linux-kernel - Re: [Patch 1/1] x86 pci: Add option to not assign BAR's if not already assigned

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Message-ID: <4C0FA3F0.6040609@sgi.com>
Date:	Wed, 09 Jun 2010 09:23:44 -0500
From:	Mike Habeck <habeck@....com>
To:	Jesse Barnes <jbarnes@...tuousgeek.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Bjorn Helgaas <bjorn.helgaas@...com>,
	Mike Travis <travis@....com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
	Jacob Pan <jacob.jun.pan@...el.com>, Tejun Heo <tj@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>, linux-pci@...r.kernel.org
Subject: Re: [Patch 1/1] x86 pci: Add option to not assign BAR's if not already
 assigned

Jesse Barnes wrote:
> On Tue, 08 Jun 2010 17:53:19 -0700
> "H. Peter Anvin" <hpa@...or.com> wrote:
> 
>> On 06/02/2010 08:53 AM, Jesse Barnes wrote:
>>>> That's what I thought, which I guess means my original question to Mike 
>>>> still stands...
>>> I thought the whole reason for this was hotplug; we don't want to
>>> exhaust I/O space unnecessarily by allocating resources for BARs the
>>> BIOS didn't assign so we can keep them around for later hotplug
>>> activity.
>>>
>>> If there's some other issue, it's not too late to drop this patch.
>>>
>> Okay, now... this means that if a device that the BIOS doesn't know
>> about, but which needs I/O addresses, then it will work if hotplugged,
>> but not if it is plugged in on system boot?
> 
> Depends on the BIOS interactions on this platform; if the kernel ends
> up doing all the allocations itself, we'll allocate space for every BAR
> unconditionally, meaning that any hotplugged device should work.

Correct, our BIOS allocates I/O space for all devices except for a
few that it knows don't use it.  On a hotplug attach the kernel
will be unconditionally allocating the I/O space for all devices..
The pci=nobar option strictly prevents the kernel from allocating
BAR resources to device BARs that the BIOS didn't assign (similar
to how the pci=norom option works for the device's ROM BAR)

> 
> But really the SGI guys should comment here.
> 

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