linux-kernel - Re: [PATCH v5 2/6] drm/bridge: Add a devm

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Date:   Sat, 5 Aug 2017 12:59:07 +0200
From:   Noralf Trønnes <noralf@...nnes.org>
To:     Ilia Mirkin <imirkin@...m.mit.edu>, Eric Anholt <eric@...olt.net>
Cc:     Daniel Vetter <daniel.vetter@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH v5 2/6] drm/bridge: Add a devm_ allocator for panel
 bridge.

(I had to switch to Daniel's Intel address to get this sent)

Den 05.08.2017 00.19, skrev Ilia Mirkin:
> On Fri, Aug 4, 2017 at 4:43 PM, Eric Anholt <eric@...olt.net> wrote:
>> Laurent Pinchart <laurent.pinchart@...asonboard.com> writes:
>>
>>> Hi Eric,
>>>
>>> (CC'ing Daniel)
>>>
>>> Thank you for the patch.
>>>
>>> On Tuesday 18 Jul 2017 14:05:06 Eric Anholt wrote:
>>>> This will let drivers reduce the error cleanup they need, in
>>>> particular the "is_panel_bridge" flag.
>>>>
>>>> v2: Slight cleanup of remove function by Andrzej
>>> I just want to point out that, in the context of Daniel's work on hot-unplug,
>>> 90% of the devm_* allocations are wrong and will get in the way. All DRM core
>>> objects that are accessible one way or another from userspace will need to be
>>> properly reference-counted and freed only when the last reference disappears,
>>> which could be well after the corresponding device is removed. I believe this
>>> could be one such objects :-/
>> Sure, if you're hotplugging, your life is pain.  For non-hotpluggable
>> devices, like our SOC platform devices (current panel-bridge consumers),
>> this still seems like an excellent simplification of memory management.
> At that point you may as well make your module non-unloadable, and
> return failure when trying to remove a device from management by the
> driver (whatever the opposite of "probe" is, I forget). Hotplugging
> doesn't only happen when physically removing, it can happen for all
> kinds of reasons... and userspace may still hold references in some of
> those cases.

If drm_open() gets a ref on dev->dev and puts it in drm_release(),
won't that delay devm_* cleanup until userspace is done?