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

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Message-ID: <1762076.f1XlaKvzUH@e123338-lin>
Date:   Fri, 18 Oct 2019 11:01:12 +0000
From:   Mihail Atanassov <Mihail.Atanassov@....com>
To:     "james qian wang (Arm Technology China)" <james.qian.wang@....com>
CC:     Brian Starkey <Brian.Starkey@....com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        David Airlie <airlied@...ux.ie>,
        Liviu Dudau <Liviu.Dudau@....com>,
        Russell King <linux@...linux.org.uk>,
        Maxime Ripard <mripard@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        nd <nd@....com>, Sean Paul <sean@...rly.run>
Subject: Re: [RFC,3/3] drm/komeda: Allow non-component drm_bridge only
 endpoints

On Friday, 18 October 2019 07:38:59 BST james qian wang (Arm Technology China) 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.
> >
> 
> My consideration is: if we support both link methods, we may suffering
> 
> - 1. bridge reference cnt problem
> - 2. maintance two link methods.
> 
> the 1) seems unavoidable, so swap all to bridage at least can avoid
> the pain of 2). that's why I thought your idea "swap all to bridage"
> is good.
> 
> Thanks
> James.
> 

Just to make sure my understanding is clear: If I respin the patch to
only use the drm_bridge i/f, you'd be happier with it and we can get it
merged?

> > 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
> 


-- 
Mihail