linux-kernel - Re: [PATCH 00/24] device link, bridge supplier <-> drm device

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Date:   Fri, 27 Apr 2018 01:09:14 +0200
From:   Peter Rosin <peda@...ntia.se>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     linux-kernel@...r.kernel.org,
        Archit Taneja <architt@...eaurora.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        David Airlie <airlied@...ux.ie>,
        Peter Senna Tschudin <peter.senna@...labora.com>,
        Martin Donnelly <martin.donnelly@...com>,
        Martyn Welch <martyn.welch@...labora.co.uk>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Sean Paul <seanpaul@...omium.org>,
        Inki Dae <inki.dae@...sung.com>,
        Joonyoung Shim <jy0922.shim@...sung.com>,
        Seung-Woo Kim <sw0312.kim@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Kukjin Kim <kgene@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        CK Hu <ck.hu@...iatek.com>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Rob Clark <robdclark@...il.com>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Vincent Abriou <vincent.abriou@...com>,
        Jyri Sarha <jsarha@...com>, dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-samsung-soc@...r.kernel.org,
        linux-mediatek@...ts.infradead.org, linux-arm-msm@...r.kernel.org,
        freedreno@...ts.freedesktop.org, linux-renesas-soc@...r.kernel.org
Subject: Re: [PATCH 00/24] device link, bridge supplier <-> drm device

On 2018-04-27 00:40, Laurent Pinchart wrote:
> Hi Peter,
> 
> Thank you for the patches.
> 
> On Friday, 27 April 2018 01:31:15 EEST Peter Rosin wrote:
>> Hi!
>>
>> It was noted by Russel King [1] that bridges (not using components)
>> might disappear unexpectedly if the owner of the bridge was unbound.
>> Jyri Sarha had previously noted the same thing with panels [2]. Jyri
>> came up with using device links to resolve the panel issue, which
>> was also my (independent) reaction to the note from Russel.
>>
>> This series builds up to the addition of that link in the last
>> patch, but in my opinion the other 23 patches do have merit on their
>> own.
>>
>> The last patch needs testing, while the others look trivial. That
>> said, I might have missed some subtlety.
> 
> I don't think this is the way to go. We have a real lifetime management 
> problem with bridges, and device links are just trying to hide the problem 
> under the carpet. They will further corner us by making a real fix much more 
> difficult to implement. I'll try to comment further in the next few days on 
> what I think a better solution would be, but in a nutshell I believe that 
> drm_bridge objects need to be refcounted, with a .release() operation to free 
> the bridge resources when the reference count drops to zero. This shouldn't be 
> difficult to implement and I'm willing to help.

Ok, sp 24/24 is dead, and maybe 23/24 too. But how do you feel about dropping
.of_node in favour of .owner? That gets rid of ugly #ifdefs...

I also have the nagging feeling that .driver_private serves very little real
purpose if there is a .owner so that you can do

	dev_get_drvdata(bridge->owner)

for the cases where the container_of macro is not appropriate.

Cheers,
Peter