linux-kernel - Re: [PATCH] PCI: Clear bridge MEM_64 flag if one child does not support it

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Date:	Tue, 9 Dec 2014 11:21:11 -0700
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Wei Yang <weiyang@...ux.vnet.ibm.com>
Cc:	Yinghai Lu <yinghai@...nel.org>,
	Marek Kordík <kordikmarek@...il.com>,
	Alexey Voronkov <zermond@...il.com>,
	Gavin Shan <gwshan@...ux.vnet.ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PCI: Clear bridge MEM_64 flag if one child does not
 support it

On Tue, Dec 9, 2014 at 12:56 AM, Wei Yang <weiyang@...ux.vnet.ibm.com> wrote:

> As you mentioned in another thread, "5b28541552ef is taking the wrong
> approach". (http://www.spinics.net/lists/linux-pci/msg37374.html) Maybe I
> don't catch it clearly. Put a 32bit prefetchable resource in a 32bit
> non-prefetchable bridge window is a bad idea?

A 32-bit prefetchable resource *can* be put in a 32-bit
non-prefetchable window, but the device won't perform as well as it
would if the resource were in a prefetchable window.

What I object to is the fact that we put a 32-bit prefetchable
resource in the non-prefetchable window and leave the 64-bit
prefetchable window unused.  This gives up performance for no benefit.

> But in my mind, if the bridge
> prefetchable window is 64bit, we can't put a 32bit prefetchable resource in
> it.

If the window is programmed to be above 4GB, of course we can't put a
32-bit resource in it.  My point is that if the bridge *supports* a
64-bit prefetchable window, we can decide where to place it.  If we
put the window below 4GB, we can put a 32-bit prefetchable resource in
it.

I think maybe you're thinking of "64-bit window" as "a window
programmed to be above 4GB."  I'm using "64-bit window" to mean "a
window that supports 64-bit addressing," i.e., one where
PCI_PREF_BASE_UPPER32 and PCI_PREF_LIMIT_UPPER32 are implemented.
That's analogous to the way we talk about 64-bit BARs.  A 64-bit BAR
is still a 64-bit BAR even if it is currently programmed to be below
4GB.

Bjorn
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