Date:	Wed, 30 Oct 2013 08:48:14 +0100
From:	David Herrmann <dh.herrmann@...il.com>
To:	Tomi Valkeinen <tomi.valkeinen@...com>
Cc:	"linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Tom Gundersen <teg@...m.no>,
	Alexandre Courbot <acourbot@...dia.com>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Stephen Warren <swarren@...dotorg.org>
Subject: Re: [PATCH 1/2] simplefb: fix unmapping fb during destruction

Hi Tomi

Could we get this in -next before the merge-window starts? Stephen
already ack'ed it.

Thanks
David

On Wed, Oct 2, 2013 at 6:23 PM, David Herrmann <dh.herrmann@...il.com> wrote:
> Hi
>
> On Wed, Oct 2, 2013 at 6:16 PM, Stephen Warren <swarren@...dotorg.org> wrote:
>> On 10/02/2013 08:58 AM, David Herrmann wrote:
>>> Unfortunately, fbdev does not create its own "struct device" for
>>> framebuffers. Instead, it attaches to the device of the parent layer. This
>>> has the side-effect that devm_* managed resources are not cleaned up on
>>> framebuffer-destruction but rather during destruction of the
>>> parent-device. In case of fbdev this might be too late, though.
>>> remove_conflicting_framebuffer() may remove fbdev devices but keep the
>>> parent device as it is.
>>>
>>> Therefore, we now use plain ioremap() and unmap the framebuffer in the
>>> fb_destroy() callback. Note that we must not free the device here as this
>>> might race with the parent-device removal. Instead, we rely on
>>> unregister_framebuffer() as barrier and we're safe.
>>
>> So, once the .fb_destroy callback has been executed, there's no other
>> callback to resurrect it? The framebuffer itself is still registered
>> until the device's remove, yet after .fb_destroy, the memory is
>> unmapped, which would be dangerous if the FB can be re-started.
>
> fbdev lifetime tracking is weird.. ->fb_destroy() is called by
> unregister_framebuffer() _after_ it got unlinked. So no, the
> framebuffer is gone and cannot be resurrected. However, the
> unregistered/dead fbdev object itself stays around until you call
> framebuffer_release(). We cannot call it from fb_destroy(), though as
> the platform_data in your platform device still points to the fbdev
> device. Therefore we keep it and wait for the platform-driver to be
> removed which then again calls unregister_framebuffer() (which will
> immediately return as the fbdev device is not registered) and then you
> can finally call framebuffer_release().
>
> Note that even though there's fb_get_info() and fb_put_info(), both
> are not exported and never used. So there is *no* fbdev ref-counting
> (which would be horribly broken anyway) and the driver is the sole
> owner of the fbdev object.
>
>> If that's not an issue, this patch seems fine, so
>> Acked-by: Stephen Warren <swarren@...dia.com>
>
> Thanks!
>
>>> I know that simplefb was supposed to stay "as simple as possible" but I really
>>> think this series is the last set of fixes I have. Unfortunately framebuffer DRM
>>> handover is mandatory so we cannot ignore it in simplefb.
>>
>> I don't think this patch adds any significant complexity, so I'm not
>> worried at least.
>
> Good to hear!
>
> Thanks
> David
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