From: Benjamin Herrenschmidt <b...@kernel.crashing.org> From: Alexey Kardashevskiy <a...@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 <a...@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]; -- 2.11.0
