Re: [PATCH v5 2/6] drm/bridge: Add a devm_ allocator for panel bridge.

From: Noralf TrÃnnes
Date: Sat Aug 05 2017 - 10:48:15 EST

Den 05.08.2017 12.59, skrev Noralf TrÃnnes:
(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@xxxxxxxxxx> wrote:
Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx> 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)

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.