linux-kernel - Re: [PATCHv8 0/5] Driver for new "VMD" device

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Date:	Fri, 15 Jan 2016 11:31:03 -0800
From:	"Veal, Bryan E." <bryan.e.veal@...el.com>
To:	Bjorn Helgaas <helgaas@...nel.org>
Cc:	Keith Busch <keith.busch@...el.com>,
	LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
	linux-pci@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Dan Williams <dan.j.williams@...el.com>,
	Jon Derrick <jonathan.derrick@...el.com>
Subject: Re: [PATCHv8 0/5] Driver for new "VMD" device

On Fri, Jan 15, 2016 at 12:19:38PM -0600, Bjorn Helgaas wrote:
> I also have a more substantive question about the flags setup.  I
> think you should not clear IORESOURCE_MEM_64.  The intent of
> IORESOURCE_MEM_64 is to describe the *capability* of a BAR, not its
> contents.  But I assume you cleared it for a reason.  vmd->resources[n]
> are not BARs, so the PCI core won't assign resources to them like it
> does for BAR, so we shouldn't care about IORESOURCE_MEM_64 for that
> reason.  Is there some other reason IORESOURCE_MEM_64 makes a
> difference there?

Hi Bjorn & Keith:

I did this to fix an issue in pre-RFC code.

The flag is subtly restrictive in one specific scenario: spec-compliant
PCIe ports lack the ability to specify a 64-bit, non-prefetchable range.
IORESOURCE_MEM_64 directs the PCI subsystem to put the address into the
64-bit *prefetchable* range. Below the port, the "prefetchable" propoerty
*is* restrictive: the addresses can't be used for non-prefetchable BARs.

Thus, in the specific case where a 64-bit non-prefetchable VMD bar happens
to contain a 32-bit address, removing the IORESOURCE_MEM_64 flag allows
the address resource to be used for *any* non-prefetchable BARs (32-bit or
64-bit) downstream.