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

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Date:   Wed, 14 Mar 2018 13:27:59 +0100
From:   Lukas Wunner <lukas@...ner.de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Tomasz Figa <tfiga@...omium.org>,
        Vivek Gautam <vivek.gautam@...eaurora.org>,
        Joerg Roedel <joro@...tes.org>,
        Rob Herring <robh+dt@...nel.org>,
        "open list:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        devicetree@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Will Deacon <will.deacon@....com>,
        Rob Clark <robdclark@...il.com>,
        Sricharan R <sricharan@...eaurora.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Archit Taneja <architt@...eaurora.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v9 1/5] driver core: Find an existing link between two
 devices

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@...ysocki.net> 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@...eaurora.org> wrote:
> >>>>>On Tue, Mar 13, 2018 at 3:45 PM, Tomasz Figa <tfiga@...omium.org> wrote:
> >>>>>>On Tue, Mar 13, 2018 at 5:55 PM, Vivek Gautam <vivek.gautam@...eaurora.org> 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?

Thanks,

Lukas