linux-kernel - Re: [PATCH v6 10/30] PCI: Introduce pci_host_bridge

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Date:	Thu, 12 Mar 2015 14:56:51 -0500
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Yijing Wang <wangyijing@...wei.com>
Cc:	Jiang Liu <jiang.liu@...ux.intel.com>, linux-pci@...r.kernel.org,
	Yinghai Lu <yinghai@...nel.org>, linux-kernel@...r.kernel.org,
	Marc Zyngier <marc.zyngier@....com>,
	linux-arm-kernel@...ts.infradead.org,
	Russell King <linux@....linux.org.uk>, x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Tony Luck <tony.luck@...el.com>, linux-ia64@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Guan Xuetao <gxt@...c.pku.edu.cn>, linux-alpha@...r.kernel.org,
	linux-m68k@...ts.linux-m68k.org, Liviu Dudau <liviu@...au.co.uk>,
	Arnd Bergmann <arnd@...db.de>,
	Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH v6 10/30] PCI: Introduce pci_host_bridge_list to manage
 host bridges

On Thu, Mar 12, 2015 at 09:03:12PM +0800, Yijing Wang wrote:
> On 2015/3/12 10:55, Bjorn Helgaas wrote:
> > On Mon, Mar 09, 2015 at 10:34:07AM +0800, Yijing Wang wrote:
> >> Introduce pci_host_bridge_list to manage pci host
> >> bridges in system, so we could detect whether
> >> the host in domain:bus is alreay registered.
> >> Then we could remove bus alreay exist test in
> >> __pci_create_root_bus().
> > 
> > It's a nice idea to move this test into the core.  While you're at it, why
> > don't you check for any overlap with the bus ranges of existing host
> > bridges?  For example, if we're trying to create a new host bridge to
> > [bus 40-7f], it should conflict with existing bridges to [bus 00-7f]
> > as well as to [bus 40-ff].  I think your current patch will detect the
> > latter conflict but not the former.
> 
> Now pci host bridge may only know its start bus number, like acpi _BBN provided,
> but does not limit the end bus number, Eg. two pci roots report _BBN 0x0 and 0x80,
> so we have two bus number resource (0, 0xff) and (0x80, 0xff), if we check it strictly,
> some pci scan would fail which currently scan success.

_BBN is not the correct source for the bridge's bus number range.  There's
a comment in acpi_pci_root_add() that explains why:

  * We need both the start and end of the downstream bus range
  * to interpret _CBA (MMCONFIG base address), so it really is
  * supposed to be in _CRS.  If we don't find it there, all we
  * can do is assume [_BBN-0xFF] or [0-0xFF].

A platform SHOULD know the start and and end bus number.  If it doesn't I
think it's the platform's responsibility to carve up the bus number range.
Maybe this can be done by trimming the range of the [bus 00-ff] bridge when
we discover another bridge that leads to bus 80.
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