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From:   "james qian wang (Arm Technology China)" <james.qian.wang@arm.com>
To:     Russell King - ARM Linux admin <linux@armlinux.org.uk>
CC:     Brian Starkey <Brian.Starkey@arm.com>,
        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>,
        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
Thread-Topic: [RFC,3/3] drm/komeda: Allow non-component drm_bridge only
 endpoints
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Date:   Fri, 18 Oct 2019 06:57:05 +0000
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On Thu, Oct 17, 2019 at 12:41:37PM +0100, Russell King - ARM Linux admin wr=
ote:
> 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 Technolo=
gy 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 Tech=
nology China) wrote:
> > > > > On Wed, Oct 16, 2019 at 04:22:07PM +0000, Brian Starkey wrote:
> > > > > >=20
> > > > > > If James is strongly against merging this, maybe we just swap
> > > > > > wholesale to bridge? But for me, the pragmatic approach would b=
e this
> > > > > > stop-gap.
> > > > > >
> > > > >=20
> > > > > This is a good idea, and I vote +ULONG_MAX :)
> > > > >=20
> > > > > and I also checked tda998x driver, it supports bridge. so swap th=
e
> > > > > wholesale to brige is perfect. :)
> > > > >=20
> > > >=20
> > > > Well, as Mihail wrote, it's definitely not perfect.
> > > >=20
> > > > Today, if you rmmod tda998x with the DPU driver still loaded,
> > > > everything will be unbound gracefully.
> > > >=20
> > > > 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 resu=
lt
> > > > in a crash.
> > >=20
> > > 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 fai=
l
> > >=20
> >=20
> > Correct, but there's no fix for that today. You can also take a look
> > at the thread linked from Mihail's cover letter.
> >=20
> > > And personally opinion, if the bridge doesn't handle the dependence.
> > > for us:
> > >=20
> > > - add such support to bridge
> >=20
> > That would certainly be helpful. I don't know if there's consensus on
> > how to do that.
> >=20
> > >   or
> > > - just do the insmod/rmmod in correct order.
> > >=20
> > > > 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?
> > > >
> > >=20
> > > This change handles two different connectors in komeda internally, co=
mpare
> > > with one interface, it increases the complexity, more risk of bug and=
 more
> > > cost of maintainance.
> > >=20
> >=20
> > 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.
> >=20
> > 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.
> >=20
> > As/when improvements are made to the bridge code we can remove the
> > component bits and not lose anything.
>=20
> 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.

Hi Russell:

Thank you, that's a good news.

Hi Brian:

Can this convince you to fully swap to bridge ?

Actually even there is no fix, we won't real encounter such rmmod problem,
since we always build the bridge/tda998 (by Y) into the image.

Thanks
James
> --=20
> RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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s up
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