Why does BIOS assign memory to 16 byte BAR

[Bharat Kumar Gogada]

> Subject: RE: Why does BIOS assign memory to 16 byte BAR
> 
> > Subject: RE: Why does BIOS assign memory to 16 byte BAR
> >
> > > Subject: Re: Why does BIOS assign memory to 16 byte BAR
> > >
> > > On Fri, Jul 22, 2016 at 10:15:46AM -0500, Bjorn Helgaas wrote:
> > > > Hi Bharat,
> > > >
> > > > On Fri, Jul 22, 2016 at 09:24:22AM +0000, Bharat Kumar Gogada wrote:
> > > > > Hi,
> > > > >
> > > > > I'm observing that on x86 BIOS successfully assigns memory if an End
> > > > > point requests BAR of size 16byte.
> > > > >
> > > > > But as per Spec:
> > > > > The minimum memory address range requested by a BAR is 128
> bytes.
> > > >
> > > > Can you provide the spec reference for this?  I don't see it in PCI
> > > > r3.0.
> > > >
> > > > PCI r3.0, sec 6.2.5.1, shows bits 4-31 of a memory BAR as writable,
> > > > which would correspond to a minimum size of 16 bytes.
> > >
> > > The reference above is to the conventional PCI spec.  I happened to trip
> > over
> > > a note in PCIe r3.0, sec 1.3.2.2, that for a PCI Express endpoint, "the
> > minimum
> > > memory address range requested by a BAR is 128 bytes."
> > >
> > > I don't think linux currently enforces this minimum.
> > >
> >
> > Hi Bjorn Thanks for the reply.
> >
> > Here is what the issue we are seeing.
> >
> > We have total memory for BAR's on our SoC of 256 MB.
> > When an End Point request individually 16 byte BAR's our root port assigns
> > memory to BAR's successfully.
> >
> > But if I have an End point which has 4 BAR's each 32 bit and request as
> > following:
> > When 1st BAR requests 1GB BAR it fails due to lack of memory. (We are
> > running this as part of SIG compliance test case)
> > 2nd BAR requests 1MB and other 2 BAR's request 16byte, but these are not
> > getting BAR's assigned. (Even though BAR space is available, since 1GB
> failed,
> > We have 256 MB still)
> >
> > We have only one End point connected to our root port.
> >
> > Here is the log:
> > [    2.319289] nwl-pcie fd0e0000.pcie: Link is UP
> > [    2.319332] PCI host bridge /amba/pcie at fd0e0000 ranges:
> > [    2.319349]   No bus range found for /amba/pcie at fd0e0000, using [bus
> 00-
> > ff]
> > [    2.319374]    IO 0xe0000000..0xe000ffff -> 0x00000000
> > [    2.319415]   MEM 0xe0100000..0xefffffff -> 0xe0100000
> > [    2.319431]   MEM 0x600000000..0x7ffffffff -> 0x600000000
> > [    2.319539] nwl-pcie fd0e0000.pcie: PCI host bridge to bus 0000:00
> > [    2.319557] pci_bus 0000:00: root bus resource [bus 00-ff]
> > [    2.319573] pci_bus 0000:00: root bus resource [io  0x0000-0xffff]
> > [    2.319589] pci_bus 0000:00: root bus resource [mem 0xe0100000-
> 0xefffffff]
> > [    2.319606] pci_bus 0000:00: root bus resource [mem 0x600000000-
> > 0x7ffffffff pref]
> > [    2.319845] pci 0000:00:00.0: cannot attach to SMMU, is it on the same
> bus?
> > [    2.319861] iommu: Adding device 0000:00:00.0 to group 1
> > [    2.320243] pci 0000:01:00.0: cannot attach to SMMU, is it on the same
> bus?
> > [    2.320258] iommu: Adding device 0000:01:00.0 to group 1
> > [    2.320313] pci 0000:00:00.0: BAR 8: no space for [mem size 0x60000000]
> > [    2.320331] pci 0000:00:00.0: BAR 8: failed to assign [mem size 0x60000000]
> > [    2.320349] pci 0000:00:00.0: BAR 6: assigned [mem 0xe0100000-
> 0xe01007ff
> > pref]
> > [    2.320374] pci 0000:01:00.0: BAR 0: no space for [mem size 0x40000000]
> > [    2.320390] pci 0000:01:00.0: BAR 0: failed to assign [mem size 0x40000000]
> > [    2.320407] pci 0000:01:00.0: BAR 4: no space for [mem size 0x00100000
> > 64bit]
> > [    2.320423] pci 0000:01:00.0: BAR 4: failed to assign [mem size 0x00100000
> > 64bit]
> > [    2.320446] pci 0000:01:00.0: BAR 2: no space for [mem size 0x00000010]
> > [    2.320461] pci 0000:01:00.0: BAR 2: failed to assign [mem size 0x00000010]
> > [    2.320477] pci 0000:01:00.0: BAR 3: no space for [mem size 0x00000010]
> > [    2.320493] pci 0000:01:00.0: BAR 3: failed to assign [mem size 0x00000010]
> > [    2.320509] pci 0000:00:00.0: PCI bridge to [bus 01-0c]
> >
> > Please let me know, what might might be the issue.
> >
> 
> Adding to the above will kernel allocate other memory BARs to an End Point
> if
> one BAR assignment fails ?
> 
> What if the End Point has multiple function and say first function BAR
> assignment failed,
> will the kernel assign BAR's to second function on same bus and device ?
> 
After debugging in function pci_bus_alloc_from_region we have two resources,
One which is taken as parameter (let's say resA), other is from  pci_bus_for_each_resource (let's say resB).

The resA contains different start address from resB, I see that resB is obtained from device resource 
from bus->resource, but im unable to understand how resA (start address & size) gets its data. 

Can any one help me understand this so that I might know reason why 16byte BAR allocation failing after
1GB request.

Regards,
Bharat