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Subject: Re: [PATCH 02/13] driver core: Remove the link if there is no
 driver with AUTO flag
From:   Yong Wu <yong.wu@mediatek.com>
To:     Evan Green <evgreen@chromium.org>
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>
Date:   Wed, 27 Feb 2019 22:33:20 +0800
In-Reply-To: <CAE=gft7mWs1jc+KQQcPuuhcmxpFRnRBTt35+uC3F20N6R554bQ@mail.gmail.com>
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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.

> 
> (To be honest, I don't really understand the case for the AUTOREMOVE
> flags at all. Is there some case where the party that set up the link
> can't tear it down? Or is this a way to devm_ify the link, where devm
> itself doesn't work because the links themselves stall out that
> mechanism?)