linux-kernel - Re: [PATCH] iommu: add a function to find an iommu group by id

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Date:	Fri, 22 Mar 2013 10:07:42 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Alexey Kardashevskiy <aik@...abs.ru>
Cc:	Joerg Roedel <joerg.roedel@....com>,
	Paul Mackerras <paulus@...ba.org>,
	linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Gibson <david@...son.dropbear.id.au>
Subject: Re: [PATCH] iommu: add a function to find an iommu group by id

On Fri, 2013-03-22 at 17:49 +1100, Alexey Kardashevskiy wrote:
> As IOMMU groups are exposed to the user space by their numbers,
> the user space can use them in various kernel APIs so the kernel
> might need an API to find a group by its ID.
> 
> As an example, QEMU VFIO on PPC64 platform needs it to associate
> a logical bus number (LIOBN) with a specific IOMMU group in order
> to support in-kernel handling of DMA map/unmap requests.
> 
> The patch adds the iommu_group_get_by_id(id) function which performs
> such search.
> 
> Signed-off-by: Alexey Kardashevskiy <aik@...abs.ru>
> ---

v2: <describe what changed>

>  drivers/iommu/iommu.c |   28 ++++++++++++++++++++++++++++
>  include/linux/iommu.h |    1 +
>  2 files changed, 29 insertions(+)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 1065a1a..531cbb5 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -204,6 +204,34 @@ again:
>  }
>  EXPORT_SYMBOL_GPL(iommu_group_alloc);
>  
> +struct iommu_group *iommu_group_get_by_id(int id)
> +{
> +	struct kobject *group_kobj;
> +	struct iommu_group *grp;

This is named "group" everywhere else in this file.

> +	const char *name;
> +
> +	if (!iommu_group_kset)
> +		return NULL;
> +
> +	name = kasprintf(GFP_KERNEL, "%d", id);
> +	if (!name)
> +		return NULL;
> +
> +	group_kobj = kset_find_obj(iommu_group_kset, name);

This does a kobject_get on group->kobj

> +	kfree(name);
> +
> +	if (!group_kobj)
> +		return NULL;
> +
> +	grp = container_of(group_kobj, struct iommu_group, kobj);
> +	BUG_ON(grp->id != id);
> +

Here you want to exchange one reference for another, so first do the
get:

> +	kobject_get(grp->devices_kobj);

Now you need to release the get from the find:

kobject_put(&group->kobj)

> +
> +	return grp;
> +}
> +EXPORT_SYMBOL_GPL(iommu_group_get_by_id);
> +
>  /**
>   * iommu_group_get_iommudata - retrieve iommu_data registered for a group
>   * @group: the group
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index f3b99e1..00e5d7d 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -113,6 +113,7 @@ struct iommu_ops {
>  extern int bus_set_iommu(struct bus_type *bus, struct iommu_ops *ops);
>  extern bool iommu_present(struct bus_type *bus);
>  extern struct iommu_domain *iommu_domain_alloc(struct bus_type *bus);
> +extern struct iommu_group *iommu_group_get_by_id(int id);
>  extern void iommu_domain_free(struct iommu_domain *domain);
>  extern int iommu_attach_device(struct iommu_domain *domain,
>  			       struct device *dev);



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