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

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Date:   Mon, 7 Aug 2017 11:25:07 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Noralf Trønnes <noralf@...nnes.org>
Cc:     Ilia Mirkin <imirkin@...m.mit.edu>, Eric Anholt <eric@...olt.net>,
        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.

On Sat, Aug 05, 2017 at 12:59:07PM +0200, Noralf Trønnes wrote:
> (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?

No. drm_device is the thing that is refcounted for userspace references
like open FD (we're not perfect about it, e.g. sysfs and dma-buf/fence
don't).

devm_ otoh is tied to the lifetime of the underlying device, and that one
can get outlived by drm_device. Or at least afaiui, devm_ stuff is nuked
on unplug, and not when the final sw reference of the struct device
disappears.

Not sure tough, it's complicated.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch