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Date: Thu, 13 May 2010 14:38:24 -0500
From: Mike Habeck <habeck@....com>
To: Bjorn Helgaas <bjorn.helgaas@...com>
Cc: Mike Travis <travis@....com>, Ingo Molnar <mingo@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
Jesse Barnes <jbarnes@...tuousgeek.org>,
Jacob Pan <jacob.jun.pan@...el.com>, Tejun Heo <tj@...nel.org>,
LKML <linux-kernel@...r.kernel.org>,
Yinghai <yinghai.lu@...cle.com>,
"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>
Subject: Re: [Patch 1/1] x86 pci: Add option to not assign BAR's if not already
assigned
Bjorn Helgaas wrote:
> On Wednesday, May 12, 2010 12:14:32 pm Mike Travis wrote:
>> Subject: [Patch 1/1] x86 pci: Add option to not assign BAR's if not already assigned
>> From: Mike Habeck <habeck@....com>
>>
>> The Linux kernel assigns BARs that a BIOS did not assign, most likely
>> to handle broken BIOSes that didn't enumerate the devices correctly.
>> On UV the BIOS purposely doesn't assign I/O BARs for certain devices/
>> drivers we know don't use them (examples, LSI SAS, Qlogic FC, ...).
>> We purposely don't assign these I/O BARs because I/O Space is a very
>> limited resource. There is only 64k of I/O Space, and in a PCIe
>> topology that space gets divided up into 4k chucks (this is due to
>> the fact that a pci-to-pci bridge's I/O decoder is aligned at 4k)...
>> Thus a system can have at most 16 cards with I/O BARs: (64k / 4k = 16)
>>
>> SGI needs to scale to >16 devices with I/O BARs. So by not assigning
>> I/O BARs on devices we know don't use them, we can do that (iff the
>> kernel doesn't go and assign these BARs that the BIOS purposely didn't
>> assign).
>
> I don't quite understand this part. If you boot with "pci=nobar",
> the BIOS doesn't assign BARs, Linux doesn't either, the drivers
> don't need them -- everything works, and that makes sense so far.
>
> Now, if you boot normally (without "pci=nobar"), what changes?
> The BIOS situation is the same, but Linux tries to assign the
> unassigned BARs. It may assign a few before running out of space,
> but the drivers still don't need those BARs. What breaks?
Nothing really breaks, it's more of a problem that the kernel uses
up the rest of the I/O Space, and starts spitting out warning
messages as it tries to assign the rest of the I/O BARs that the
BIOS didn't assign, something like:
pci 0010:03:00.0: BAR 5: can't allocate I/O resource [0x0-0x7f]
pci 0012:05:00.0: BAR 5: can't allocate I/O resource [0x0-0x7f]
...
And in using up all the I/O space, I think that could prevent a
hotplug attach of a pci device requiring I/O space (although I
believe most BIOSes pad the bridge decoders to support that).
I'm not to familiar with how pci hotplug works on x86 so I may
be wrong in what I just stated.
>
>> This patch will not assign a resource to a device BAR if that BAR was
>> not assigned by the BIOS, and the kernel cmdline option 'pci=nobar'
>> was specified. This patch is closely modeled after the 'pci=norom'
>> option that currently exists in the tree.
>
> Can't we figure out whether we need this ourselves? Using a command-
> line option just guarantees that we'll forever be writing customer
> advisories about this issue.
>
> This issue is not specific to x86, so I don't really like having
> the implementation be x86-specific.
I agree this isn't a x86 specific issue but given the 'norom'
cmdline option is basically doing the same thing (but for pci
Expansion ROM BARs) this code was modeled after it.
>
> Do we know anything about how other OSes handle this case of I/O
> space exhaustion?
>
> I'm a little bit nervous about Linux's current strategy of assigning
> resources to things before we even know whether we're going to use
> them. We don't support dynamic PCI resource reassignment, so maybe
> we don't have any choice in this case, but generally I prefer the
> lazy approach.
>
> Bjorn
>
>> Signed-off-by: Mike Habeck <habeck@....com>
>> Signed-off-by: Mike Travis <travis@....com>
>> ---
>> Documentation/kernel-parameters.txt | 2 ++
>> arch/x86/include/asm/pci_x86.h | 1 +
>> arch/x86/pci/common.c | 20 ++++++++++++++++++++
>> 3 files changed, 23 insertions(+)
>>
>> --- linux.orig/Documentation/kernel-parameters.txt
>> +++ linux/Documentation/kernel-parameters.txt
>> @@ -1935,6 +1935,8 @@ and is between 256 and 4096 characters.
>> norom [X86] Do not assign address space to
>> expansion ROMs that do not already have
>> BIOS assigned address ranges.
>> + nobar [X86] Do not assign address space to the
>> + BARs that weren't assigned by the BIOS.
>> irqmask=0xMMMM [X86] Set a bit mask of IRQs allowed to be
>> assigned automatically to PCI devices. You can
>> make the kernel exclude IRQs of your ISA cards
>> --- linux.orig/arch/x86/include/asm/pci_x86.h
>> +++ linux/arch/x86/include/asm/pci_x86.h
>> @@ -30,6 +30,7 @@
>> #define PCI_HAS_IO_ECS 0x40000
>> #define PCI_NOASSIGN_ROMS 0x80000
>> #define PCI_ROOT_NO_CRS 0x100000
>> +#define PCI_NOASSIGN_BARS 0x200000
>>
>> extern unsigned int pci_probe;
>> extern unsigned long pirq_table_addr;
>> --- linux.orig/arch/x86/pci/common.c
>> +++ linux/arch/x86/pci/common.c
>> @@ -125,6 +125,23 @@ void __init dmi_check_skip_isa_align(voi
>> static void __devinit pcibios_fixup_device_resources(struct pci_dev *dev)
>> {
>> struct resource *rom_r = &dev->resource[PCI_ROM_RESOURCE];
>> + struct resource *bar_r;
>> + int bar;
>> +
>> + if (pci_probe & PCI_NOASSIGN_BARS) {
>> + /*
>> + * If the BIOS did not assign the BAR, zero out the
>> + * resource so the kernel doesn't attmept to assign
>> + * it later on in pci_assign_unassigned_resources
>> + */
>> + for (bar = 0; bar <= PCI_STD_RESOURCE_END; bar++) {
>> + bar_r = &dev->resource[bar];
>> + if (bar_r->start == 0 && bar_r->end != 0) {
>> + bar_r->flags = 0;
>> + bar_r->end = 0;
>> + }
>> + }
>> + }
>>
>> if (pci_probe & PCI_NOASSIGN_ROMS) {
>> if (rom_r->parent)
>> @@ -509,6 +526,9 @@ char * __devinit pcibios_setup(char *st
>> } else if (!strcmp(str, "norom")) {
>> pci_probe |= PCI_NOASSIGN_ROMS;
>> return NULL;
>> + } else if (!strcmp(str, "nobar")) {
>> + pci_probe |= PCI_NOASSIGN_BARS;
>> + return NULL;
>> } else if (!strcmp(str, "assign-busses")) {
>> pci_probe |= PCI_ASSIGN_ALL_BUSSES;
>> return NULL;
>>
--
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