Hi Alex, On 2/15/2017 4:43 PM, Sinan Kaya wrote: > On 2/15/2017 2:36 PM, Alex Williamson wrote: >> On Tue, 14 Feb 2017 22:53:35 -0500 >> okaya@xxxxxxxxxxxxxx wrote: >> >>> On 2017-02-14 18:51, Alex Williamson wrote: >>>> On Tue, 14 Feb 2017 16:25:22 -0500 >>>> Sinan Kaya <okaya@xxxxxxxxxxxxxx> wrote: >>>> >>>>> The ACS requirement has been obscured in the current code and is only >>>>> known by certain individuals who happen to read the code. Print out a >>>>> warning with ACS path failure when ACS requirement is not met. >>>>> >>>>> Signed-off-by: Sinan Kaya <okaya@xxxxxxxxxxxxxx> >>>>> --- >>>>> drivers/iommu/iommu.c | 3 +++ >>>>> 1 file changed, 3 insertions(+) >>>>> >>>>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c >>>>> index dbe7f65..049ee0a 100644 >>>>> --- a/drivers/iommu/iommu.c >>>>> +++ b/drivers/iommu/iommu.c >>>>> @@ -811,6 +811,9 @@ struct iommu_group *pci_device_group(struct device >>>>> *dev) >>>>> if (IS_ERR(group)) >>>>> return NULL; >>>>> >>>>> + if (pci_is_root_bus(bus)) >>>>> + dev_warn_once(&pdev->dev, "using shared group due to ACS path >>>>> failure\n"); >>>>> + >>>>> return group; >>>>> } >>>>> >>>> >>>> The premise here is flawed. An IOMMU group based at the root bus >>>> doesn't necessarily imply a lack of ACS. There are devices on root >>>> buses, integrated endpoints and root ports. Naturally an IOMMU group >>>> for these devices needs to be based at the root bus. Additionally, >>>> there can be IOMMU groups developed around a lack of ACS that don't >>>> intersect with the root bus. Since this is a warn_once, the false >>>> positives for root bus devices are going to be enumerated first. On an >>>> Intel system there's typically a device as 00.0 that will always be >>>> pointlessly listed first. Also, it's not clear that grouping devices >>>> together is always wrong, as Robin pointed out in the EHCI/OHCI >>>> example. Lack of ACS on downtream ports is likely to cause problems, >>>> especially if that downstream port exposes a slot. Maybe that would be >>>> a good place to start. Also, what is someone supposed to do when they >>>> see this error? If we can hope they'll look for the error in the code >>>> (unlikely) a big comment with useful external links might be >>>> necessary. Based on how easily vendors ignore kernel warnings, I'm >>>> dubious there's any value to this path though. Thanks, >>> >>> Maybe, a better solution would be to add some sentences into vfio.txt >>> documentation. >>> >>> I'm ready to drop this patch. I just don't want ACS requirement to be >>> hidden between the source code. >>> I posted V2 to linux-pci maillist but forgot to CC the iommu group. [PATCH V2] PCI: add QCOM root port quirks for ACS I dropped the second patch (this one I'm replying to) as discussed. I did minor cleanups in the first commit including 1- commit message change 2- replace dev_info_once with dev_info https://patchwork.codeaurora.org/patch/177033/ Sinan -- Sinan Kaya Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.
