Re: [Patch 1/1] x86 pci: Add option to not assign BAR's if not already assigned

[Mike Travis <travis@sgi.com>]

> 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;
>>>