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

[Vivek Gautam]

Hi Lukasz,


On 3/14/2018 5:57 PM, Lukas Wunner wrote:
> On Wed, Mar 14, 2018 at 12:14:15PM +0000, Robin Murphy 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:
> > > > > > > On Tue, Mar 13, 2018 at 3:45 PM, Tomasz Figa <tfiga@xxxxxxxxxxxx> wrote:
> > > > > > > > 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.
> > 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.
> Makes sense to me.  How about an additional flag which autoremoves the
> link on provider unbind?

If I understand this correctly, if we create the device link with 
DL_FLAG_AUTOREMOVE, the link is deleted after a consumer unbind. During 
a supplier unbind all we get is a WARN_ON with DL_FLAG_AUTOREMOVE. I 
guess that's an intended behavior?

If this is the case, then the consumer/supplier drivers just don't have 
to take care of deleting the device link explicitly.
Is my understanding correct?

regards
Vivek

> Thanks,
>
> Lukas