x86 pci: Add option to not assign BAR's if not already assigned
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From: H. Peter Anvin on 13 May 2010 15:10 On 05/13/2010 11:56 AM, Bjorn Helgaas wrote: > > 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. > Lazy has its own pitfalls. In particular, the issue here is about allocation of bridge devices, which may cause all kinds of issues with bridges further up. Consider: A / \ B C | | D E .... where A, B, and C are bridges, and D and E are devices. Now the driver initializes D, and requests address space. As a result, the OS configures bridge B to have a 4K window, and accordingly the same for bridge A. Now you want to initialize device E. This means the window for bridge A has to be widened, because bridge C is going to need its own 4K window. If the linearly consecutive address space is not available, it now means reconfiguring bridge B and device D. 4K granularity really hurts. It made sense when bridges were relatively rare, but in PCI-express world that is no longer the case... -hpa -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Mike Travis on 13 May 2010 15:20 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(a)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(a)sgi.com> >> Signed-off-by: Mike Travis <travis(a)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; >> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Mike Habeck on 13 May 2010 15:40 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(a)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? 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(a)sgi.com> >> Signed-off-by: Mike Travis <travis(a)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; >> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: H. Peter Anvin on 13 May 2010 16:20 On 05/13/2010 01:02 PM, Bjorn Helgaas wrote: > > Yep, that's definitely a problem, and I don't have a good solution. > > HP (and probably SGI) had a nice hardware solution for ia64 -- > address translation across the host bridge, so each bridge could > have its own 64K I/O space. But I don't see that coming in the > x86 PC arena. > I would agree, that is unlikely (not that I have any information on this subject, mind you.) Much more likely would be a bridge extension capability for finer-grained I/O space routing. -hpa -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Mike Habeck on 13 May 2010 16:30
Bjorn Helgaas wrote: > On Thursday, May 13, 2010 01:12:21 pm 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(a)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. > > It sounds like your BIOS treats some devices specially, so I assumed > it would leave the first sixteen devices unassigned, but would assign > the last eight, including the bridge windows leading to them. In that > case, I would expect Linux to preserve the resources of the last > eight devices, since they're already assigned, and assign anything > left over to the first sixteen. Correct, the BIOS special cases devices that have an I/O BAR but aren't used because the same functionality that the I/O BAR provides is available via the MEM BAR. So if the device driver only uses the MEM BAR then the BIOS will only assign resources to it, not to the unused I/O BAR And yes, linux would preserve the resources of the last eight devices in your example above > > Are you saying that Linux clobbers the resources of the last eight > devices in the process of assigning the first sixteen? If so, I'd > say that's a Linux bug. No, Linux does not clobbers the resources > > Or are the last eight hot-added cards that the BIOS never had a > chance to assign? That's definitely a problem. Yes, that is one of the problems we envision seeing. > >>> 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? > > It feels specific now, but in five years, I bet it won't be so > unusual. I really don't want to force customers to figure out > when they need this. > >>> 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. > > It's of interest to me. I spend a lot of time pulling generic > out of architecture-specific places. If there's stuff that we > know is generic from the beginning, we shouldn't make work for > ourselves by making it x86-specific. > >>> 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. > > Like I said, we maybe don't have a choice in this case, but I'd like > to have a clearer understanding of the problem and how other OSes > deal with it before we start applying band-aids that will hurt when > we pull them off later. > > Bjorn -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |