Re: [PATCH v8 4/5] iommu/arm-smmu: Add the device_link between masters and smmu

[Robin Murphy <robin.murphy@arm.com>]

On 09/03/18 07:11, Vivek Gautam wrote:
> On Thu, Mar 8, 2018 at 10:29 AM, Vivek Gautam
> <vivek.gautam@codeaurora.org> wrote:
>> On Wed, Mar 7, 2018 at 6:17 PM, Robin Murphy <robin.murphy@arm.com> wrote:
>>> On 02/03/18 10:10, Vivek Gautam wrote:
>>>>
>>>> From: Sricharan R <sricharan@codeaurora.org>
>>>>
>>>> Finally add the device link between the master device and
>>>> smmu, so that the smmu gets runtime enabled/disabled only when the
>>>> master needs it. This is done from add_device callback which gets
>>>> called once when the master is added to the smmu.
>>>>
>>>> Signed-off-by: Sricharan R <sricharan@codeaurora.org>
>>>> Signed-off-by: Vivek Gautam <vivek.gautam@codeaurora.org>
>>>> ---
>>>>    drivers/iommu/arm-smmu.c | 21 +++++++++++++++++++++
>>>>    1 file changed, 21 insertions(+)
>>>>
>>>> diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
>>>> index 3d6a1875431f..bb1ea82c1003 100644
>>>> --- a/drivers/iommu/arm-smmu.c
>>>> +++ b/drivers/iommu/arm-smmu.c
>>>> @@ -217,6 +217,9 @@ struct arm_smmu_device {
>>>>          /* IOMMU core code handle */
>>>>          struct iommu_device             iommu;
>>>> +
>>>> +       /* runtime PM link to master */
>>>> +       struct device_link *link;
>>>
>>>
>>> Just the one?
> 
> we will either have to count all the devices that are present on the
> iommu bus, or
> maintain a list to which all the links can be added.
> But to add the list, we will have to initialize a LIST_HEAD in struct
> device_link
> as well.
> 
> Or, I think we don't even need to maintain a pointer to link with smmu.
> In arm_smmu_remove_device(), we can find out the correct link, and delete it.
> 
>          list_for_each_entry(link, &dev->links.suppliers, c_node)
>                  if (link->supplier == smmu->dev);
>                             device_link_del(link);
> 
> Should that be fine?
> 
> Rafael, does the above snippet looks right to you? Context: smmu->dev
> is the supplier, and dev is the consumer. We want to find the link,
> and delete it.

Actually, looking at the existing code, it seems like device_link_add() 
will in fact look up and return any existing link between a given 
supplier and consumer - is that intentional API behaviour that users may 
rely on to avoid keeping track of explicit link pointers? (or 
conversely, might it be reasonable to factor out a device_link_find() 
function?)

Robin.