
-----Original Message----- From: linux-pci-owner@vger.kernel.org [mailto:linux-pci- owner@vger.kernel.org] On Behalf Of Bjorn Helgaas Sent: 21 November 2016 16:47 To: Gabriele Paoloni Cc: Bjorn Helgaas; linux-pci@vger.kernel.org; linux- acpi@vger.kernel.org; linux-kernel@vger.kernel.org; linux-arm- kernel@lists.infradead.org; linaro-acpi@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:
Hi Bjorn
-----Original Message----- From: Bjorn Helgaas [mailto:helgaas@kernel.org] Sent: 18 November 2016 17:54 To: Gabriele Paoloni Cc: Bjorn Helgaas; linux-pci@vger.kernel.org; linux- acpi@vger.kernel.org; linux-kernel@vger.kernel.org; linux-arm- kernel@lists.infradead.org; linaro-acpi@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:
-----Original Message----- From: linux-kernel-owner@vger.kernel.org [mailto:linux-kernel- owner@vger.kernel.org] On Behalf Of Bjorn Helgaas Sent: 17 November 2016 18:00
+Static tables like MCFG, HPET, ECDT, etc., are *not*
mechanisms
for
+reserving address space! The static tables are for things the
OS
+needs to know early in boot, before it can parse the ACPI
namespace.
+If a new table is defined, an old OS needs to operate
correctly
even
+though it ignores the table. _CRS allows that because it is
generic
+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
so that the PNP driver can reserve these resources.
Yes.
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 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
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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