Subject: Re: [PATCH kernel] powerpc/powernv/ioda2: Add devices only from buses which belong to PE

From: Gavin Shan <[email-masked]>
Date: Mon, 15 Jun 2015 10:11:41 +1000

On Fri, Jun 12, 2015 at 08:06:43AM -0700, Nishanth Aravamudan wrote:
>On 12.06.2015 [16:47:03 +1000], Gavin Shan wrote:
>> On Fri, Jun 12, 2015 at 04:19:17PM +1000, Alexey Kardashevskiy wrote:
>> >The existing code puts all devices from a root PE to the same IOMMU group.
>> >However it is a possible situation when subordinate buses belong to
>> >separate PEs, in this case devices from these subordinate buses
>> >should be added to lower level PE rather to the root PE.
>> >
>> >This limits pnv_ioda_setup_bus_dma() invocation to only PEs which own
>> >all subordinate buses.
>> >
>> >Suggested-by: Gavin Shan <[email-masked]>
>> >Signed-off-by: Alexey Kardashevskiy <[email-masked]>
>> 
>> Alexey, I think it's good candidate for stable.
>
>Why? This commit message doesn't imply there is a serious bug just a
>"possible" (does that mean theoretical?) situtation and the following:
>

The problem depends on the PCI topology. Below topology, existing on
firestone machine, is one of the cases that wrong IOMMU group is used
for devices behind the upstream port of the PCIe switch: The PE for
Bus#2 has same IOMMU group as that one of the PE for "Root Bus" wrongly
here.

                        |
                        +---------------+       (Root Bus)
                        |               |
                  [ Root Port ]    [ System Peripheral ]
                        +
                        |			(Bus#1)
                        +
                  [ Up Port ]
			+
			|
           +------------+-----------+           (Bus#2)
           |                        |
     [ Dn port]                [ Dn Port]

>> >This would be nice to have together with the DDW patchset.
>> >This does not fix anything DDW patchset did, it fixes IOMMU
>> >groups management which is essential for the whole feature to work.
>
>implies it is both related and independent of DDW?
>

The problem is independent of DDW.

Thanks,
Gavin

>-Nish