Re: [PATCH 2/2] acpi based pci gap caluculation v2

[Alok kataria]

Hi Yakui,

On Tue, Jun 24, 2008 at 10:39 PM, Zhao Yakui <yakui.zhao@xxxxxxxxx> 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@xxxxxxxxx> 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 ?

Thanks,
Alok
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