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

From: Brian Starkey <Brian.Starkey@arm.com>
To: "james qian wang (Arm Technology China)" <james.qian.wang@arm.com>
Cc: Mihail Atanassov <Mihail.Atanassov@arm.com>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	David Airlie <airlied@linux.ie>,
	Liviu Dudau <Liviu.Dudau@arm.com>,
	Russell King <linux@armlinux.org.uk>,
	Maxime Ripard <mripard@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	nd <nd@arm.com>, Sean Paul <sean@poorly.run>
Subject: Re: [RFC,3/3] drm/komeda: Allow non-component drm_bridge only endpoints
Date: Thu, 17 Oct 2019 10:48:12 +0000	[thread overview]
Message-ID: <20191017104812.6qpuzoh5bx5i2y3m@DESKTOP-E1NTVVP.localdomain> (raw)
In-Reply-To: <20191017102055.GA8308@jamwan02-TSP300>

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.

> So my suggestion is keeping on one single interface in komeda, no
> matter it is bridge or component, but I'd like it only one, but not
> them both in komeda.

If we can put the effort into fixing bridges then I guess that's the
best approach for everyone :-) Might not be easy though!

-Brian

> 
> Thanks
> James
> 
> > Thanks,
> > -Brian