linux-kernel - Re: [PATCH 02/13] driver core: Remove the link if there is no driver with AUTO flag

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Date:   Wed, 27 Feb 2019 22:33:20 +0800
From:   Yong Wu <yong.wu@...iatek.com>
To:     Evan Green <evgreen@...omium.org>
CC:     Joerg Roedel <joro@...tes.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Tomasz Figa <tfiga@...gle.com>,
        Will Deacon <will.deacon@....com>,
        <linux-mediatek@...ts.infradead.org>,
        <srv_heupstream@...iatek.com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <iommu@...ts.linux-foundation.org>, Arnd Bergmann <arnd@...db.de>,
        <yingjoe.chen@...iatek.com>, <youlin.pei@...iatek.com>,
        Nicolas Boichat <drinkcat@...omium.org>
Subject: Re: [PATCH 02/13] driver core: Remove the link if there is no
 driver with AUTO flag

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@...iatek.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@...uxfoundation.org>
> > Signed-off-by: Yong Wu <yong.wu@...iatek.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?)