linux-kernel - Re: Why does BIOS assign memory to 16 byte BAR

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Date:	Fri, 22 Jul 2016 10:51:33 -0500
From:	Bjorn Helgaas <helgaas@...nel.org>
To:	Bharat Kumar Gogada <bharat.kumar.gogada@...inx.com>
Cc:	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Arnd Bergmann <arnd@...db.de>, nofooter <nofooter@...inx.com>,
	nofooter2 <nofooter2@...inx.com>
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.

Bjorn