Subject: Re: [PATCH 2/2] acpi based pci gap caluculation v2
From: Zhao Yakui <yakui.zhao@intel.com>
To: Alok kataria <alokkataria1@gmail.com>
Cc: akataria@vmware.com, lenb@kernel.org, Ingo Molnar <mingo@elte.hu>,
       linux-acpi <linux-acpi@vger.kernel.org>,
       LKML <linux-kernel@vger.kernel.org>
In-Reply-To: <35f686220806242304q43987059xb203dd1ac75b7583@mail.gmail.com>
References: <1214333326.27577.28.camel@promb-2n-dhcp368.eng.vmware.com>
	 <1214362159.9800.28.camel@yakui_zhao.sh.intel.com>
	 <35f686220806242117p4a442982hb459a6b76312f391@mail.gmail.com>
	 <1214372365.9800.42.camel@yakui_zhao.sh.intel.com>
	 <35f686220806242304q43987059xb203dd1ac75b7583@mail.gmail.com>
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Date: Wed, 25 Jun 2008 16:38:15 +0800
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On Tue, 2008-06-24 at 23:04 -0700, Alok kataria wrote:
> Hi Yakui,
> 
> On Tue, Jun 24, 2008 at 10:39 PM, Zhao Yakui <yakui.zhao@intel.com> wrote:
> > On Tue, 2008-06-24 at 21:17 -0700, Alok kataria wrote:
> >> On Tue, Jun 24, 2008 at 7:49 PM, Zhao Yakui <yakui.zhao@intel.com> wrote:
> >> > On Tue, 2008-06-24 at 11:48 -0700, Alok Kataria wrote:
> >> >> Evaluates the _CRS object under PCI0 looking for producer resources.
> >> >> Then searches the e820 memory space for a gap within these producer resources.
> >> >>
> >> >> Allows us to find a gap for the unclaimed pci resources or MMIO resources
> >> >> for hotplug devices within the BIOS allowed pci regions.
> >> >>
> >> > It seems reasonable.
> >> > But if the resource obtained from the PCI0 _CRS method is incorrect, we
> >> > will get the incorrect pci_mem_start.
> >>
> >> Hi Yakui,
> >>
> >> What do you mean by the PCI0 _CRS being incorrect ? Why would the BIOS
> >> give a incorrect _CRS object ?
> >> Also we don't just take the value given from the _CRS method, we still
> >> read the e820_map to search for an unallocated resource. So even if
> >> (by chance) the _CRS method returns incorrect value we would still
> >> figure out if there is a collision with an already allocated resource.
> > In the patch the address obtained from the _CRS object will be passed
> > into the function of e820_search_gap. In such case maybe we will get the
> > pci_mem_start different with the e820_setup_gap.
> 
> True..the whole idea behind doing this patch was to get a correct
> (different) value for pci_mem_start.
> We read the _CRS object over here to make sure that we assign the
> pci_mem_start from the address range which is reserved by the BIOS for
> PCI devices.
> 
> Also this reading of _CRS object would be done before we start
> initializing the pci devices, i.e. before we start using the value of
> pci_mem_start, so the original value assigned by pci_setup_gap is just
> overwritten by this function. So that should be fine IMHO.
> Also we would still want the call for e820_setup_gap because there can
> be systems with no acpi support or acpi disabled
> 
> 
> >
> >> >
> >> > At the same time after the patch is applied, pci_mem_start will be
> >> > parsed in two different ways.
> >>
> >> Yes pci_mem_start would be initialized in 2 different ways but we
> >> still have to parse the e820_map the old way because there could be
> >> systems without ACPI.
> >>
> >> > If the result is different, maybe the
> >> > incorrect pci_mem_start will be used.
> >>
> >> Yeah, The result is different in my case. Though my BIOS reserves
> >> hotpluggable memory region, kernel doesn't respect that right now and
> >> just parses the e820_map to calculate the gap and pci_mem_start value.
> >> I have explained the problem in this mail.
> >>
> >> http://marc.info/?l=linux-acpi&m=121391675711763&w=2
> >>
> >> Maybe nobody has seen this problem yet, because there are no systems
> >> out there with less than 4GB memory to start with and which allow
> >> memory hotplug.
> >>
> >> But still i don't understand what do you mean by, we can get incorrect
> >> pci_mem_start, in which case ?
> >
> > In the function of setup_arch the pci_mem_start will be parsed by
> > searching the e820 table.  After the patch is applied, we will parse the
> > pci_mem_start again in the function of pci_acpi_scan_init and it will
> > override the value parsed in the function of setup_arch. If the
> > pci_mem_start is incorrect in the second case, maybe it will have side
> > effect.
> 
> Yes it will override. But how can the value be incorrect in the second
> case. As explained in my previous mail we still parse the e820_map to
> check if we have unclaimed resources between start_address (that read
> from  _CRS) to 2^32.  So even if this start_address is wrong we would
> catch that during parsing e820_map. But again why would the _CRS
> return incorrect values, are you talking about errors in BIOS ?
The pci_mem_start is still gotten by parsing the E820 table.But the
input parameter start_addr will be used in the function of
e820_search_gap.
  If the following is the resource start address returned by the PCI0
_CRS object , maybe the different pci_mem_start will be gotten.
    0xE0000000
    0xE4000000

  At the same time if several start address is returned by the _CRS
object, the e820 table will be parsed several times. 

Thanks.
  Yakui

> Thanks,
> Alok

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