linux-kernel - Re: [PATCH RESEND v2 1/2] drm/xen-front: Add support for Xen PV display frontend

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Date:   Tue, 20 Mar 2018 14:47:16 +0100
From:   Daniel Vetter <daniel@...ll.ch>
To:     Oleksandr Andrushchenko <andr2000@...il.com>
Cc:     Daniel Vetter <daniel@...ll.ch>, xen-devel@...ts.xenproject.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Dave Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Sean Paul <seanpaul@...omium.org>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Juergen Gross <jgross@...e.com>, boris.ostrovsky@...cle.com,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        "Oleksandr_Andrushchenko@...m.com" <Oleksandr_Andrushchenko@...m.com>
Subject: Re: [PATCH RESEND v2 1/2] drm/xen-front: Add support for Xen PV
 display frontend

On Tue, Mar 20, 2018 at 01:58:01PM +0200, Oleksandr Andrushchenko wrote:
> On 03/19/2018 05:28 PM, Daniel Vetter wrote:
> > There should be no difference between immediate removal and delayed
> > removal of the drm_device from the xenbus pov. The lifetimes of the
> > front-end (drm_device) and backend (the xen bus thing) are entirely
> > decoupled:
> Well, they are not decoupled for simplicity of handling,
> please see below
> > 
> > So for case 2 you only have 1 case:
> > 
> > - drm_dev_unplug
> > - tear down the entire xenbus backend completely
> > - all xenbus access will be caught with drm_dev_entre/exit (well right
> > now drm_dev_is_unplugged) checks, including any access to your private
> > drm_device data
> > - once drm_device->open_count == 0 the core will tear down the
> > drm_device instance and call your optional drm_driver->release
> > callback.
> > 
> > So past drm_dev_unplug the drm_device is in zombie state and the only
> > thing that will happen is a) it rejects all ioctls and anything else
> > userspace might ask it to do and b) gets releases once the last
> > userspace reference is gone.
> I have re-worked the driver with this in mind [1]
> So, I now use drm_dev_unplug and destroy the DRM device
> on drm_driver.release.
> In context of unplug work I also merged xen_drm_front_drv.c and
> xen_drm_front.c as these are too coupled together now.
> 
> Could you please take a look and tell me if this is what you mean?
> > 
> > If the backend comes up again, you create a _new_ drm_device instance
> > (while the other one is still in the process of eventually getting
> > released).
> We only have a single xenbus instance, so this way I'll need
> to handle list of such zombies. For that reason I prefer to
> wait until the DRM device is destroyed, telling the backend
> to hold on until then (via going into XenbusStateReconfiguring state).

Why exactly do you need to keep track of your drm_devices from the xenbus?
Once unplugged, there should be no connection with the "hw" for your
device, in neither direction. Maybe I need to look again, but this still
smells funny and not like something you should ever do.

> Another drawback of such approach is that I'll have different
> minors at run-time, e.g. card0, card1, etc.
> For software which has /dev/dri/card0 hardcoded it may be a problem.
> But this is minor, IMO

Fix userspace :-)

But yeah unlikely this is a problem, hotplugging is fairly old thing.

> > In short, your driver code should never have a need to look at
> > drm_device->open_count. I hope this explains it a bit better.
> > -Daniel
> > 
> Yes, you are correct: at [1] I am not touching drm_device->open_count
> anymore and everything just happens synchronously

> [1] https://github.com/andr2000/linux/commits/drm_tip_pv_drm_v3

Please just resend, makes it easier to comment inline.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch