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

[Robin Murphy]

On 09/03/18 07:11, Vivek Gautam wrote:
> On Thu, Mar 8, 2018 at 10:29 AM, Vivek Gautam
> <vivek.gautam@xxxxxxxxxxxxxx> wrote:
> > On Wed, Mar 7, 2018 at 6:17 PM, Robin Murphy <robin.murphy@xxxxxxx> wrote:
> > > On 02/03/18 10:10, Vivek Gautam wrote:
> > > > From: Sricharan R <sricharan@xxxxxxxxxxxxxx>
> > > >
> > > > 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@xxxxxxxxxxxxxx>
> > > > Signed-off-by: Vivek Gautam <vivek.gautam@xxxxxxxxxxxxxx>
> > > > ---
> > > >    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.