linux-kernel - Re: [PATCH V5 3/4] x86/PCI: Stop enabling ECS for AMD CPUs after Fam16h

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Message-ID: <20140523150518.GW27560@rric.localhost>
Date:	Fri, 23 May 2014 17:05:18 +0200
From:	Robert Richter <rric@...nel.org>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Suravee Suthikulanit <suravee.suthikulpanit@....com>,
	Borislav Petkov <bp@...en8.de>,
	Daniel J Blueman <daniel@...ascale.com>,
	Andreas Herrmann <herrmann.der.user@...glemail.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Aravind Gopalakrishnan <Aravind.Gopalakrishnan@....com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Borislav Petkov <bp@...e.de>,
	Myron Stowe <myron.stowe@...hat.com>
Subject: Re: [PATCH V5 3/4] x86/PCI: Stop enabling ECS for AMD CPUs after
 Fam16h

On 23.05.14 07:01:41, Bjorn Helgaas wrote:
> [I guess I've been using the wrong term here.  I think "ECS" just
> refers to the extended config space itself, and I should have been
> saying "IO ECS" or "EnableCf8ExtCfg".]
> 
> My understanding was that if we don't enable IO ECS and we don't have
> MCFG, we will not be able to access extended config space.  The system
> can certainly boot without extended config space, but some drivers may
> not work correctly, so it would be a regression from the user point of
> view.

No, I got you right. If we disable IO ECS there is no fallback if MCFG
fails... and this may cause a regression then.

I might be completely wrong here, but as I remember IO ECS only
affects access to cpu devices (bus 0, slot 0x18-0x1f). Thus, esp. PCIe
extended config space access works since a different host controller
handles this. So only cpu devices would see a regression and thus cpu
bringup code. I don't think ECS (either MCFG or IO ECS) is needed
anymore for cpu bringup (this is true for IBS, but I don't know of
other cpu features requiring ECS, though family 16h might introduced
new ones).

IMHO device drivers are not affected and wont get broken.

-Robert
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