linux-kernel - Re: [PATCH 2/2] thunderbolt: Initialize after IOMMUs

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Date:   Wed, 5 Sep 2018 12:46:02 +0300
From:   Mika Westerberg <mika.westerberg@...ux.intel.com>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     linux-kernel@...r.kernel.org,
        Andreas Noever <andreas.noever@...il.com>,
        Michael Jamet <michael.jamet@...el.com>,
        Yehezkel Bernat <YehezkelShB@...il.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>
Subject: Re: [PATCH 2/2] thunderbolt: Initialize after IOMMUs

On Wed, Sep 05, 2018 at 10:47:46AM +0200, Lukas Wunner wrote:
> On Mon, Sep 03, 2018 at 04:20:12PM +0300, Mika Westerberg wrote:
> > If IOMMU is enabled and Thunderbolt driver is built into the kernel
> > image, it will be probed before IOMMUs are attached to the PCI bus.
> > Because of this DMA mappings the driver does will not go through IOMMU
> > and start failing right after IOMMUs are enabled.
> > 
> > For this reason move the Thunderbolt driver initialization happen at
> > rootfs level.
> > 
> > Signed-off-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
> > ---
> >  drivers/thunderbolt/nhi.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/thunderbolt/nhi.c b/drivers/thunderbolt/nhi.c
> > index 88cff05a1808..5cd6bdfa068f 100644
> > --- a/drivers/thunderbolt/nhi.c
> > +++ b/drivers/thunderbolt/nhi.c
> > @@ -1191,5 +1191,5 @@ static void __exit nhi_unload(void)
> >  	tb_domain_exit();
> >  }
> >  
> > -fs_initcall(nhi_init);
> > +rootfs_initcall(nhi_init);
> >  module_exit(nhi_unload);
> 
> What if the rootfs is located on a Thunderbolt-attached drive and
> the thunderbolt driver needs to establish tunnels to that drive
> before rootfs can be accessed?  Doesn't the above break such a setup?

No, then you put the driver as part of your initrd.

> I think the dependency on the IOMMU should be open coded by returning
> -EPROBE_DEFER from the ->probe hook if it's not yet attached.
> Shuffling around initcall order is just applying duct tape.

It is not a dependency. The same thing can happen with any other driver
if they happen to initialize any DMA with the device before IOMMUs are
initialized.

> Commit acb40d841257 already changed module_init() to fs_initcall()
> and now it has to be changed again.  Shows how fragile this is.

It is a bit fragile but I don't see any other way to handle this than
trusting on the link ordering. Both -EPROBE_DEFER and device_links are
out of the question AFAICT.