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Subject: Re: [PATCH 02/13] driver core: Remove the link if there is no driver
 with AUTO flag
To:     Evan Green <evgreen@chromium.org>, Yong Wu <yong.wu@mediatek.com>
Cc:     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>
References: <1546318276-18993-1-git-send-email-yong.wu@mediatek.com>
 <1546318276-18993-3-git-send-email-yong.wu@mediatek.com>
 <CAE=gft7mWs1jc+KQQcPuuhcmxpFRnRBTt35+uC3F20N6R554bQ@mail.gmail.com>
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 <CAE=gft4iHRo8VQot26dH2NaRNLtxxRYWBmSpjkoL2BE_i6eXQw@mail.gmail.com>
From:   Matthias Brugger <matthias.bgg@gmail.com>
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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?
> 

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 :)