Why does BIOS assign memory to 16 byte BAR

Bjorn Helgaas
2016-07-22T15:51:33Z

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.

Bjorn