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

[Noralf TrÃnnes]

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)
{
[...]
    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.