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

[Bjorn Helgaas]

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@xxxxxxx>
> 
> 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?

> 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.

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@xxxxxxx>
> Signed-off-by: Mike Travis <travis@xxxxxxx>
> ---
>  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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