Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1757580Ab3GER3G (ORCPT <rfc822;w@1wt.eu>);
	Fri, 5 Jul 2013 13:29:06 -0400
Received: from mail-oa0-f41.google.com ([209.85.219.41]:56079 "EHLO
	mail-oa0-f41.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751579Ab3GER3B (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 5 Jul 2013 13:29:01 -0400
MIME-Version: 1.0
In-Reply-To: <20130704045316.GC3244@hli22-desktop>
References: <1372864578-6925-1-git-send-email-haicheng.li@linux.intel.com>
 <CAErSpo5dR_2tUDFoFO49N3Ut569+6Cfosx8Q=91U_w-8mvFVkg@mail.gmail.com> <20130704045316.GC3244@hli22-desktop>
From: Bjorn Helgaas <bhelgaas@google.com>
Date: Fri, 5 Jul 2013 11:28:40 -0600
Message-ID: <CAErSpo73pzpK1-401m9+EOiV_t13KY_DQa9WDU+5Gq+LidGpYg@mail.gmail.com>
Subject: Re: [PATCH 1/3] PCI: Add hide_device support to pci subsystem.
To: Haicheng Li <haicheng.li@linux.intel.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Haicheng Li <haicheng.lee@gmail.com>,
        "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>
Content-Type: text/plain; charset=ISO-8859-1
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 3070
Lines: 58

On Wed, Jul 3, 2013 at 10:53 PM, Haicheng Li
<haicheng.li@linux.intel.com> wrote:
> On Wed, Jul 03, 2013 at 10:09:32AM -0600, Bjorn Helgaas wrote:
>> On Wed, Jul 3, 2013 at 9:16 AM, Haicheng Li <haicheng.li@linux.intel.com> wrote:
>> > With more and more SOCs having pci device integrated into chip (e.g. Intel
>> > Atom series), it's useful to add an interface to cleanly hide pci devices from
>> > pci device scanning, which is because:
>> >
>> > 1. phone or tablet OEMs may choose disabling some pci device in the SOC,
>> >    such as camera ISP in Intel Atom Z2580 chip, and etc.
>> > 2. if such disabled devices are not cleanly removed from pci device tree,
>> >    then pci-core will still try to operate on the relative device control
>> >    registers while S3 suspend and resume.
>> > 3. so hiding such devices from early begining will not only reduce the kernel
>> >    boot time, but also optimize the latency of system suspend and resume.
>>
>> Normally the chip provides a way to disable devices by writing a
>> configuration register.  Then the device doesn't respond when Linux
>> enumerates devices, so nothing special is required in the kernel.
> Agreed, this is true.
>
>> What's different about the Z2580?  I'd be surprised if Intel forgot to
>> include such a register.
> A pci shim faked by firmware was introduced to help easily port Linux onto Z2XXX
> SOC chips, which enumerates both real and fake PCI devices inside the SOC (The camera ISP
> I mentioned above is a real PCI device in this case)
>
> A detailed tech talk about this technology by Jacob Pan in elc2010 can be found online
> here: http://elinux.org/images/e/ee/Jacob-Pan-x86MID-elc2010.pdf
>
>> Maybe the firmware just isn't smart enough
>> to disable the device?  If so, it would be better to fix the firmware
>> than to add kludges in the kernel.
> On PC or server, end-user/OEM can disable/hide a pci device easily thru BIOS setting or
> by hacking BIOS code directly when they find some device is broken or useless.
>
> However on phone or tablet equipment, there is no BIOS-setting alike UI exposed to end-user/developer
> to disable broken device easily (and physically removing the device is not doable on Phone or tablet)
>
> OTOH, this i/f is really *helpful* for kernel developer to power-on a new platform, debug system
> problem, or to do performance tuning of suspend/resume. At least it makes my daily job easier:).
>
> So if people are strongly against this, I would still suggest accept this i/f as a debug i/f
> at least:).
>
>> > To hide pci devices, just pass such parameters to kernel at boot stage:
>> >         pci=hide=[<domain>:]<bus>:<slot>.<func>[; ...]

This doesn't seem generally useful to me.  It might be useful as a
development aid, and you can easily maintain it as an out-of-tree
patch for that.

Bjorn
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/