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

[Alexey Kardashevskiy]

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.


> 
> 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