Re: [RFC,3/3] drm/komeda: Allow non-component drm_bridge only endpoints - Russell King

From: Russell King - ARM Linux admin <linux@armlinux.org.uk>
To: Daniel Vetter <daniel@ffwll.ch>
Cc: Brian Starkey <Brian.Starkey@arm.com>, nd <nd@arm.com>,
	David Airlie <airlied@linux.ie>,
	Liviu Dudau <Liviu.Dudau@arm.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"james qian wang (Arm Technology China)"
	<james.qian.wang@arm.com>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	Mihail Atanassov <Mihail.Atanassov@arm.com>,
	Sean Paul <sean@poorly.run>
Subject: Re: [RFC,3/3] drm/komeda: Allow non-component drm_bridge only endpoints
Date: Tue, 22 Oct 2019 09:48:26 +0100	[thread overview]
Message-ID: <20191022084826.GP25745@shell.armlinux.org.uk> (raw)
In-Reply-To: <20191022084210.GX11828@phenom.ffwll.local>

On Tue, Oct 22, 2019 at 10:42:10AM +0200, Daniel Vetter wrote:
> 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.

I had a patches, which is why I raised the problem with the core:

6961edfee26d bridge hacks using device links

but it never went further than an experiment at the time because of the
problems in the core.  As it was a hack, it never got posted.  Seems
that kernel tree (for the cubox) is still 5.2 based, so has a lot of
patches and might need updating to a more recent base before anything
can be tested.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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