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From:   Evan Green <evgreen@chromium.org>
Date:   Tue, 5 Mar 2019 11:03:49 -0800
Message-ID: <CAE=gft4iHRo8VQot26dH2NaRNLtxxRYWBmSpjkoL2BE_i6eXQw@mail.gmail.com>
Subject: Re: [PATCH 02/13] driver core: Remove the link if there is no driver
 with AUTO flag
To:     Yong Wu <yong.wu@mediatek.com>
Cc:     Joerg Roedel <joro@8bytes.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        Rob Herring <robh+dt@kernel.org>,
        Robin Murphy <robin.murphy@arm.com>,
        Tomasz Figa <tfiga@google.com>,
        Will Deacon <will.deacon@arm.com>,
        linux-mediatek@lists.infradead.org, srv_heupstream@mediatek.com,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        linux-arm-kernel@lists.infradead.org,
        iommu@lists.linux-foundation.org, Arnd Bergmann <arnd@arndb.de>,
        yingjoe.chen@mediatek.com, youlin.pei@mediatek.com,
        Nicolas Boichat <drinkcat@chromium.org>
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On Wed, Feb 27, 2019 at 6:33 AM Yong Wu <yong.wu@mediatek.com> wrote:
>
> On Mon, 2019-02-25 at 15:53 -0800, Evan Green wrote:
> > On Mon, Dec 31, 2018 at 8:52 PM Yong Wu <yong.wu@mediatek.com> wrote:
> > >
> > > DL_FLAG_AUTOREMOVE_CONSUMER/SUPPLIER means "Remove the link
> > > automatically on consumer/supplier driver unbind", that means we should
> > > remove whole the device_link when there is no this driver no matter what
> > > the ref_count of the link is.
> > >
> > > CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > Signed-off-by: Yong Wu <yong.wu@mediatek.com>
> > > ---
> > > The ref_count of our device_link normally is over 1. When the consumer
> > > device driver is removed, whole the device_link should be removed.
> > > Thus, I add this patch.
> > > ---
> >
> > I will admit to reading about device links for the first time while
> > reviewing this patch, but I don't really get this. Why use a kref at
> > all if we're just going to ignore its value? For instance, I see that
> > if you call device_link_add() with the same supplier and consumer, it
> > uses the kref to return the same link. That machinery is broken with
> > your change. Although I don't see any uses of it, you might also
> > expect a supplier or consumer could do a kref_get() on the link it got
> > back from device_link_add(), and have a reasonable expectation that
> > the link wouldn't be freed out from under it. This would also be
> > broken.
> >
> > Can you explain why your device_links normally have a reference count
> > >1,
>
> I use device link between the smi-larb device and the iommu-consumer
> device. Take a example, smi-larb1 have 4 VDEC ports. From 4/13 in this
> patchset, we use device_link to link the VDEC device and the smi-larb1
> device in the function(mtk_iommu_config). since there are 4 ports, it
> will call device_link_add 4 times.
>
> >
> > and why those additional references can't be cleaned up in an
> > orderly fashion?
>
> If the iommu-consume device(like VDEC above) is removed, It should enter
> device_links_driver_cleanup which only ref_put one time. I guess whole
> the link should be removed at that time.

It seems like Robin had some suggestions about using
mtk_iommu_add_device() rather than the attach_dev() to set the links
up, and then track them for removal in the corresponding
remove_device() callback. Then you wouldn't need this change, right?