Re: [PATCH v9 1/5] driver core: Find an existing link between two devices

[Robin Murphy]

Hi Rafael,

On 14/03/18 11:57, Rafael J. Wysocki wrote:
> On Wednesday, March 14, 2018 12:50:54 PM CET Tomasz Figa wrote:
> > On Wed, Mar 14, 2018 at 8:12 PM, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
> > > On Tuesday, March 13, 2018 12:23:34 PM CET Tomasz Figa wrote:
> > > > On Tue, Mar 13, 2018 at 7:34 PM, Vivek Gautam
> > > > <vivek.gautam@xxxxxxxxxxxxxx> wrote:
> > > > > Hi Tomasz,
> > > > >
> > > > > On Tue, Mar 13, 2018 at 3:45 PM, Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote:
> > > > > > Hi Vivek,
> > > > > >
> > > > > > Thanks for the patch.
> > > > > >
> > > > > > On Tue, Mar 13, 2018 at 5:55 PM, Vivek Gautam
> > > > > > <vivek.gautam@xxxxxxxxxxxxxx> wrote:
> > > > > > > The lists managing the device-links can be traversed to
> > > > > > > find the link between two devices. The device_link_add() APIs
> > > > > > > does traverse these lists to check if there's already a link
> > > > > > > setup between the two devices.
> > > > > > > So, add a new APIs, device_link_find(), to find an existing
> > > > > > > device link between two devices - suppliers and consumers.
> > > > > >
> > > > > > I'm wondering if this API would be useful for anything else that the
> > > > > > problem we're trying to solve with deleting links without storing them
> > > > > > anywhere. Perhaps a device_link_del_dev(consumer, supplier) would be a
> > > > > > better alternative?
> > > > >
> > > > > Yea, that sounds simpler i think. Will add this API instead of
> > > > > find_link(). Thanks.
> > > >
> > > > Perhaps let's wait for a moment to see if there are other opinions. :)
> > > >
> > > > Rafael, Lucas, any thoughts?
> > >
> > > It is not clear to me what the device_link_del_dev(consumer, supplier) would do.
> >
> > It would delete a link between consumer and supplier.
>
> If there's one I suppose.
>
> I'm wondering if you are somehow trying to address the same problem as the
> device links reference counting patch from Lukas that has been queued up for 4.17
> already.

Not quite - the issue here is that we have one supplier with an 
arbitrarily large number of consumers, and would prefer that supplier 
not to have to spend a whole bunch of memory to store all the struct 
device_link pointers for the sole reason of having something to give to 
device_link_del() at the end, given that the device links code is 
already keeping track of everything internally anyway.

The current API would permit doing this:

	iommu_attach(dev) {
		...
		if (!device_link_add(dev, iommu, IOMMU_LINK_FLAGS))
			return -ENODEV;
		...
	}

	iommu_detach(dev) {
		...
		// Will return the existing link from earlier
		link = device_link_add(dev, iommu, IOMMU_LINK_FLAGS);
		device_link_del(link);
		// Needed once refcounting is in place
		//device_link_del(link);
		...
	}

but it looks so wacky and non-obvious that we'd like to encapsulate the 
same behaviour into a more formal interface (my personal naming 
preference would be device_link_remove(consumer, supplier)).

Robin.