[PATCH kernel] powerpc/powernv: Update comment about shifting IOV BAR

[Alexey Kardashevskiy]

On 14/09/17 13:07, Benjamin Herrenschmidt wrote:
> On Thu, 2017-09-14 at 12:45 +1000, Alexey Kardashevskiy wrote:
>> On 31/08/17 13:34, Alexey Kardashevskiy wrote:
>>> From: Benjamin Herrenschmidt <benh at kernel.crashing.org>
>>
>> Oops, this was not right :)
>>
>> Anyway, Ben, please comment. Thanks.
> 
> This is incorrect, we can do hotplug behind switches afaik.

Do we have an actual system which allows this? Anyway, what we do now is
wrong and it needs what? Reserve that hole? I'd like to update the comment
for now, at least, and state what bad thing can happen and what we expect.


>>
>>>
>>> From: Alexey Kardashevskiy <aik at ozlabs.ru>
>>>
>>> This updates the comment about creating a hole in /proc/iomem which
>>> should not be normally happening but it does in the powernv platform
>>> due the way MMIO M64 BARs are organised in the IODA2-capable hardware.
>>>
>>> Signed-off-by: Alexey Kardashevskiy <aik at ozlabs.ru>
>>> ---
>>>
>>> It has been mentioned multiple times (last one -
>>> https://www.spinics.net/lists/linux-pci/msg64084.html ) that the comment
>>> is not informative enough for people not particularly familiar with
>>> the POWER8 IO hardware.
>>>
>>> This attempt aims to:
>>> 1. explain why we shift the resource
>>> 2. explain why nothing can use that hole as a resource while it is "free"
>>> (I am not sure that this is the case actually)
>>>
>>> Please comment, everyone, let's have this very well documented while
>>> I remember these bits :) Thanks.
>>> ---
>>>  arch/powerpc/platforms/powernv/pci-ioda.c | 10 +++++++---
>>>  1 file changed, 7 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/powerpc/platforms/powernv/pci-ioda.c b/arch/powerpc/platforms/powernv/pci-ioda.c
>>> index 48de308224d6..c4a36ae78c95 100644
>>> --- a/arch/powerpc/platforms/powernv/pci-ioda.c
>>> +++ b/arch/powerpc/platforms/powernv/pci-ioda.c
>>> @@ -1002,9 +1002,13 @@ static int pnv_pci_vf_resource_shift(struct pci_dev *dev, int offset)
>>>  	}
>>>  
>>>  	/*
>>> -	 * After doing so, there would be a "hole" in the /proc/iomem when
>>> -	 * offset is a positive value. It looks like the device return some
>>> -	 * mmio back to the system, which actually no one could use it.
>>> +	 * Since M64 BAR shares segments among all possible 256 PEs,
>>> +	 * we have to shift the beginning of PF IOV BAR to make it start from
>>> +	 * the segment which belongs to the PE number assigned to the first VF.
>>> +	 * This creates a "hole" in the /proc/iomem which could be used for
>>> +	 * allocating other resources, however this is not expected to happen
>>> +	 * on IODA as the only possibility would be a PCI hotplug and IODA
>>> +	 * hardware only allows it on a slot with dedicated PHB.
>>>  	 */
>>>  	for (i = 0; i < PCI_SRIOV_NUM_BARS; i++) {
>>>  		res = &dev->resource[i + PCI_IOV_RESOURCES];
>>>
>>
>>


-- 
Alexey