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Date:   Thu, 31 Oct 2019 15:42:47 +0000
From:   Will Deacon <will@kernel.org>
To:     Joerg Roedel <joro@8bytes.org>
Cc:     iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
        Robin Murphy <robin.murphy@arm.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Subject: Re: [PATCH 5/7] iommu/arm-smmu-v3: Allow building as a module
Message-ID: <20191031154247.GB28061@willie-the-truck>
References: <20191030145112.19738-1-will@kernel.org>
 <20191030145112.19738-6-will@kernel.org>
 <20191030193148.GA8432@8bytes.org>
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Hi Joerg,

On Wed, Oct 30, 2019 at 08:31:48PM +0100, Joerg Roedel wrote:
> On Wed, Oct 30, 2019 at 02:51:10PM +0000, Will Deacon wrote:
> > By removing the redundant call to 'pci_request_acs()' we can allow the
> > ARM SMMUv3 driver to be built as a module.
> > 
> > Signed-off-by: Will Deacon <will@kernel.org>
> > ---
> >  drivers/iommu/Kconfig       | 2 +-
> >  drivers/iommu/arm-smmu-v3.c | 1 -
> >  2 files changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
> > index e3842eabcfdd..7583d47fc4d5 100644
> > --- a/drivers/iommu/Kconfig
> > +++ b/drivers/iommu/Kconfig
> > @@ -388,7 +388,7 @@ config ARM_SMMU_DISABLE_BYPASS_BY_DEFAULT
> >  	  config.
> >  
> >  config ARM_SMMU_V3
> > -	bool "ARM Ltd. System MMU Version 3 (SMMUv3) Support"
> > +	tristate "ARM Ltd. System MMU Version 3 (SMMUv3) Support"
> >  	depends on ARM64
> >  	select IOMMU_API
> >  	select IOMMU_IO_PGTABLE_LPAEa
> 
> Sorry for the stupid question, but what prevents the iommu module from
> being unloaded when there are active users? There are no symbol
> dependencies to endpoint device drivers, because the interface is only
> exposed through the iommu-api, right? Is some sort of manual module
> reference counting needed?

Generally, I think unloading the IOMMU driver module while there are
active users is a pretty bad idea, much like unbinding the driver via
/sys in the same situation would also be fairly daft. However, I *think*
the code in __device_release_driver() tries to deal with this by
iterating over the active consumers and ->remove()ing them first.

I'm without hardware access at the moment, so I haven't been able to
test this myself. We could nobble the module_exit() hook, but there's
still the "force unload" option depending on the .config.

Will