[PATCH 1/5] iommu: Allow taking a reference on a group directly

[Robin Murphy]

On 09/11/16 17:25, Will Deacon wrote:
> On Wed, Nov 09, 2016 at 12:47:24PM +0000, Robin Murphy wrote:
>> iommu_group_get_for_dev() expects that the IOMMU driver's device_group
>> callback return a group with a reference held for the given device.
>> Whilst allocating a new group is fine, and pci_device_group() correctly
>> handles reusing an existing group, there is no general means for IOMMU
>> drivers doing their own group lookup to take additional references on an
>> existing group pointer without having to also store device pointers or
>> resort to elaborate trickery.
>>
>> Add an IOMMU-driver-specific function to fill the hole.
>>
>> Signed-off-by: Robin Murphy <robin.murphy at arm.com>
>> ---
>>  drivers/iommu/iommu.c | 14 ++++++++++++++
>>  include/linux/iommu.h |  1 +
>>  2 files changed, 15 insertions(+)
>>
>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
>> index 9a2f1960873b..b0b052bc6bb5 100644
>> --- a/drivers/iommu/iommu.c
>> +++ b/drivers/iommu/iommu.c
>> @@ -552,6 +552,20 @@ struct iommu_group *iommu_group_get(struct device *dev)
>>  EXPORT_SYMBOL_GPL(iommu_group_get);
>>  
>>  /**
>> + * __iommu_group_get - Increment reference on a group
>> + * @group: the group to use, must not be NULL
>> + *
>> + * This function may be called by internal iommu driver group management
>> + * when the context of a struct device pointer is not available.  It is
>> + * not for general use.  Returns the given group for convenience.
>> + */
>> +struct iommu_group *__iommu_group_get(struct iommu_group *group)
>> +{
>> +	kobject_get(group->devices_kobj);
>> +	return group;
>> +}
> 
> This probably either wants sticking in a header or exporting to modules.
> That said, why do we need the underscores and the comment about internal
> group management? That's pretty much already the case for iommu_group_get.

The definition of struct iommu_group is private to iommu.c, so any
touching of the members has to be in here. The comment is to contrast
with iommu_group_get()'s "This function is called by iommu drivers and
users". This one is explicitly not for users of the API (DMA mapping,
VFIO, etc.), as they really have no business messing with refcounts
directly, and should always be operating in the context of a device;
it's only for the benefit of anyone *implementing* the API. And since
IOMMU drivers aren't modular (yet... ;)) there's no cause for an export.

> Of course, removing the underscores gives you a naming conflict, but we
> could just call it something like "iommu_group_get_ref".

Ideally, this would be the iommu_group_get() to precisely match
iommu_group_put(), and the existing function would renamed something
like iommu_dev_group_get() (or perhaps even all external uses converted
over to iommu_group_get_for_dev()), but that would be an awful lot of
churn for little obvious benefit. Similarly, I nearly added the below
hunk, but it didn't seem worth the bother.

Robin.

----->8-----
diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 9a2f1960873b..89d509c59019 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -545,7 +545,7 @@ struct iommu_group *iommu_group_get(struct device *dev)
 	struct iommu_group *group = dev->iommu_group;

 	if (group)
-		kobject_get(group->devices_kobj);
+		__iommu_group_get(group);

 	return group;
 }