linux-kernel - Re: [PATCH 1/5] PCI: Don't check resource_size() in pci_bus_alloc

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Date:	Tue, 11 Mar 2014 16:02:32 -0600
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Paul Bolle <pebolle@...cali.nl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/5] PCI: Don't check resource_size() in pci_bus_alloc_resource()

On Tue, Mar 11, 2014 at 3:39 PM, Yinghai Lu <yinghai@...nel.org> wrote:
> On Tue, Mar 11, 2014 at 10:42 AM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
>> Paul reported that after f75b99d5a77d ("PCI: Enforce bus address limits in
>> resource allocation") on a 32-bit kernel (CONFIG_PHYS_ADDR_T_64BIT not
>> set), intel-gtt complained "can't ioremap flush page - no chipset
>> flushing".  In addition, other PCI resource allocations, e.g., for bridge
>> windows, failed.
>>
>> This happens because we incorrectly skip bus resources of
>> [mem 0x00000000-0xffffffff] because we think they are of size zero.
>> When resource_size_t is 32 bits wide, resource_size() on
>> [mem 0x00000000-0xffffffff] returns 0 because (r->end - r->start + 1)
>> overflows.
>>
>> Therefore, we can't use "resource_size() == 0" to decide that allocation
>> from this resource will fail.  allocate_resource() should fail anyway if it
>> can't satisfy the address constraints, so we should just depend on that.
>>
>> A [mem 0x00000000-0xffffffff] bus resource is obviously not really valid,
>> but we do fall back to it as a default when we don't have information about
>> host bridge apertures.
>>
>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=71611
>> Fixes: f75b99d5a77d PCI: Enforce bus address limits in resource allocation
>> Reported-and-tested-by: Paul Bolle <pebolle@...cali.nl>
>> Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
>> ---
>>  drivers/pci/bus.c |    2 --
>>  1 file changed, 2 deletions(-)
>>
>> diff --git a/drivers/pci/bus.c b/drivers/pci/bus.c
>> index 00660cc502c5..38901665c770 100644
>> --- a/drivers/pci/bus.c
>> +++ b/drivers/pci/bus.c
>> @@ -162,8 +162,6 @@ static int pci_bus_alloc_from_region(struct pci_bus *bus, struct resource *res,
>>
>>                 avail = *r;
>>                 pci_clip_resource_to_region(bus, &avail, region);
>> -               if (!resource_size(&avail))
>> -                       continue;
>>
>>                 /*
>>                  * "min" is typically PCIBIOS_MIN_IO or PCIBIOS_MIN_MEM to
>>
>
> how about replacing that with:
>
>         if (avail.start > avail.end)
>                   continue;
>
> so we do not need to go deep into allocate_resource()

We could, but why bother?  This isn't a performance path, so there's
no need to add more code to optimize it.  Adding code means there's
more for human readers to look at and understand, so I prefer not to
add it unless it's needed.

Bjorn
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