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SubjectRe: [Patch 1/1] x86 pci: Add option to not assign BAR's if not already assigned
> Oops, I added Yinghai to the CC: list, but I forgot to add
> linux-pci@vger.kernel.org. Please add that on any future replies.

[added.]

Mike Travis wrote:
>
>
> 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@sgi.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?
>
> The problem arises because we run out of address spaces to assign.
>
> Say you have 24 cards, and the 1st 16 do not use I/O BARs. If
> you assign the available 16 address spaces to cards that may not
> need them, then the final 8 cards will not be available.
>
> This avoids this problem by not wasting I/O address spaces when
> they are not going to be used.
>
>>
>>> 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.
>
> I think since this is so specific (like the potential of having
> more than 16 cards would be something the customer would know),
> I think it's better to error on the safe side. If a BIOS does
> not recognize an add in card (for whatever reason), and does
> not assign the I/O BAR, then it would be up to the kernel to
> do that. Wouldn't you get more customer complaints about non-working
> I/O, than someone with > 16 PCI cards not being able to use them
> all?
>
>>
>> This issue is not specific to x86, so I don't really like having
>> the implementation be x86-specific.
>
> We were going for as light a touch as possible, as there is not
> time to verify other arches. I'd be glad to submit a follow on
> patch dealing with the generic case and depend on others for
> testing, if that's of interest.
>
> Note we also modeled the option to be identical in operation to
> the pci=norom option, which is a similar x86 specific function.
>
>>
>> Do we know anything about how other OSes handle this case of I/O
>> space exhaustion?
>
> 16+ PCI devices is a fairly large amount. Are there any other PC's
> that handle this much I/O?
>>
>> 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.
>
> That's a great idea if it can work. Unfortunately, we are all tied
> to the way BIOS sets up the system, and for UV systems I don't think
> dynamic provisioning would work. There's too much infrastructure
> that all has to cooperate by the time the system is fully functional.
>
>>
>> Bjorn
>
> Thanks for the feedback.
>
> Mike
>
>>
>>> Signed-off-by: Mike Habeck <habeck@sgi.com>
>>> Signed-off-by: Mike Travis <travis@sgi.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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