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

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Date:   Sat, 5 Aug 2017 16:47:27 +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>,
        Thierry Reding <thierry.reding@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>
Subject: Re: [PATCH v5 2/6] drm/bridge: Add a devm_ allocator for panel
 bridge.


Den 05.08.2017 12.59, skrev Noralf Trønnes:
> (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?
>

It seems plausible looking at the code:

void device_initialize(struct device *dev)
{
[...]
     kobject_init(&dev->kobj, &device_ktype);
[...]
}

static struct kobj_type device_ktype = {
     .release    = device_release,
};

/**
  * device_release - free device structure.
  * @kobj: device's kobject.
  *
  * This is called once the reference count for the object
  * reaches 0. We forward the call to the device's release
  * method, which should handle actually freeing the structure.
  */
static void device_release(struct kobject *kobj)
{
[...]
     devres_release_all(dev);
[...]
}

Last put call chain:
put_device() -> kobject_put() -> kref_put() -> kobject_release() ->
kobject_cleanup() -> device_release() -> devres_release_all()

But I haven't actually tried it, so I might be mistaken.