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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:     Matthias Brugger <matthias.bgg@gmail.com>,
        Joerg Roedel <joro@8bytes.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "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, 13 Mar 2019 17:08:23 +0800
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On Tue, 2019-03-12 at 16:17 -0700, Evan Green wrote:
> On Tue, Mar 12, 2019 at 7:21 AM Matthias Brugger <matthias.bgg@gmail.com> wrote:
> >
> >
> >
> > On 05/03/2019 20:03, Evan Green wrote:
> > > 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?

Hi Evan,  sorry for reply you so late. I have not got time to try
this(Put it in the add_device), But I guess it works.
At that time the ref_cnt here should be 1, then this patch is
unnecessary.

> > >
> >
> > FYI, Evan the patch is queued for v5.1-rc1 as
> > 0fe6f7874d46 ("driver core: Remove the link if there is no driver with AUTO flag")
> >
> > So if you think there is something wrong with it, then please provide a fix or
> > raise awareness :)
> 
> Oh. Thanks for the heads-up Matthias. It's pretty weird that we have
> the kref there whose count we just completely ignore. I'll try to find
> some time to submit a patch.

Thanks very much if you improve this.

> -Evan