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From: Bjorn Helgaas <email@example.com>
To: Logan Gunthorpe <firstname.lastname@example.org>
Cc: Doug Meyer <email@example.com>,
Bjorn Helgaas <firstname.lastname@example.org>, Jon Mason <email@example.com>,
"Jiang, Dave" <firstname.lastname@example.org>,
Allen Hubbe <email@example.com>,
Alex Williamson <firstname.lastname@example.org>
Subject: Re: [PATCH 2/2] NTB: PCI Quirk to Enable Switchtec NT Functionality with IOMMU On
Date: Wed, 23 May 2018 08:33:21 -0500 [thread overview]
Message-ID: <20180523133321.GA150632@bhelgaas-glaptop.roam.corp.google.com> (raw)
On Tue, May 22, 2018 at 04:23:13PM -0600, Logan Gunthorpe wrote:
> On 22/05/18 03:51 PM, Bjorn Helgaas wrote:
> > I don't think the question of when the aliases need to be added is
> > quite closed. Logan said "it seems pci_add_dma_alias() must be called
> > before the driver is initialized and therefore in a quirk", but that
> > doesn't make clear *why* the alias needs to be added before the driver
> > is initialized. The alias shouldn't be needed until the device does a
> > DMA, and it shouldn't do that until after the driver initializes.
> No, Doug tried it in the driver first and it didn't work. The symbol is
> also not exported which was probably done because it can't be used in
> the driver.
> > I suspect the reason the existing quirks are in drivers/pci/quirks.c
> > is because the IOMMU driver is in the host OS, but the host may not
> > have a driver for the device if the device is passed through to a
> > guest OS. In that case, the only way to add the alias is by using a
> > quirk that is always built into the host OS.
> Digging into the code a bit, it's not because it must be done by the
> Host OS but because it must be done before the IOMMU groups are created.
> The IOMMU code registers a bus_notifier and creates the groups based on
> the dma_alias mask when it receives the BUS_NOTIFY_ADD_DEVICE event.
> This event is notified in device_add() just before a call to
> bus_probe_device(). Therefore, if a driver attempts to use
> pci_add_dma_alias() as part of it's probe routine, it will be too late
> as the IOMMU has already setup the groups based on the original version
> of the dma_alias_mask.
This (and Alex's) analysis is very useful and I'd like to capture it
somehow, perhaps by expanding the poor pci_add_dma_alias() function
comment I added with f0af9593372a ("PCI: Add pci_add_dma_alias() to
The admonition to "call early" without any details about *how* early
or *why* it needs to be called early is not really very useful.
If we added your analysis, it would be a great help to anybody who
reworks IOMMU groups in the future.
> I suspect this is by design as the groups must be created before and any
> dma_maps are done on the device and some drivers may create dma_maps
> during probe.
next prev parent reply other threads:[~2018-05-23 13:33 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-17 12:00 [PATCH 0/2] PCI Quirk Patchset for Microsemi Switchtec NTB dmeyer
2018-05-17 12:00 ` [PATCH 1/2] NTB: Migrate PCI Constants to Cannonical PCI Header dmeyer
2018-05-17 13:25 ` Bjorn Helgaas
2018-05-17 12:00 ` [PATCH 2/2] NTB: PCI Quirk to Enable Switchtec NT Functionality with IOMMU On dmeyer
2018-05-17 13:45 ` Bjorn Helgaas
2018-05-17 16:06 ` Logan Gunthorpe
[not found] ` <CA+GK6en+g+T9H0sOMdVXv-_aD3rRcuzZ1JdfK0moEoTuuJnrqQ@mail.gmail.com>
2018-05-22 21:51 ` Bjorn Helgaas
2018-05-22 22:13 ` Alex Williamson
2018-05-22 22:23 ` Logan Gunthorpe
2018-05-23 13:33 ` Bjorn Helgaas [this message]
2018-05-23 20:21 ` Logan Gunthorpe
2018-05-17 15:48 ` Logan Gunthorpe
2018-05-22 21:08 ` Doug Meyer
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