linux-kernel - Re: [RFC,3/3] drm/komeda: Allow non-component drm

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Message-ID: <20191022084210.GX11828@phenom.ffwll.local>
Date:   Tue, 22 Oct 2019 10:42:10 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Brian Starkey <Brian.Starkey@....com>, nd <nd@....com>,
        David Airlie <airlied@...ux.ie>,
        Liviu Dudau <Liviu.Dudau@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "james qian wang (Arm Technology China)" <james.qian.wang@....com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        Mihail Atanassov <Mihail.Atanassov@....com>,
        Sean Paul <sean@...rly.run>
Subject: Re: [RFC,3/3] drm/komeda: Allow non-component drm_bridge only
 endpoints

On Thu, Oct 17, 2019 at 12:41:37PM +0100, Russell King - ARM Linux admin wrote:
> On Thu, Oct 17, 2019 at 10:48:12AM +0000, Brian Starkey wrote:
> > On Thu, Oct 17, 2019 at 10:21:03AM +0000, james qian wang (Arm Technology China) wrote:
> > > On Thu, Oct 17, 2019 at 08:20:56AM +0000, Brian Starkey wrote:
> > > > On Thu, Oct 17, 2019 at 03:07:59AM +0000, james qian wang (Arm Technology China) wrote:
> > > > > On Wed, Oct 16, 2019 at 04:22:07PM +0000, Brian Starkey wrote:
> > > > > > 
> > > > > > If James is strongly against merging this, maybe we just swap
> > > > > > wholesale to bridge? But for me, the pragmatic approach would be this
> > > > > > stop-gap.
> > > > > >
> > > > > 
> > > > > This is a good idea, and I vote +ULONG_MAX :)
> > > > > 
> > > > > and I also checked tda998x driver, it supports bridge. so swap the
> > > > > wholesale to brige is perfect. :)
> > > > > 
> > > > 
> > > > Well, as Mihail wrote, it's definitely not perfect.
> > > > 
> > > > Today, if you rmmod tda998x with the DPU driver still loaded,
> > > > everything will be unbound gracefully.
> > > > 
> > > > If we swap to bridge, then rmmod'ing tda998x (or any other bridge
> > > > driver the DPU is using) with the DPU driver still loaded will result
> > > > in a crash.
> > > 
> > > I haven't read the bridge code, but seems this is a bug of drm_bridge,
> > > since if the bridge is still in using by others, the rmmod should fail
> > > 
> > 
> > Correct, but there's no fix for that today. You can also take a look
> > at the thread linked from Mihail's cover letter.
> > 
> > > And personally opinion, if the bridge doesn't handle the dependence.
> > > for us:
> > > 
> > > - add such support to bridge
> > 
> > That would certainly be helpful. I don't know if there's consensus on
> > how to do that.
> > 
> > >   or
> > > - just do the insmod/rmmod in correct order.
> > > 
> > > > So, there really are proper benefits to sticking with the component
> > > > code for tda998x, which is why I'd like to understand why you're so
> > > > against this patch?
> > > >
> > > 
> > > This change handles two different connectors in komeda internally, compare
> > > with one interface, it increases the complexity, more risk of bug and more
> > > cost of maintainance.
> > > 
> > 
> > Well, it's only about how to bind the drivers - two different methods
> > of binding, not two different connectors. I would argue that carrying
> > our out-of-tree patches to support both platforms is a larger
> > maintenance burden.
> > 
> > Honestly this looks like a win-win to me. We get the superior approach
> > when its supported, and still get to support bridges which are more
> > common.
> > 
> > As/when improvements are made to the bridge code we can remove the
> > component bits and not lose anything.
> 
> There was an idea a while back about using the device links code to
> solve the bridge issue - but at the time the device links code wasn't
> up to the job.  I think that's been resolved now, but I haven't been
> able to confirm it.  I did propose some patches for bridge at the
> time but they probably need updating.

I think the only patches that existed where for panel, and we only
discussed the bridge case. At least I can only find patches for panel,not
bridge, but might be missing something.

Either way I think device core is fixed now, so would be really great if
someone can give this another stab, and make drm_bridge/panel easier to
use without fireworks on unload.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch