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

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Message-ID: <CAE=gft7mWs1jc+KQQcPuuhcmxpFRnRBTt35+uC3F20N6R554bQ@mail.gmail.com>
Date:   Mon, 25 Feb 2019 15:53:44 -0800
From:   Evan Green <evgreen@...omium.org>
To:     Yong Wu <yong.wu@...iatek.com>
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, 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, and why those additional references can't be cleaned up in an
orderly fashion?

(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?)