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

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Date:   Wed, 5 Sep 2018 10:47:46 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Mika Westerberg <mika.westerberg@...ux.intel.com>
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 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?

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.

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

Thanks,

Lukas